[PATCH tip/core/rcu 0/40] RCU ->gp_seq conversion for v4.19

[PATCH tip/core/rcu 0/40] RCU ->gp_seq conversion for v4.19

[Paul E. McKenney]

Hello!

This series moves from the two-variable ->gpnum and ->completed to
a single-variable ->gp_seq approach.  The key point is that ->gpnum
and ->completed are either identical or ->gpnum is only one greater,
which means that we are using a full unsigned long were a single bit
would do.  This series therefore applies the pre-existing grace-period
sequence-number code used by expedited grace periods, _rcu_barrier(),
and SRCU.  This change also has the beneficial effect of allowing the
grace-period state to be sampled with a single load, which will be used
to improve the grace-period start code.  The individual commits are
as follows:

1.	Introduce grace-period sequence numbers (->gp_seq) to normal RCU.

2-17.	Make core RCU code use ->gp_seq instead of ->gpnum and ->completed.

18.	Move from ->need_future_gp[] to ->gp_seq_needed, which allows
	simpler grace-period-already-requested checks.

19.	Use the new ->gp_seq_needed to make rcu_nocb_wait_gp() check if
	a given grace has already been requested, thus reducing overhead
	for no-CBs CPUs.

20-26.	Move RCU tracepoints to ->gp_seq.

27.	Remove ->gpnum and ->completed.

28.	Make simple callback acceleration refer to rdp->gp_seq_needed.

29-31.	Update documentation to reflect ->gp_seq.

32.	Don't funnel-lock above the leaf rcu_node structure if a grace period
	is in progress, thus saving an upper-level rcu_node ->lock
	acquisition.

33.	Produce the last CleanupMore trace only if the corresponding
	grace-period request was quite recent, courtesy of Joel Fernandes.

34.	Fix cpustart tracepoint ->gp_seq number, courtesy of Joel Fernandes.

35.	Make rcu_start_this_gp() check for grace period already started.

36.	Correctly handle grace-period sequence (->gp_seq) wrap in
	rcutorture.

37.	Regularize resetting of rcu_data wrap indicator.

38-39.	Rename grace-period-request variables and parameters, courtesy
	of Joel Fernandes.

40.	Check current node rather than leaf node for requested grace
	period already being in progress.

							Thanx, Paul

------------------------------------------------------------------------

 Documentation/RCU/Design/Data-Structures/Data-Structures.html          |  118 -
 Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Memory-Ordering.html |   22 
 Documentation/RCU/Design/Memory-Ordering/TreeRCU-gp-cleanup.svg        |  123 -
 Documentation/RCU/Design/Memory-Ordering/TreeRCU-gp-init-1.svg         |   16 
 Documentation/RCU/Design/Memory-Ordering/TreeRCU-gp-init-3.svg         |   56 
 Documentation/RCU/Design/Memory-Ordering/TreeRCU-gp.svg                |  237 +--
 Documentation/RCU/Design/Memory-Ordering/TreeRCU-qs.svg                |   12 
 Documentation/RCU/stallwarn.txt                                        |   24 
 include/trace/events/rcu.h                                             |  114 -
 kernel/rcu/rcu.h                                                       |   70 -
 kernel/rcu/rcuperf.c                                                   |   44 
 kernel/rcu/rcutorture.c                                                |   93 -
 kernel/rcu/srcutree.c                                                  |    5 
 kernel/rcu/tree.c                                                      |  678 ++++------
 kernel/rcu/tree.h                                                      |   40 
 kernel/rcu/tree_plugin.h                                               |   54 
 16 files changed, 832 insertions(+), 874 deletions(-)

[PATCH tip/core/rcu 01/40] rcu: Introduce grace-period sequence numbers

[Paul E. McKenney]

This commit adds grace-period sequence numbers (->gp_seq) to the
rcu_state, rcu_node, and rcu_data structures, and updates them.
It also checks for consistency between rsp->gpnum and rsp->gp_seq.
These ->gp_seq counters will eventually replace the existing ->gpnum
and ->completed counters, allowing a single memory access to determine
whether or not a grace period is in progress and if so, which one.
This in turn will enable changes that will reduce ->lock contention on
the leaf rcu_node structures.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 kernel/rcu/tree.c | 16 +++++++++++++++-
 kernel/rcu/tree.h |  3 +++
 2 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 2db3475c2924..c50bb88dcab0 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -97,6 +97,7 @@ struct rcu_state sname##_state = { \
 	.gp_state = RCU_GP_IDLE, \
 	.gpnum = 0UL - 300UL, \
 	.completed = 0UL - 300UL, \
+	.gp_seq = (0UL - 300UL) << RCU_SEQ_CTR_SHIFT, \
 	.barrier_mutex = __MUTEX_INITIALIZER(sname##_state.barrier_mutex), \
 	.name = RCU_STATE_NAME(sname), \
 	.abbr = sabbr, \
@@ -1849,6 +1850,8 @@ static bool __note_gp_changes(struct rcu_state *rsp, struct rcu_node *rnp,
 		WRITE_ONCE(rdp->gpwrap, false);
 		rcu_gpnum_ovf(rnp, rdp);
 	}
+	if (rdp->gp_seq != rnp->gp_seq)
+		rdp->gp_seq = rnp->gp_seq;
 	return ret;
 }
 
@@ -1910,7 +1913,10 @@ static bool rcu_gp_init(struct rcu_state *rsp)
 	/* Advance to a new grace period and initialize state. */
 	record_gp_stall_check_time(rsp);
 	/* Record GP times before starting GP, hence smp_store_release(). */
+	WARN_ON_ONCE(rsp->gpnum << RCU_SEQ_CTR_SHIFT != rsp->gp_seq);
 	smp_store_release(&rsp->gpnum, rsp->gpnum + 1);
+	smp_mb(); /* Pairs with barriers in stall-warning code. */
+	rcu_seq_start(&rsp->gp_seq);
 	trace_rcu_grace_period(rsp->name, rsp->gpnum, TPS("start"));
 	raw_spin_unlock_irq_rcu_node(rnp);
 
@@ -1984,6 +1990,7 @@ static bool rcu_gp_init(struct rcu_state *rsp)
 		WRITE_ONCE(rnp->gpnum, rsp->gpnum);
 		if (WARN_ON_ONCE(rnp->completed != rsp->completed))
 			WRITE_ONCE(rnp->completed, rsp->completed);
+		WRITE_ONCE(rnp->gp_seq, rsp->gp_seq);
 		if (rnp == rdp->mynode)
 			(void)__note_gp_changes(rsp, rnp, rdp);
 		rcu_preempt_boost_start_gp(rnp);
@@ -2050,6 +2057,7 @@ static void rcu_gp_cleanup(struct rcu_state *rsp)
 {
 	unsigned long gp_duration;
 	bool needgp = false;
+	unsigned long new_gp_seq;
 	struct rcu_data *rdp;
 	struct rcu_node *rnp = rcu_get_root(rsp);
 	struct swait_queue_head *sq;
@@ -2079,12 +2087,15 @@ static void rcu_gp_cleanup(struct rcu_state *rsp)
 	 * all of the rcu_node structures before the beginning of the next
 	 * grace period is recorded in any of the rcu_node structures.
 	 */
+	new_gp_seq = rsp->gp_seq;
+	rcu_seq_end(&new_gp_seq);
 	rcu_for_each_node_breadth_first(rsp, rnp) {
 		raw_spin_lock_irq_rcu_node(rnp);
 		if (WARN_ON_ONCE(rcu_preempt_blocked_readers_cgp(rnp)))
 			dump_blkd_tasks(rnp, 10);
 		WARN_ON_ONCE(rnp->qsmask);
 		WRITE_ONCE(rnp->completed, rsp->gpnum);
+		WRITE_ONCE(rnp->gp_seq, new_gp_seq);
 		rdp = this_cpu_ptr(rsp->rda);
 		if (rnp == rdp->mynode)
 			needgp = __note_gp_changes(rsp, rnp, rdp) || needgp;
@@ -2098,10 +2109,11 @@ static void rcu_gp_cleanup(struct rcu_state *rsp)
 		rcu_gp_slow(rsp, gp_cleanup_delay);
 	}
 	rnp = rcu_get_root(rsp);
-	raw_spin_lock_irq_rcu_node(rnp); /* Order GP before ->completed update. */
+	raw_spin_lock_irq_rcu_node(rnp); /* GP before rsp->gp_seq update. */
 
 	/* Declare grace period done. */
 	WRITE_ONCE(rsp->completed, rsp->gpnum);
+	rcu_seq_end(&rsp->gp_seq);
 	trace_rcu_grace_period(rsp->name, rsp->completed, TPS("end"));
 	rsp->gp_state = RCU_GP_IDLE;
 	/* Check for GP requests since above loop. */
@@ -3609,6 +3621,7 @@ rcu_init_percpu_data(int cpu, struct rcu_state *rsp)
 	rdp->beenonline = true;	 /* We have now been online. */
 	rdp->gpnum = rnp->completed; /* Make CPU later note any new GP. */
 	rdp->completed = rnp->completed;
+	rdp->gp_seq = rnp->gp_seq;
 	rdp->cpu_no_qs.b.norm = true;
 	rdp->rcu_qs_ctr_snap = per_cpu(rcu_dynticks.rcu_qs_ctr, cpu);
 	rdp->core_needs_qs = false;
@@ -3988,6 +4001,7 @@ static void __init rcu_init_one(struct rcu_state *rsp)
 						   &rcu_fqs_class[i], fqs[i]);
 			rnp->gpnum = rsp->gpnum;
 			rnp->completed = rsp->completed;
+			rnp->gp_seq = rsp->gp_seq;
 			rnp->completedqs = rsp->completed;
 			rnp->qsmask = 0;
 			rnp->qsmaskinit = 0;
diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index 3c1942174c56..50a28d1cf5a1 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -87,6 +87,7 @@ struct rcu_node {
 	unsigned long completed; /* Last GP completed for this node. */
 				/*  This will either be equal to or one */
 				/*  behind the root rcu_node's gpnum. */
+	unsigned long gp_seq;	/* Track rsp->rcu_gp_seq. */
 	unsigned long completedqs; /* All QSes done for this node. */
 	unsigned long qsmask;	/* CPUs or groups that need to switch in */
 				/*  order for current grace period to proceed.*/
@@ -211,6 +212,7 @@ struct rcu_data {
 					/*  in order to detect GP end. */
 	unsigned long	gpnum;		/* Highest gp number that this CPU */
 					/*  is aware of having started. */
+	unsigned long	gp_seq;		/* Track rsp->rcu_gp_seq counter. */
 	unsigned long	rcu_qs_ctr_snap;/* Snapshot of rcu_qs_ctr to check */
 					/*  for rcu_all_qs() invocations. */
 	union rcu_noqs	cpu_no_qs;	/* No QSes yet for this CPU. */
@@ -343,6 +345,7 @@ struct rcu_state {
 						/* Subject to priority boost. */
 	unsigned long gpnum;			/* Current gp number. */
 	unsigned long completed;		/* # of last completed gp. */
+	unsigned long gp_seq;			/* Grace-period sequence #. */
 	struct task_struct *gp_kthread;		/* Task for grace periods. */
 	struct swait_queue_head gp_wq;		/* Where GP task waits. */
 	short gp_flags;				/* Commands for GP task. */
-- 
2.17.1

[PATCH tip/core/rcu 02/40] rcu: Move rcu_gp_slow() to ->gp_seq

[Paul E. McKenney]

This commit moves rcu_gp_slow() to ->gp_seq.  This function only uses
the grace-period number to modulate delay, so rcu_seq_ctr(rsp->gp_seq)
gets the same effect, at least in cases where the delay is to happen
more than four times per wrap of an unsigned long.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 kernel/rcu/tree.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index c50bb88dcab0..90346ffd721f 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1879,7 +1879,8 @@ static void note_gp_changes(struct rcu_state *rsp, struct rcu_data *rdp)
 static void rcu_gp_slow(struct rcu_state *rsp, int delay)
 {
 	if (delay > 0 &&
-	    !(rsp->gpnum % (rcu_num_nodes * PER_RCU_NODE_PERIOD * delay)))
+	    !(rcu_seq_ctr(rsp->gp_seq) %
+	      (rcu_num_nodes * PER_RCU_NODE_PERIOD * delay)))
 		schedule_timeout_uninterruptible(delay);
 }
 
-- 
2.17.1

[PATCH tip/core/rcu 03/40] rcu: Make rcutorture's batches-completed API use ->gp_seq

[Paul E. McKenney]

The rcutorture test invokes rcu_batches_started(),
rcu_batches_completed(), rcu_batches_started_bh(),
rcu_batches_completed_bh(), rcu_batches_started_sched(), and
rcu_batches_completed_sched() to do grace-period consistency checks,
and rcuperf uses the _completed variants for statistics.
These functions use ->gpnum and ->completed.  This commit therefore
replaces them with rcu_get_gp_seq(), rcu_bh_get_gp_seq(), and
rcu_sched_get_gp_seq(), adjusting rcutorture and rcuperf to make
use of them.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 kernel/rcu/rcu.h        | 18 +++++----------
 kernel/rcu/rcuperf.c    | 26 ++++++++-------------
 kernel/rcu/rcutorture.c | 50 +++++++++++++++-------------------------
 kernel/rcu/tree.c       | 51 ++++++++++-------------------------------
 4 files changed, 45 insertions(+), 100 deletions(-)

diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
index db0870acfdff..f0907f9f6cd0 100644
--- a/kernel/rcu/rcu.h
+++ b/kernel/rcu/rcu.h
@@ -463,12 +463,9 @@ void srcutorture_get_gp_data(enum rcutorture_type test_type,
 #endif
 
 #ifdef CONFIG_TINY_RCU
-static inline unsigned long rcu_batches_started(void) { return 0; }
-static inline unsigned long rcu_batches_started_bh(void) { return 0; }
-static inline unsigned long rcu_batches_started_sched(void) { return 0; }
-static inline unsigned long rcu_batches_completed(void) { return 0; }
-static inline unsigned long rcu_batches_completed_bh(void) { return 0; }
-static inline unsigned long rcu_batches_completed_sched(void) { return 0; }
+static inline unsigned long rcu_get_gp_seq(void) { return 0; }
+static inline unsigned long rcu_bh_get_gp_seq(void) { return 0; }
+static inline unsigned long rcu_sched_get_gp_seq(void) { return 0; }
 static inline unsigned long rcu_exp_batches_completed(void) { return 0; }
 static inline unsigned long rcu_exp_batches_completed_sched(void) { return 0; }
 static inline unsigned long
@@ -480,12 +477,9 @@ static inline void show_rcu_gp_kthreads(void) { }
 #else /* #ifdef CONFIG_TINY_RCU */
 extern unsigned long rcutorture_testseq;
 extern unsigned long rcutorture_vernum;
-unsigned long rcu_batches_started(void);
-unsigned long rcu_batches_started_bh(void);
-unsigned long rcu_batches_started_sched(void);
-unsigned long rcu_batches_completed(void);
-unsigned long rcu_batches_completed_bh(void);
-unsigned long rcu_batches_completed_sched(void);
+unsigned long rcu_get_gp_seq(void);
+unsigned long rcu_bh_get_gp_seq(void);
+unsigned long rcu_sched_get_gp_seq(void);
 unsigned long rcu_exp_batches_completed(void);
 unsigned long rcu_exp_batches_completed_sched(void);
 unsigned long srcu_batches_completed(struct srcu_struct *sp);
diff --git a/kernel/rcu/rcuperf.c b/kernel/rcu/rcuperf.c
index df29119b2013..2b5a613afcf3 100644
--- a/kernel/rcu/rcuperf.c
+++ b/kernel/rcu/rcuperf.c
@@ -138,8 +138,7 @@ struct rcu_perf_ops {
 	void (*cleanup)(void);
 	int (*readlock)(void);
 	void (*readunlock)(int idx);
-	unsigned long (*started)(void);
-	unsigned long (*completed)(void);
+	unsigned long (*get_gp_seq)(void);
 	unsigned long (*exp_completed)(void);
 	void (*async)(struct rcu_head *head, rcu_callback_t func);
 	void (*gp_barrier)(void);
@@ -179,8 +178,7 @@ static struct rcu_perf_ops rcu_ops = {
 	.init		= rcu_sync_perf_init,
 	.readlock	= rcu_perf_read_lock,
 	.readunlock	= rcu_perf_read_unlock,
-	.started	= rcu_batches_started,
-	.completed	= rcu_batches_completed,
+	.get_gp_seq	= rcu_get_gp_seq,
 	.exp_completed	= rcu_exp_batches_completed,
 	.async		= call_rcu,
 	.gp_barrier	= rcu_barrier,
@@ -209,8 +207,7 @@ static struct rcu_perf_ops rcu_bh_ops = {
 	.init		= rcu_sync_perf_init,
 	.readlock	= rcu_bh_perf_read_lock,
 	.readunlock	= rcu_bh_perf_read_unlock,
-	.started	= rcu_batches_started_bh,
-	.completed	= rcu_batches_completed_bh,
+	.get_gp_seq	= rcu_bh_get_gp_seq,
 	.exp_completed	= rcu_exp_batches_completed_sched,
 	.async		= call_rcu_bh,
 	.gp_barrier	= rcu_barrier_bh,
@@ -266,8 +263,7 @@ static struct rcu_perf_ops srcu_ops = {
 	.init		= rcu_sync_perf_init,
 	.readlock	= srcu_perf_read_lock,
 	.readunlock	= srcu_perf_read_unlock,
-	.started	= NULL,
-	.completed	= srcu_perf_completed,
+	.get_gp_seq	= srcu_perf_completed,
 	.exp_completed	= srcu_perf_completed,
 	.async		= srcu_call_rcu,
 	.gp_barrier	= srcu_rcu_barrier,
@@ -295,8 +291,7 @@ static struct rcu_perf_ops srcud_ops = {
 	.cleanup	= srcu_sync_perf_cleanup,
 	.readlock	= srcu_perf_read_lock,
 	.readunlock	= srcu_perf_read_unlock,
-	.started	= NULL,
-	.completed	= srcu_perf_completed,
+	.get_gp_seq	= srcu_perf_completed,
 	.exp_completed	= srcu_perf_completed,
 	.async		= srcu_call_rcu,
 	.gp_barrier	= srcu_rcu_barrier,
@@ -325,8 +320,7 @@ static struct rcu_perf_ops sched_ops = {
 	.init		= rcu_sync_perf_init,
 	.readlock	= sched_perf_read_lock,
 	.readunlock	= sched_perf_read_unlock,
-	.started	= rcu_batches_started_sched,
-	.completed	= rcu_batches_completed_sched,
+	.get_gp_seq	= rcu_sched_get_gp_seq,
 	.exp_completed	= rcu_exp_batches_completed_sched,
 	.async		= call_rcu_sched,
 	.gp_barrier	= rcu_barrier_sched,
@@ -353,8 +347,7 @@ static struct rcu_perf_ops tasks_ops = {
 	.init		= rcu_sync_perf_init,
 	.readlock	= tasks_perf_read_lock,
 	.readunlock	= tasks_perf_read_unlock,
-	.started	= rcu_no_completed,
-	.completed	= rcu_no_completed,
+	.get_gp_seq	= rcu_no_completed,
 	.async		= call_rcu_tasks,
 	.gp_barrier	= rcu_barrier_tasks,
 	.sync		= synchronize_rcu_tasks,
@@ -447,8 +440,7 @@ rcu_perf_writer(void *arg)
 			b_rcu_perf_writer_started =
 				cur_ops->exp_completed() / 2;
 		} else {
-			b_rcu_perf_writer_started =
-				cur_ops->completed();
+			b_rcu_perf_writer_started = cur_ops->get_gp_seq();
 		}
 	}
 
@@ -505,7 +497,7 @@ rcu_perf_writer(void *arg)
 						cur_ops->exp_completed() / 2;
 				} else {
 					b_rcu_perf_writer_finished =
-						cur_ops->completed();
+						cur_ops->get_gp_seq();
 				}
 				if (shutdown) {
 					smp_mb(); /* Assign before wake. */
diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
index 5604bfac8df4..1f66597c7783 100644
--- a/kernel/rcu/rcutorture.c
+++ b/kernel/rcu/rcutorture.c
@@ -264,8 +264,7 @@ struct rcu_torture_ops {
 	int (*readlock)(void);
 	void (*read_delay)(struct torture_random_state *rrsp);
 	void (*readunlock)(int idx);
-	unsigned long (*started)(void);
-	unsigned long (*completed)(void);
+	unsigned long (*get_gp_seq)(void);
 	void (*deferred_free)(struct rcu_torture *p);
 	void (*sync)(void);
 	void (*exp_sync)(void);
@@ -305,10 +304,10 @@ static void rcu_read_delay(struct torture_random_state *rrsp)
 	 * force_quiescent_state. */
 
 	if (!(torture_random(rrsp) % (nrealreaders * 2000 * longdelay_ms))) {
-		started = cur_ops->completed();
+		started = cur_ops->get_gp_seq();
 		ts = rcu_trace_clock_local();
 		mdelay(longdelay_ms);
-		completed = cur_ops->completed();
+		completed = cur_ops->get_gp_seq();
 		do_trace_rcu_torture_read(cur_ops->name, NULL, ts,
 					  started, completed);
 	}
@@ -400,8 +399,7 @@ static struct rcu_torture_ops rcu_ops = {
 	.readlock	= rcu_torture_read_lock,
 	.read_delay	= rcu_read_delay,
 	.readunlock	= rcu_torture_read_unlock,
-	.started	= rcu_batches_started,
-	.completed	= rcu_batches_completed,
+	.get_gp_seq	= rcu_get_gp_seq,
 	.deferred_free	= rcu_torture_deferred_free,
 	.sync		= synchronize_rcu,
 	.exp_sync	= synchronize_rcu_expedited,
@@ -442,8 +440,7 @@ static struct rcu_torture_ops rcu_bh_ops = {
 	.readlock	= rcu_bh_torture_read_lock,
 	.read_delay	= rcu_read_delay,  /* just reuse rcu's version. */
 	.readunlock	= rcu_bh_torture_read_unlock,
-	.started	= rcu_batches_started_bh,
-	.completed	= rcu_batches_completed_bh,
+	.get_gp_seq	= rcu_bh_get_gp_seq,
 	.deferred_free	= rcu_bh_torture_deferred_free,
 	.sync		= synchronize_rcu_bh,
 	.exp_sync	= synchronize_rcu_bh_expedited,
@@ -486,8 +483,7 @@ static struct rcu_torture_ops rcu_busted_ops = {
 	.readlock	= rcu_torture_read_lock,
 	.read_delay	= rcu_read_delay,  /* just reuse rcu's version. */
 	.readunlock	= rcu_torture_read_unlock,
-	.started	= rcu_no_completed,
-	.completed	= rcu_no_completed,
+	.get_gp_seq	= rcu_no_completed,
 	.deferred_free	= rcu_busted_torture_deferred_free,
 	.sync		= synchronize_rcu_busted,
 	.exp_sync	= synchronize_rcu_busted,
@@ -575,8 +571,7 @@ static struct rcu_torture_ops srcu_ops = {
 	.readlock	= srcu_torture_read_lock,
 	.read_delay	= srcu_read_delay,
 	.readunlock	= srcu_torture_read_unlock,
-	.started	= NULL,
-	.completed	= srcu_torture_completed,
+	.get_gp_seq	= srcu_torture_completed,
 	.deferred_free	= srcu_torture_deferred_free,
 	.sync		= srcu_torture_synchronize,
 	.exp_sync	= srcu_torture_synchronize_expedited,
@@ -613,8 +608,7 @@ static struct rcu_torture_ops srcud_ops = {
 	.readlock	= srcu_torture_read_lock,
 	.read_delay	= srcu_read_delay,
 	.readunlock	= srcu_torture_read_unlock,
-	.started	= NULL,
-	.completed	= srcu_torture_completed,
+	.get_gp_seq	= srcu_torture_completed,
 	.deferred_free	= srcu_torture_deferred_free,
 	.sync		= srcu_torture_synchronize,
 	.exp_sync	= srcu_torture_synchronize_expedited,
@@ -651,8 +645,7 @@ static struct rcu_torture_ops sched_ops = {
 	.readlock	= sched_torture_read_lock,
 	.read_delay	= rcu_read_delay,  /* just reuse rcu's version. */
 	.readunlock	= sched_torture_read_unlock,
-	.started	= rcu_batches_started_sched,
-	.completed	= rcu_batches_completed_sched,
+	.get_gp_seq	= rcu_sched_get_gp_seq,
 	.deferred_free	= rcu_sched_torture_deferred_free,
 	.sync		= synchronize_sched,
 	.exp_sync	= synchronize_sched_expedited,
@@ -690,8 +683,7 @@ static struct rcu_torture_ops tasks_ops = {
 	.readlock	= tasks_torture_read_lock,
 	.read_delay	= rcu_read_delay,  /* just reuse rcu's version. */
 	.readunlock	= tasks_torture_read_unlock,
-	.started	= rcu_no_completed,
-	.completed	= rcu_no_completed,
+	.get_gp_seq	= rcu_no_completed,
 	.deferred_free	= rcu_tasks_torture_deferred_free,
 	.sync		= synchronize_rcu_tasks,
 	.exp_sync	= synchronize_rcu_tasks,
@@ -1104,10 +1096,7 @@ static void rcu_torture_timer(struct timer_list *unused)
 	unsigned long long ts;
 
 	idx = cur_ops->readlock();
-	if (cur_ops->started)
-		started = cur_ops->started();
-	else
-		started = cur_ops->completed();
+	started = cur_ops->get_gp_seq();
 	ts = rcu_trace_clock_local();
 	p = rcu_dereference_check(rcu_torture_current,
 				  rcu_read_lock_bh_held() ||
@@ -1131,7 +1120,7 @@ static void rcu_torture_timer(struct timer_list *unused)
 		/* Should not happen, but... */
 		pipe_count = RCU_TORTURE_PIPE_LEN;
 	}
-	completed = cur_ops->completed();
+	completed = cur_ops->get_gp_seq();
 	if (pipe_count > 1) {
 		do_trace_rcu_torture_read(cur_ops->name, &p->rtort_rcu, ts,
 					  started, completed);
@@ -1139,8 +1128,8 @@ static void rcu_torture_timer(struct timer_list *unused)
 	}
 	__this_cpu_inc(rcu_torture_count[pipe_count]);
 	completed = completed - started;
-	if (cur_ops->started)
-		completed++;
+	if (completed > ULONG_MAX >> 1)
+		completed = 0; /* Not all gp_seq have full range. */
 	if (completed > RCU_TORTURE_PIPE_LEN) {
 		/* Should not happen, but... */
 		completed = RCU_TORTURE_PIPE_LEN;
@@ -1187,10 +1176,7 @@ rcu_torture_reader(void *arg)
 				mod_timer(&t, jiffies + 1);
 		}
 		idx = cur_ops->readlock();
-		if (cur_ops->started)
-			started = cur_ops->started();
-		else
-			started = cur_ops->completed();
+		started = cur_ops->get_gp_seq();
 		ts = rcu_trace_clock_local();
 		p = rcu_dereference_check(rcu_torture_current,
 					  rcu_read_lock_bh_held() ||
@@ -1212,7 +1198,7 @@ rcu_torture_reader(void *arg)
 			/* Should not happen, but... */
 			pipe_count = RCU_TORTURE_PIPE_LEN;
 		}
-		completed = cur_ops->completed();
+		completed = cur_ops->get_gp_seq();
 		if (pipe_count > 1) {
 			do_trace_rcu_torture_read(cur_ops->name, &p->rtort_rcu,
 						  ts, started, completed);
@@ -1220,8 +1206,8 @@ rcu_torture_reader(void *arg)
 		}
 		__this_cpu_inc(rcu_torture_count[pipe_count]);
 		completed = completed - started;
-		if (cur_ops->started)
-			completed++;
+		if (completed > ULONG_MAX >> 1)
+			completed = 0; /* Not all gp_seq have full range. */
 		if (completed > RCU_TORTURE_PIPE_LEN) {
 			/* Should not happen, but... */
 			completed = RCU_TORTURE_PIPE_LEN;
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 90346ffd721f..0961f6c61894 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -530,58 +530,31 @@ static void force_quiescent_state(struct rcu_state *rsp);
 static int rcu_pending(void);
 
 /*
- * Return the number of RCU batches started thus far for debug & stats.
+ * Return the number of RCU GPs completed thus far for debug & stats.
  */
-unsigned long rcu_batches_started(void)
+unsigned long rcu_get_gp_seq(void)
 {
-	return rcu_state_p->gpnum;
+	return rcu_seq_ctr(READ_ONCE(rcu_state_p->gp_seq));
 }
-EXPORT_SYMBOL_GPL(rcu_batches_started);
+EXPORT_SYMBOL_GPL(rcu_get_gp_seq);
 
 /*
- * Return the number of RCU-sched batches started thus far for debug & stats.
+ * Return the number of RCU-sched GPs completed thus far for debug & stats.
  */
-unsigned long rcu_batches_started_sched(void)
+unsigned long rcu_sched_get_gp_seq(void)
 {
-	return rcu_sched_state.gpnum;
+	return rcu_seq_ctr(READ_ONCE(rcu_sched_state.gp_seq));
 }
-EXPORT_SYMBOL_GPL(rcu_batches_started_sched);
+EXPORT_SYMBOL_GPL(rcu_sched_get_gp_seq);
 
 /*
- * Return the number of RCU BH batches started thus far for debug & stats.
+ * Return the number of RCU-bh GPs completed thus far for debug & stats.
  */
-unsigned long rcu_batches_started_bh(void)
+unsigned long rcu_bh_get_gp_seq(void)
 {
-	return rcu_bh_state.gpnum;
+	return rcu_seq_ctr(READ_ONCE(rcu_bh_state.gp_seq));
 }
-EXPORT_SYMBOL_GPL(rcu_batches_started_bh);
-
-/*
- * Return the number of RCU batches completed thus far for debug & stats.
- */
-unsigned long rcu_batches_completed(void)
-{
-	return rcu_state_p->completed;
-}
-EXPORT_SYMBOL_GPL(rcu_batches_completed);
-
-/*
- * Return the number of RCU-sched batches completed thus far for debug & stats.
- */
-unsigned long rcu_batches_completed_sched(void)
-{
-	return rcu_sched_state.completed;
-}
-EXPORT_SYMBOL_GPL(rcu_batches_completed_sched);
-
-/*
- * Return the number of RCU BH batches completed thus far for debug & stats.
- */
-unsigned long rcu_batches_completed_bh(void)
-{
-	return rcu_bh_state.completed;
-}
-EXPORT_SYMBOL_GPL(rcu_batches_completed_bh);
+EXPORT_SYMBOL_GPL(rcu_bh_get_gp_seq);
 
 /*
  * Return the number of RCU expedited batches completed thus far for
-- 
2.17.1

[PATCH tip/core/rcu 04/40] rcu: Convert rcu_check_gp_kthread_starvation() to GP sequence number

[Paul E. McKenney]

This commit switches rcu_check_gp_kthread_starvation() from printing
->gpnum and ->completed to printing ->gp_seq upon detecting a starving
RCU grace-period kthread during an RCU CPU stall warning.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 kernel/rcu/tree.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 0961f6c61894..b9e69bbbecf5 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1276,9 +1276,9 @@ static void rcu_check_gp_kthread_starvation(struct rcu_state *rsp)
 	j = jiffies;
 	gpa = READ_ONCE(rsp->gp_activity);
 	if (j - gpa > 2 * HZ) {
-		pr_err("%s kthread starved for %ld jiffies! g%lu c%lu f%#x %s(%d) ->state=%#lx ->cpu=%d\n",
+		pr_err("%s kthread starved for %ld jiffies! g%ld f%#x %s(%d) ->state=%#lx ->cpu=%d\n",
 		       rsp->name, j - gpa,
-		       rsp->gpnum, rsp->completed,
+		       (long)rcu_seq_current(&rsp->gp_seq),
 		       rsp->gp_flags,
 		       gp_state_getname(rsp->gp_state), rsp->gp_state,
 		       rsp->gp_kthread ? rsp->gp_kthread->state : ~0,
-- 
2.17.1

[PATCH tip/core/rcu 05/40] rcu: Make quiescent-state reporting use ->gp_seq

[Paul E. McKenney]

This commit switches the functions reporting quiescent states from
use of ->gpnum to ->gp_seq.  In either case, the point is to handle
races where a given grace period ends before a quiescent state can
be reported.  Failing to catch these races would result in too-short
grace periods, hence the checking.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 kernel/rcu/tree.c | 18 +++++++++---------
 1 file changed, 9 insertions(+), 9 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index b9e69bbbecf5..001596f05111 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2242,7 +2242,7 @@ static void rcu_report_qs_rsp(struct rcu_state *rsp, unsigned long flags)
  * must be represented by the same rcu_node structure (which need not be a
  * leaf rcu_node structure, though it often will be).  The gps parameter
  * is the grace-period snapshot, which means that the quiescent states
- * are valid only if rnp->gpnum is equal to gps.  That structure's lock
+ * are valid only if rnp->gp_seq is equal to gps.  That structure's lock
  * must be held upon entry, and it is released before return.
  */
 static void
@@ -2257,7 +2257,7 @@ rcu_report_qs_rnp(unsigned long mask, struct rcu_state *rsp,
 
 	/* Walk up the rcu_node hierarchy. */
 	for (;;) {
-		if (!(rnp->qsmask & mask) || rnp->gpnum != gps) {
+		if (!(rnp->qsmask & mask) || rnp->gp_seq != gps) {
 
 			/*
 			 * Our bit has already been cleared, or the
@@ -2335,8 +2335,8 @@ static void rcu_report_unblock_qs_rnp(struct rcu_state *rsp,
 		return;
 	}
 
-	/* Report up the rest of the hierarchy, tracking current ->gpnum. */
-	gps = rnp->gpnum;
+	/* Report up the rest of the hierarchy, tracking current ->gp_seq. */
+	gps = rnp->gp_seq;
 	mask = rnp->grpmask;
 	raw_spin_unlock_rcu_node(rnp);	/* irqs remain disabled. */
 	raw_spin_lock_rcu_node(rnp_p);	/* irqs already disabled. */
@@ -2357,8 +2357,8 @@ rcu_report_qs_rdp(int cpu, struct rcu_state *rsp, struct rcu_data *rdp)
 
 	rnp = rdp->mynode;
 	raw_spin_lock_irqsave_rcu_node(rnp, flags);
-	if (rdp->cpu_no_qs.b.norm || rdp->gpnum != rnp->gpnum ||
-	    rnp->completed == rnp->gpnum || rdp->gpwrap) {
+	if (rdp->cpu_no_qs.b.norm || rdp->gp_seq != rnp->gp_seq ||
+	    rdp->gpwrap) {
 
 		/*
 		 * The grace period in which this quiescent state was
@@ -2383,7 +2383,7 @@ rcu_report_qs_rdp(int cpu, struct rcu_state *rsp, struct rcu_data *rdp)
 		 */
 		needwake = rcu_accelerate_cbs(rsp, rnp, rdp);
 
-		rcu_report_qs_rnp(mask, rsp, rnp, rnp->gpnum, flags);
+		rcu_report_qs_rnp(mask, rsp, rnp, rnp->gp_seq, flags);
 		/* ^^^ Released rnp->lock */
 		if (needwake)
 			rcu_gp_kthread_wake(rsp);
@@ -2688,8 +2688,8 @@ static void force_qs_rnp(struct rcu_state *rsp, int (*f)(struct rcu_data *rsp))
 			}
 		}
 		if (mask != 0) {
-			/* Idle/offline CPUs, report (releases rnp->lock. */
-			rcu_report_qs_rnp(mask, rsp, rnp, rnp->gpnum, flags);
+			/* Idle/offline CPUs, report (releases rnp->lock). */
+			rcu_report_qs_rnp(mask, rsp, rnp, rnp->gp_seq, flags);
 		} else {
 			/* Nothing to do here, so just drop the lock. */
 			raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
-- 
2.17.1

[PATCH tip/core/rcu 06/40] rcu: Convert conditional grace-period primitives to ->gp_seq

[Paul E. McKenney]

This commit converts get_state_synchronize_rcu(), cond_synchronize_rcu(),
get_state_synchronize_sched(), and cond_synchronize_sched() from ->gpnum
and ->completed to ->gp_seq.  Note that this also introduces a full
memory barrier in the already-done paths off cond_synchronize_rcu() and
cond_synchronize_sched(), as work with LKMM indicates that the earlier
smp_load_acquire() were insufficiently strong in some situations where
these two functions were called just as the grace period ended.  In such
cases, these two functions would not gain the benefit of memory ordering
at the end of the grace period.

Please note that the performance impact is negligible, as you shouldn't
be using either function anywhere near a fastpath in any case.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 kernel/rcu/tree.c | 42 ++++++++++--------------------------------
 1 file changed, 10 insertions(+), 32 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 001596f05111..1a3a7d402a20 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3180,16 +3180,10 @@ unsigned long get_state_synchronize_rcu(void)
 {
 	/*
 	 * Any prior manipulation of RCU-protected data must happen
-	 * before the load from ->gpnum.
+	 * before the load from ->gp_seq.
 	 */
 	smp_mb();  /* ^^^ */
-
-	/*
-	 * Make sure this load happens before the purportedly
-	 * time-consuming work between get_state_synchronize_rcu()
-	 * and cond_synchronize_rcu().
-	 */
-	return smp_load_acquire(&rcu_state_p->gpnum);
+	return rcu_seq_snap(&rcu_state_p->gp_seq);
 }
 EXPORT_SYMBOL_GPL(get_state_synchronize_rcu);
 
@@ -3209,15 +3203,10 @@ EXPORT_SYMBOL_GPL(get_state_synchronize_rcu);
  */
 void cond_synchronize_rcu(unsigned long oldstate)
 {
-	unsigned long newstate;
-
-	/*
-	 * Ensure that this load happens before any RCU-destructive
-	 * actions the caller might carry out after we return.
-	 */
-	newstate = smp_load_acquire(&rcu_state_p->completed);
-	if (ULONG_CMP_GE(oldstate, newstate))
+	if (!rcu_seq_done(&rcu_state_p->gp_seq, oldstate))
 		synchronize_rcu();
+	else
+		smp_mb(); /* Ensure GP ends before subsequent accesses. */
 }
 EXPORT_SYMBOL_GPL(cond_synchronize_rcu);
 
@@ -3232,16 +3221,10 @@ unsigned long get_state_synchronize_sched(void)
 {
 	/*
 	 * Any prior manipulation of RCU-protected data must happen
-	 * before the load from ->gpnum.
+	 * before the load from ->gp_seq.
 	 */
 	smp_mb();  /* ^^^ */
-
-	/*
-	 * Make sure this load happens before the purportedly
-	 * time-consuming work between get_state_synchronize_sched()
-	 * and cond_synchronize_sched().
-	 */
-	return smp_load_acquire(&rcu_sched_state.gpnum);
+	return rcu_seq_snap(&rcu_sched_state.gp_seq);
 }
 EXPORT_SYMBOL_GPL(get_state_synchronize_sched);
 
@@ -3261,15 +3244,10 @@ EXPORT_SYMBOL_GPL(get_state_synchronize_sched);
  */
 void cond_synchronize_sched(unsigned long oldstate)
 {
-	unsigned long newstate;
-
-	/*
-	 * Ensure that this load happens before any RCU-destructive
-	 * actions the caller might carry out after we return.
-	 */
-	newstate = smp_load_acquire(&rcu_sched_state.completed);
-	if (ULONG_CMP_GE(oldstate, newstate))
+	if (!rcu_seq_done(&rcu_sched_state.gp_seq, oldstate))
 		synchronize_sched();
+	else
+		smp_mb(); /* Ensure GP ends before subsequent accesses. */
 }
 EXPORT_SYMBOL_GPL(cond_synchronize_sched);
 
-- 
2.17.1

[PATCH tip/core/rcu 07/40] rcu: Move RCU's grace-period-change code to ->gp_seq

[Paul E. McKenney]

This commit moves __note_gp_changes(), note_gp_changes(), and
__rcu_pending() to ->gp_seq, creating new rcu_seq_completed_gp() and
rcu_seq_new_gp() functions for this purpose.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
[ paulmck: Reinstate "cpuend: trace as suggested by Joel Fernandes. ]
---
 kernel/rcu/rcu.h  | 17 +++++++++++++++++
 kernel/rcu/tree.c | 39 ++++++++++++++++-----------------------
 2 files changed, 33 insertions(+), 23 deletions(-)

diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
index f0907f9f6cd0..7568a3fd0815 100644
--- a/kernel/rcu/rcu.h
+++ b/kernel/rcu/rcu.h
@@ -116,6 +116,23 @@ static inline bool rcu_seq_done(unsigned long *sp, unsigned long s)
 	return ULONG_CMP_GE(READ_ONCE(*sp), s);
 }
 
+/*
+ * Has a grace period completed since the time the old gp_seq was collected?
+ */
+static inline bool rcu_seq_completed_gp(unsigned long old, unsigned long new)
+{
+	return ULONG_CMP_LT(old, new & ~RCU_SEQ_STATE_MASK);
+}
+
+/*
+ * Has a grace period started since the time the old gp_seq was collected?
+ */
+static inline bool rcu_seq_new_gp(unsigned long old, unsigned long new)
+{
+	return ULONG_CMP_LT((old + RCU_SEQ_STATE_MASK) & ~RCU_SEQ_STATE_MASK,
+			    new);
+}
+
 /*
  * debug_rcu_head_queue()/debug_rcu_head_unqueue() are used internally
  * by call_rcu() and rcu callback execution, and are therefore not part of the
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 1a3a7d402a20..a8c19363c979 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1790,24 +1790,23 @@ static bool __note_gp_changes(struct rcu_state *rsp, struct rcu_node *rnp,
 
 	raw_lockdep_assert_held_rcu_node(rnp);
 
-	/* Handle the ends of any preceding grace periods first. */
-	if (rdp->completed == rnp->completed &&
-	    !unlikely(READ_ONCE(rdp->gpwrap))) {
-
-		/* No grace period end, so just accelerate recent callbacks. */
-		ret = rcu_accelerate_cbs(rsp, rnp, rdp);
-
-	} else {
-
-		/* Advance callbacks. */
-		ret = rcu_advance_cbs(rsp, rnp, rdp);
+	if (rdp->gp_seq == rnp->gp_seq)
+		return false; /* Nothing to do. */
 
+	/* Handle the ends of any preceding grace periods first. */
+	if (rcu_seq_completed_gp(rdp->gp_seq, rnp->gp_seq) ||
+	    unlikely(READ_ONCE(rdp->gpwrap))) {
+		ret = rcu_advance_cbs(rsp, rnp, rdp); /* Advance callbacks. */
 		/* Remember that we saw this grace-period completion. */
 		rdp->completed = rnp->completed;
-		trace_rcu_grace_period(rsp->name, rdp->gpnum, TPS("cpuend"));
+		trace_rcu_grace_period(rsp->name, rdp->gp_seq, TPS("cpuend"));
+	} else {
+		ret = rcu_accelerate_cbs(rsp, rnp, rdp); /* Recent callbacks. */
 	}
 
-	if (rdp->gpnum != rnp->gpnum || unlikely(READ_ONCE(rdp->gpwrap))) {
+	/* Now handle the beginnings of any new-to-this-CPU grace periods. */
+	if (rcu_seq_new_gp(rdp->gp_seq, rnp->gp_seq) ||
+	    unlikely(READ_ONCE(rdp->gpwrap))) {
 		/*
 		 * If the current grace period is waiting for this CPU,
 		 * set up to detect a quiescent state, otherwise don't
@@ -1823,8 +1822,7 @@ static bool __note_gp_changes(struct rcu_state *rsp, struct rcu_node *rnp,
 		WRITE_ONCE(rdp->gpwrap, false);
 		rcu_gpnum_ovf(rnp, rdp);
 	}
-	if (rdp->gp_seq != rnp->gp_seq)
-		rdp->gp_seq = rnp->gp_seq;
+	rdp->gp_seq = rnp->gp_seq;  /* Remember new grace-period state. */
 	return ret;
 }
 
@@ -1836,8 +1834,7 @@ static void note_gp_changes(struct rcu_state *rsp, struct rcu_data *rdp)
 
 	local_irq_save(flags);
 	rnp = rdp->mynode;
-	if ((rdp->gpnum == READ_ONCE(rnp->gpnum) &&
-	     rdp->completed == READ_ONCE(rnp->completed) &&
+	if ((rdp->gp_seq == rcu_seq_current(&rnp->gp_seq) &&
 	     !unlikely(READ_ONCE(rdp->gpwrap))) || /* w/out lock. */
 	    !raw_spin_trylock_rcu_node(rnp)) { /* irqs already off, so later. */
 		local_irq_restore(flags);
@@ -3283,12 +3280,8 @@ static int __rcu_pending(struct rcu_state *rsp, struct rcu_data *rdp)
 	    !rcu_segcblist_restempty(&rdp->cblist, RCU_NEXT_READY_TAIL))
 		return 1;
 
-	/* Has another RCU grace period completed?  */
-	if (READ_ONCE(rnp->completed) != rdp->completed) /* outside lock */
-		return 1;
-
-	/* Has a new RCU grace period started? */
-	if (READ_ONCE(rnp->gpnum) != rdp->gpnum ||
+	/* Have RCU grace period completed or started?  */
+	if (rcu_seq_current(&rnp->gp_seq) != rdp->gp_seq ||
 	    unlikely(READ_ONCE(rdp->gpwrap))) /* outside lock */
 		return 1;
 
-- 
2.17.1

[PATCH tip/core/rcu 08/40] rcu: Convert rcu_gpnum_ovf() to ->gp_seq

[Paul E. McKenney]

This commit converts rcu_gpnum_ovf() to use ->gp_seq instead of ->gpnum.
Same size unsigned long, so same approach.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 kernel/rcu/tree.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index a8c19363c979..eda23797a7f4 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1089,14 +1089,15 @@ static int rcu_is_cpu_rrupt_from_idle(void)
 /*
  * We are reporting a quiescent state on behalf of some other CPU, so
  * it is our responsibility to check for and handle potential overflow
- * of the rcu_node ->gpnum counter with respect to the rcu_data counters.
+ * of the rcu_node ->gp_seq counter with respect to the rcu_data counters.
  * After all, the CPU might be in deep idle state, and thus executing no
  * code whatsoever.
  */
 static void rcu_gpnum_ovf(struct rcu_node *rnp, struct rcu_data *rdp)
 {
 	raw_lockdep_assert_held_rcu_node(rnp);
-	if (ULONG_CMP_LT(READ_ONCE(rdp->gpnum) + ULONG_MAX / 4, rnp->gpnum))
+	if (ULONG_CMP_LT(rcu_seq_current(&rdp->gp_seq) + ULONG_MAX / 4,
+			 rnp->gp_seq))
 		WRITE_ONCE(rdp->gpwrap, true);
 	if (ULONG_CMP_LT(rdp->rcu_iw_gpnum + ULONG_MAX / 4, rnp->gpnum))
 		rdp->rcu_iw_gpnum = rnp->gpnum + ULONG_MAX / 4;
-- 
2.17.1

[PATCH tip/core/rcu 09/40] rcu: Move rcu_implicit_dynticks_qs() to ->gp_seq

[Paul E. McKenney]

This commit makes rcu_implicit_dynticks_qs() use ->gp_seq, with the
exception of tracing, which will be converted later.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 kernel/rcu/tree.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index eda23797a7f4..eef3ce54114f 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1178,7 +1178,7 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
 	ruqp = per_cpu_ptr(&rcu_dynticks.rcu_urgent_qs, rdp->cpu);
 	if (time_after(jiffies, rdp->rsp->gp_start + jtsq) &&
 	    READ_ONCE(rdp->rcu_qs_ctr_snap) != per_cpu(rcu_dynticks.rcu_qs_ctr, rdp->cpu) &&
-	    READ_ONCE(rdp->gpnum) == rnp->gpnum && !rdp->gpwrap) {
+	    rcu_seq_current(&rdp->gp_seq) == rnp->gp_seq && !rdp->gpwrap) {
 		trace_rcu_fqs(rdp->rsp->name, rdp->gpnum, rdp->cpu, TPS("rqc"));
 		rcu_gpnum_ovf(rnp, rdp);
 		return 1;
-- 
2.17.1

[PATCH tip/core/rcu 10/40] rcu: Move rcu_try_advance_all_cbs() to ->gp_seq

[Paul E. McKenney]

This commit makes rcu_try_advance_all_cbs() use ->gp_seq, with the
exception of tracing, which will be converted later.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 kernel/rcu/tree_plugin.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index e387ea712758..d893899b72f4 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -1467,7 +1467,8 @@ static bool __maybe_unused rcu_try_advance_all_cbs(void)
 		 * completed since we last checked and there are
 		 * callbacks not yet ready to invoke.
 		 */
-		if ((rdp->completed != rnp->completed ||
+		if ((rcu_seq_completed_gp(rdp->gp_seq,
+					  rcu_seq_current(&rnp->gp_seq)) ||
 		     unlikely(READ_ONCE(rdp->gpwrap))) &&
 		    rcu_segcblist_pend_cbs(&rdp->cblist))
 			note_gp_changes(rsp, rdp);
-- 
2.17.1

[PATCH tip/core/rcu 11/40] rcu: Move rcu_nocb_gp_get() to ->gp_seq

[Paul E. McKenney]

This commit makes rcu_try_advance_all_cbs() use ->gp_seq.  It uses
rcu_seq_ctr() in order to shift away the state bits, so that the
low-order bits of the result may safely be used to index ->nocb_gp_wq[].

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 kernel/rcu/tree_plugin.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index d893899b72f4..5b10904669c5 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -1852,7 +1852,7 @@ static void rcu_nocb_gp_cleanup(struct swait_queue_head *sq)
 
 static struct swait_queue_head *rcu_nocb_gp_get(struct rcu_node *rnp)
 {
-	return &rnp->nocb_gp_wq[rnp->completed & 0x1];
+	return &rnp->nocb_gp_wq[rcu_seq_ctr(rnp->gp_seq) & 0x1];
 }
 
 static void rcu_init_one_nocb(struct rcu_node *rnp)
-- 
2.17.1

[PATCH tip/core/rcu 12/40] rcu: Move rcu_gp_in_progress() to ->gp_seq

[Paul E. McKenney]

This commit makes rcu_gp_in_progress() use ->gp_seq instead of
->completed and ->gpnum.  The READ_ONCE() invocations are buried
in rcu_seq_current().

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 kernel/rcu/tree.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index eef3ce54114f..fc889c58964c 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -219,7 +219,7 @@ unsigned long rcu_rnp_online_cpus(struct rcu_node *rnp)
  */
 static int rcu_gp_in_progress(struct rcu_state *rsp)
 {
-	return READ_ONCE(rsp->completed) != READ_ONCE(rsp->gpnum);
+	return rcu_seq_state(rcu_seq_current(&rsp->gp_seq));
 }
 
 /*
-- 
2.17.1

[PATCH tip/core/rcu 13/40] rcu: Convert ->rcu_iw_gpnum to ->gp_seq

[Paul E. McKenney]

This commit switches the interrupt-disabled detection mechanism to
->gp_seq.  This mechanism is used as part of RCU CPU stall warnings,
and detects cases where the stall is due to a CPU having interrupts
disabled.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 kernel/rcu/tree.c        | 12 ++++++------
 kernel/rcu/tree.h        |  2 +-
 kernel/rcu/tree_plugin.h |  2 +-
 3 files changed, 8 insertions(+), 8 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index fc889c58964c..e42510dd0b1f 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1099,8 +1099,8 @@ static void rcu_gpnum_ovf(struct rcu_node *rnp, struct rcu_data *rdp)
 	if (ULONG_CMP_LT(rcu_seq_current(&rdp->gp_seq) + ULONG_MAX / 4,
 			 rnp->gp_seq))
 		WRITE_ONCE(rdp->gpwrap, true);
-	if (ULONG_CMP_LT(rdp->rcu_iw_gpnum + ULONG_MAX / 4, rnp->gpnum))
-		rdp->rcu_iw_gpnum = rnp->gpnum + ULONG_MAX / 4;
+	if (ULONG_CMP_LT(rdp->rcu_iw_gp_seq + ULONG_MAX / 4, rnp->gp_seq))
+		rdp->rcu_iw_gp_seq = rnp->gp_seq + ULONG_MAX / 4;
 }
 
 /*
@@ -1134,7 +1134,7 @@ static void rcu_iw_handler(struct irq_work *iwp)
 	rnp = rdp->mynode;
 	raw_spin_lock_rcu_node(rnp);
 	if (!WARN_ON_ONCE(!rdp->rcu_iw_pending)) {
-		rdp->rcu_iw_gpnum = rnp->gpnum;
+		rdp->rcu_iw_gp_seq = rnp->gp_seq;
 		rdp->rcu_iw_pending = false;
 	}
 	raw_spin_unlock_rcu_node(rnp);
@@ -1231,11 +1231,11 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
 	if (jiffies - rdp->rsp->gp_start > rcu_jiffies_till_stall_check() / 2) {
 		resched_cpu(rdp->cpu);
 		if (IS_ENABLED(CONFIG_IRQ_WORK) &&
-		    !rdp->rcu_iw_pending && rdp->rcu_iw_gpnum != rnp->gpnum &&
+		    !rdp->rcu_iw_pending && rdp->rcu_iw_gp_seq != rnp->gp_seq &&
 		    (rnp->ffmask & rdp->grpmask)) {
 			init_irq_work(&rdp->rcu_iw, rcu_iw_handler);
 			rdp->rcu_iw_pending = true;
-			rdp->rcu_iw_gpnum = rnp->gpnum;
+			rdp->rcu_iw_gp_seq = rnp->gp_seq;
 			irq_work_queue_on(&rdp->rcu_iw, rdp->cpu);
 		}
 	}
@@ -3572,7 +3572,7 @@ rcu_init_percpu_data(int cpu, struct rcu_state *rsp)
 	rdp->rcu_qs_ctr_snap = per_cpu(rcu_dynticks.rcu_qs_ctr, cpu);
 	rdp->core_needs_qs = false;
 	rdp->rcu_iw_pending = false;
-	rdp->rcu_iw_gpnum = rnp->gpnum - 1;
+	rdp->rcu_iw_gp_seq = rnp->gp_seq - 1;
 	trace_rcu_grace_period(rsp->name, rdp->gpnum, TPS("cpuonl"));
 	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 }
diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index 50a28d1cf5a1..6d6cbc8b3a9c 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -286,7 +286,7 @@ struct rcu_data {
 	/* ->rcu_iw* fields protected by leaf rcu_node ->lock. */
 	struct irq_work rcu_iw;		/* Check for non-irq activity. */
 	bool rcu_iw_pending;		/* Is ->rcu_iw pending? */
-	unsigned long rcu_iw_gpnum;	/* ->gpnum associated with ->rcu_iw. */
+	unsigned long rcu_iw_gp_seq;	/* ->gp_seq associated with ->rcu_iw. */
 
 	int cpu;
 	struct rcu_state *rsp;
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 5b10904669c5..bc32e1f434a6 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -1763,7 +1763,7 @@ static void print_cpu_stall_info(struct rcu_state *rsp, int cpu)
 		ticks_value = rsp->gpnum - rdp->gpnum;
 	}
 	print_cpu_stall_fast_no_hz(fast_no_hz, cpu);
-	delta = rdp->mynode->gpnum - rdp->rcu_iw_gpnum;
+	delta = rcu_seq_ctr(rdp->mynode->gp_seq - rdp->rcu_iw_gp_seq);
 	pr_err("\t%d-%c%c%c%c: (%lu %s) idle=%03x/%ld/%ld softirq=%u/%u fqs=%ld %s\n",
 	       cpu,
 	       "O."[!!cpu_online(cpu)],
-- 
2.17.1

[PATCH tip/core/rcu 14/40] rcu: Convert ->completedqs to ->gp_seq

[Paul E. McKenney]

This commit switches the quiescent-state no-backtracking checks from
->gpnum and ->completed to ->gp_seq.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 kernel/rcu/tree.c        | 4 ++--
 kernel/rcu/tree_plugin.h | 4 ++--
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index e42510dd0b1f..85d14ae49ffc 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2278,7 +2278,7 @@ rcu_report_qs_rnp(unsigned long mask, struct rcu_state *rsp,
 			raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 			return;
 		}
-		rnp->completedqs = rnp->gpnum;
+		rnp->completedqs = rnp->gp_seq;
 		mask = rnp->grpmask;
 		if (rnp->parent == NULL) {
 
@@ -3948,7 +3948,7 @@ static void __init rcu_init_one(struct rcu_state *rsp)
 			rnp->gpnum = rsp->gpnum;
 			rnp->completed = rsp->completed;
 			rnp->gp_seq = rsp->gp_seq;
-			rnp->completedqs = rsp->completed;
+			rnp->completedqs = rsp->gp_seq;
 			rnp->qsmask = 0;
 			rnp->qsmaskinit = 0;
 			rnp->grplo = j * cpustride;
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index bc32e1f434a6..2a81d243521f 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -262,7 +262,7 @@ static void rcu_preempt_ctxt_queue(struct rcu_node *rnp, struct rcu_data *rdp)
 	 */
 	if (!rnp->gp_tasks && (blkd_state & RCU_GP_BLKD)) {
 		rnp->gp_tasks = &t->rcu_node_entry;
-		WARN_ON_ONCE(rnp->completedqs == rnp->gpnum);
+		WARN_ON_ONCE(rnp->completedqs == rnp->gp_seq);
 	}
 	if (!rnp->exp_tasks && (blkd_state & RCU_EXP_BLKD))
 		rnp->exp_tasks = &t->rcu_node_entry;
@@ -537,7 +537,7 @@ void rcu_read_unlock_special(struct task_struct *t)
 		WARN_ON_ONCE(rnp != t->rcu_blocked_node);
 		WARN_ON_ONCE(!rcu_is_leaf_node(rnp));
 		empty_norm = !rcu_preempt_blocked_readers_cgp(rnp);
-		WARN_ON_ONCE(rnp->completedqs == rnp->gpnum &&
+		WARN_ON_ONCE(rnp->completedqs == rnp->gp_seq &&
 			     (!empty_norm || rnp->qsmask));
 		empty_exp = sync_rcu_preempt_exp_done(rnp);
 		smp_mb(); /* ensure expedited fastpath sees end of RCU c-s. */
-- 
2.17.1

[PATCH tip/core/rcu 15/40] rcu: Convert grace-period requests to ->gp_seq

[Paul E. McKenney]

This commit converts the grace-period request code paths from ->completed
and ->gpnum to ->gp_seq.  The need_future_gp_element() macro encapsulates
the shift operation required to use ->gp_seq as an index to the
->need_future_gp[] array.  The rcu_cbs_completed() function is removed
in favor of the rcu_seq_snap() function.  The rcu_start_this_gp()
gets some temporary consistency checks and uses rcu_seq_done(),
rcu_seq_current(), rcu_seq_state(), and rcu_gp_in_progress() in place
of the earlier open-coded comparisons of ->gpnum and ->completed.
The rcu_future_gp_cleanup() function replaces use of ->completed
with ->gp_seq.  The rcu_accelerate_cbs() function replaces a call to
rcu_cbs_completed() with one to rcu_seq_snap().  The rcu_advance_cbs()
function replaces an access to >completed with one to ->gp_seq and adds
some temporary warnings.  The rcu_nocb_wait_gp() function replaces a
call to rcu_cbs_completed() with one to rcu_seq_snap() and an open-coded
comparison with rcu_seq_done().

The temporary warnings will be removed when the various ->gpnum and
->completed fields are removed.  Their purpose is to locate code who
might still be using ->gpnum and ->completed.  (Much easier that way
than trying to trace down the causes of too-short grace periods and
grace-period hangs!)

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 kernel/rcu/tree.c        | 84 ++++++++++------------------------------
 kernel/rcu/tree.h        |  2 +-
 kernel/rcu/tree_plugin.h |  6 +--
 3 files changed, 24 insertions(+), 68 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 85d14ae49ffc..6f9a564e82d3 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1548,52 +1548,6 @@ void rcu_cpu_stall_reset(void)
 		WRITE_ONCE(rsp->jiffies_stall, jiffies + ULONG_MAX / 2);
 }
 
-/*
- * Determine the value that ->completed will have at the end of the
- * next subsequent grace period.  This is used to tag callbacks so that
- * a CPU can invoke callbacks in a timely fashion even if that CPU has
- * been dyntick-idle for an extended period with callbacks under the
- * influence of RCU_FAST_NO_HZ.
- *
- * The caller must hold rnp->lock with interrupts disabled.
- */
-static unsigned long rcu_cbs_completed(struct rcu_state *rsp,
-				       struct rcu_node *rnp)
-{
-	raw_lockdep_assert_held_rcu_node(rnp);
-
-	/*
-	 * If RCU is idle, we just wait for the next grace period.
-	 * But we can only be sure that RCU is idle if we are looking
-	 * at the root rcu_node structure -- otherwise, a new grace
-	 * period might have started, but just not yet gotten around
-	 * to initializing the current non-root rcu_node structure.
-	 */
-	if (rcu_get_root(rsp) == rnp && rnp->gpnum == rnp->completed)
-		return rnp->completed + 1;
-
-	/*
-	 * If the current rcu_node structure believes that RCU is
-	 * idle, and if the rcu_state structure does not yet reflect
-	 * the start of a new grace period, then the next grace period
-	 * will suffice.  The memory barrier is needed to accurately
-	 * sample the rsp->gpnum, and pairs with the second lock
-	 * acquisition in rcu_gp_init(), which is augmented with
-	 * smp_mb__after_unlock_lock() for this purpose.
-	 */
-	if (rnp->gpnum == rnp->completed) {
-		smp_mb(); /* See above block comment. */
-		if (READ_ONCE(rsp->gpnum) == rnp->completed)
-			return rnp->completed + 1;
-	}
-
-	/*
-	 * Otherwise, wait for a possible partial grace period and
-	 * then the subsequent full grace period.
-	 */
-	return rnp->completed + 2;
-}
-
 /* Trace-event wrapper function for trace_rcu_future_grace_period.  */
 static void trace_rcu_this_gp(struct rcu_node *rnp, struct rcu_data *rdp,
 			      unsigned long c, const char *s)
@@ -1629,16 +1583,16 @@ static bool rcu_start_this_gp(struct rcu_node *rnp, struct rcu_data *rdp,
 	 * not be released.
 	 */
 	raw_lockdep_assert_held_rcu_node(rnp);
+	WARN_ON_ONCE(c & 0x2); /* Catch any lingering use of ->gpnum. */
+	WARN_ON_ONCE(((rnp->completed << RCU_SEQ_CTR_SHIFT) >> RCU_SEQ_CTR_SHIFT) != rcu_seq_ctr(rnp->gp_seq)); /* Catch any ->completed/->gp_seq mismatches. */
 	trace_rcu_this_gp(rnp, rdp, c, TPS("Startleaf"));
 	for (rnp_root = rnp; 1; rnp_root = rnp_root->parent) {
 		if (rnp_root != rnp)
 			raw_spin_lock_rcu_node(rnp_root);
-		WARN_ON_ONCE(ULONG_CMP_LT(rnp_root->gpnum +
-					  need_future_gp_mask(), c));
 		if (need_future_gp_element(rnp_root, c) ||
-		    ULONG_CMP_GE(rnp_root->gpnum, c) ||
+		    rcu_seq_done(&rnp_root->gp_seq, c) ||
 		    (rnp != rnp_root &&
-		     rnp_root->gpnum != rnp_root->completed)) {
+		     rcu_seq_state(rcu_seq_current(&rnp_root->gp_seq)))) {
 			trace_rcu_this_gp(rnp_root, rdp, c, TPS("Prestarted"));
 			goto unlock_out;
 		}
@@ -1650,7 +1604,7 @@ static bool rcu_start_this_gp(struct rcu_node *rnp, struct rcu_data *rdp,
 	}
 
 	/* If GP already in progress, just leave, otherwise start one. */
-	if (rnp_root->gpnum != rnp_root->completed) {
+	if (rcu_gp_in_progress(rsp)) {
 		trace_rcu_this_gp(rnp_root, rdp, c, TPS("Startedleafroot"));
 		goto unlock_out;
 	}
@@ -1675,7 +1629,7 @@ static bool rcu_start_this_gp(struct rcu_node *rnp, struct rcu_data *rdp,
  */
 static bool rcu_future_gp_cleanup(struct rcu_state *rsp, struct rcu_node *rnp)
 {
-	unsigned long c = rnp->completed;
+	unsigned long c = rnp->gp_seq;
 	bool needmore;
 	struct rcu_data *rdp = this_cpu_ptr(rsp->rda);
 
@@ -1703,14 +1657,14 @@ static void rcu_gp_kthread_wake(struct rcu_state *rsp)
 }
 
 /*
- * If there is room, assign a ->completed number to any callbacks on
- * this CPU that have not already been assigned.  Also accelerate any
- * callbacks that were previously assigned a ->completed number that has
- * since proven to be too conservative, which can happen if callbacks get
- * assigned a ->completed number while RCU is idle, but with reference to
- * a non-root rcu_node structure.  This function is idempotent, so it does
- * not hurt to call it repeatedly.  Returns an flag saying that we should
- * awaken the RCU grace-period kthread.
+ * If there is room, assign a ->gp_seq number to any callbacks on this
+ * CPU that have not already been assigned.  Also accelerate any callbacks
+ * that were previously assigned a ->gp_seq number that has since proven
+ * to be too conservative, which can happen if callbacks get assigned a
+ * ->gp_seq number while RCU is idle, but with reference to a non-root
+ * rcu_node structure.  This function is idempotent, so it does not hurt
+ * to call it repeatedly.  Returns an flag saying that we should awaken
+ * the RCU grace-period kthread.
  *
  * The caller must hold rnp->lock with interrupts disabled.
  */
@@ -1736,7 +1690,7 @@ static bool rcu_accelerate_cbs(struct rcu_state *rsp, struct rcu_node *rnp,
 	 * accelerating callback invocation to an earlier grace-period
 	 * number.
 	 */
-	c = rcu_cbs_completed(rsp, rnp);
+	c = rcu_seq_snap(&rsp->gp_seq);
 	if (rcu_segcblist_accelerate(&rdp->cblist, c))
 		ret = rcu_start_this_gp(rnp, rdp, c);
 
@@ -1751,7 +1705,7 @@ static bool rcu_accelerate_cbs(struct rcu_state *rsp, struct rcu_node *rnp,
 /*
  * Move any callbacks whose grace period has completed to the
  * RCU_DONE_TAIL sublist, then compact the remaining sublists and
- * assign ->completed numbers to any callbacks in the RCU_NEXT_TAIL
+ * assign ->gp_seq numbers to any callbacks in the RCU_NEXT_TAIL
  * sublist.  This function is idempotent, so it does not hurt to
  * invoke it repeatedly.  As long as it is not invoked -too- often...
  * Returns true if the RCU grace-period kthread needs to be awakened.
@@ -1768,10 +1722,10 @@ static bool rcu_advance_cbs(struct rcu_state *rsp, struct rcu_node *rnp,
 		return false;
 
 	/*
-	 * Find all callbacks whose ->completed numbers indicate that they
+	 * Find all callbacks whose ->gp_seq numbers indicate that they
 	 * are ready to invoke, and put them into the RCU_DONE_TAIL sublist.
 	 */
-	rcu_segcblist_advance(&rdp->cblist, rnp->completed);
+	rcu_segcblist_advance(&rdp->cblist, rnp->gp_seq);
 
 	/* Classify any remaining callbacks. */
 	return rcu_accelerate_cbs(rsp, rnp, rdp);
@@ -1889,6 +1843,8 @@ static bool rcu_gp_init(struct rcu_state *rsp)
 	smp_store_release(&rsp->gpnum, rsp->gpnum + 1);
 	smp_mb(); /* Pairs with barriers in stall-warning code. */
 	rcu_seq_start(&rsp->gp_seq);
+	if (WARN_ON_ONCE(((rnp->completed << RCU_SEQ_CTR_SHIFT) >> RCU_SEQ_CTR_SHIFT) != rcu_seq_ctr(rnp->gp_seq))) /* Catch any ->completed/->gp_seq mismatches. */
+		pr_info("%s ->completed: %#lx (%#lx) ->gp_seq %#lx (%#lx)\n", __func__, rnp->completed, (rnp->completed << RCU_SEQ_CTR_SHIFT) >> RCU_SEQ_CTR_SHIFT, rnp->gp_seq, rcu_seq_ctr(rnp->gp_seq));
 	trace_rcu_grace_period(rsp->name, rsp->gpnum, TPS("start"));
 	raw_spin_unlock_irq_rcu_node(rnp);
 
diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index 6d6cbc8b3a9c..a21d403a6010 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -174,7 +174,7 @@ struct rcu_node {
 #define need_future_gp_mask() \
 	(ARRAY_SIZE(((struct rcu_node *)NULL)->need_future_gp) - 1)
 #define need_future_gp_element(rnp, c) \
-	((rnp)->need_future_gp[(c) & need_future_gp_mask()])
+	((rnp)->need_future_gp[(c >> RCU_SEQ_CTR_SHIFT) & need_future_gp_mask()])
 #define need_any_future_gp(rnp)						\
 ({									\
 	int __i;							\
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 2a81d243521f..2036dc7426ac 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -2105,7 +2105,7 @@ static void rcu_nocb_wait_gp(struct rcu_data *rdp)
 	struct rcu_node *rnp = rdp->mynode;
 
 	raw_spin_lock_irqsave_rcu_node(rnp, flags);
-	c = rcu_cbs_completed(rdp->rsp, rnp);
+	c = rcu_seq_snap(&rdp->rsp->gp_seq);
 	needwake = rcu_start_this_gp(rnp, rdp, c);
 	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 	if (needwake)
@@ -2118,8 +2118,8 @@ static void rcu_nocb_wait_gp(struct rcu_data *rdp)
 	trace_rcu_this_gp(rnp, rdp, c, TPS("StartWait"));
 	for (;;) {
 		swait_event_interruptible(
-			rnp->nocb_gp_wq[c & 0x1],
-			(d = ULONG_CMP_GE(READ_ONCE(rnp->completed), c)));
+			rnp->nocb_gp_wq[rcu_seq_ctr(c) & 0x1],
+			(d = rcu_seq_done(&rnp->gp_seq, c)));
 		if (likely(d))
 			break;
 		WARN_ON(signal_pending(current));
-- 
2.17.1

[PATCH tip/core/rcu 16/40] rcu: Make RCU CPU stall warnings use ->gp_seq

[Paul E. McKenney]

This commit makes the RCU CPU stall-warning code in print_other_cpu_stall(),
print_cpu_stall(), and check_cpu_stall() use ->gp_seq instead of ->gpnum
and ->completed.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 kernel/rcu/tree.c        | 51 ++++++++++++++++++++--------------------
 kernel/rcu/tree_plugin.h |  8 +++----
 2 files changed, 30 insertions(+), 29 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 6f9a564e82d3..9452842e0590 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1340,7 +1340,7 @@ static inline void panic_on_rcu_stall(void)
 		panic("RCU Stall\n");
 }
 
-static void print_other_cpu_stall(struct rcu_state *rsp, unsigned long gpnum)
+static void print_other_cpu_stall(struct rcu_state *rsp, unsigned long gp_seq)
 {
 	int cpu;
 	unsigned long flags;
@@ -1350,6 +1350,8 @@ static void print_other_cpu_stall(struct rcu_state *rsp, unsigned long gpnum)
 	struct rcu_node *rnp = rcu_get_root(rsp);
 	long totqlen = 0;
 
+	WARN_ON_ONCE(gp_seq & 0x2); /* Remove when ->gpnum removed. */
+
 	/* Kick and suppress, if so configured. */
 	rcu_stall_kick_kthreads(rsp);
 	if (rcu_cpu_stall_suppress)
@@ -1380,17 +1382,16 @@ static void print_other_cpu_stall(struct rcu_state *rsp, unsigned long gpnum)
 	for_each_possible_cpu(cpu)
 		totqlen += rcu_segcblist_n_cbs(&per_cpu_ptr(rsp->rda,
 							    cpu)->cblist);
-	pr_cont("(detected by %d, t=%ld jiffies, g=%ld, c=%ld, q=%lu)\n",
+	pr_cont("(detected by %d, t=%ld jiffies, g=%ld, q=%lu)\n",
 	       smp_processor_id(), (long)(jiffies - rsp->gp_start),
-	       (long)rsp->gpnum, (long)rsp->completed, totqlen);
+	       (long)rcu_seq_current(&rsp->gp_seq), totqlen);
 	if (ndetected) {
 		rcu_dump_cpu_stacks(rsp);
 
 		/* Complain about tasks blocking the grace period. */
 		rcu_print_detail_task_stall(rsp);
 	} else {
-		if (READ_ONCE(rsp->gpnum) != gpnum ||
-		    READ_ONCE(rsp->completed) == gpnum) {
+		if (rcu_seq_current(&rsp->gp_seq) != gp_seq) {
 			pr_err("INFO: Stall ended before state dump start\n");
 		} else {
 			j = jiffies;
@@ -1442,9 +1443,9 @@ static void print_cpu_stall(struct rcu_state *rsp)
 	for_each_possible_cpu(cpu)
 		totqlen += rcu_segcblist_n_cbs(&per_cpu_ptr(rsp->rda,
 							    cpu)->cblist);
-	pr_cont(" (t=%lu jiffies g=%ld c=%ld q=%lu)\n",
+	pr_cont(" (t=%lu jiffies g=%ld q=%lu)\n",
 		jiffies - rsp->gp_start,
-		(long)rsp->gpnum, (long)rsp->completed, totqlen);
+		(long)rcu_seq_current(&rsp->gp_seq), totqlen);
 
 	rcu_check_gp_kthread_starvation(rsp);
 
@@ -1471,8 +1472,8 @@ static void print_cpu_stall(struct rcu_state *rsp)
 
 static void check_cpu_stall(struct rcu_state *rsp, struct rcu_data *rdp)
 {
-	unsigned long completed;
-	unsigned long gpnum;
+	unsigned long gs1;
+	unsigned long gs2;
 	unsigned long gps;
 	unsigned long j;
 	unsigned long jn;
@@ -1488,28 +1489,28 @@ static void check_cpu_stall(struct rcu_state *rsp, struct rcu_data *rdp)
 	/*
 	 * Lots of memory barriers to reject false positives.
 	 *
-	 * The idea is to pick up rsp->gpnum, then rsp->jiffies_stall,
-	 * then rsp->gp_start, and finally rsp->completed.  These values
-	 * are updated in the opposite order with memory barriers (or
-	 * equivalent) during grace-period initialization and cleanup.
-	 * Now, a false positive can occur if we get an new value of
-	 * rsp->gp_start and a old value of rsp->jiffies_stall.  But given
-	 * the memory barriers, the only way that this can happen is if one
-	 * grace period ends and another starts between these two fetches.
-	 * Detect this by comparing rsp->completed with the previous fetch
-	 * from rsp->gpnum.
+	 * The idea is to pick up rsp->gp_seq, then rsp->jiffies_stall,
+	 * then rsp->gp_start, and finally another copy of rsp->gp_seq.
+	 * These values are updated in the opposite order with memory
+	 * barriers (or equivalent) during grace-period initialization
+	 * and cleanup.  Now, a false positive can occur if we get an new
+	 * value of rsp->gp_start and a old value of rsp->jiffies_stall.
+	 * But given the memory barriers, the only way that this can happen
+	 * is if one grace period ends and another starts between these
+	 * two fetches.  This is detected by comparing the second fetch
+	 * of rsp->gp_seq with the previous fetch from rsp->gp_seq.
 	 *
 	 * Given this check, comparisons of jiffies, rsp->jiffies_stall,
 	 * and rsp->gp_start suffice to forestall false positives.
 	 */
-	gpnum = READ_ONCE(rsp->gpnum);
-	smp_rmb(); /* Pick up ->gpnum first... */
+	gs1 = READ_ONCE(rsp->gp_seq);
+	smp_rmb(); /* Pick up ->gp_seq first... */
 	js = READ_ONCE(rsp->jiffies_stall);
 	smp_rmb(); /* ...then ->jiffies_stall before the rest... */
 	gps = READ_ONCE(rsp->gp_start);
-	smp_rmb(); /* ...and finally ->gp_start before ->completed. */
-	completed = READ_ONCE(rsp->completed);
-	if (ULONG_CMP_GE(completed, gpnum) ||
+	smp_rmb(); /* ...and finally ->gp_start before ->gp_seq again. */
+	gs2 = READ_ONCE(rsp->gp_seq);
+	if (gs1 != gs2 ||
 	    ULONG_CMP_LT(j, js) ||
 	    ULONG_CMP_GE(gps, js))
 		return; /* No stall or GP completed since entering function. */
@@ -1527,7 +1528,7 @@ static void check_cpu_stall(struct rcu_state *rsp, struct rcu_data *rdp)
 		   cmpxchg(&rsp->jiffies_stall, js, jn) == js) {
 
 		/* They had a few time units to dump stack, so complain. */
-		print_other_cpu_stall(rsp, gpnum);
+		print_other_cpu_stall(rsp, gs2);
 	}
 }
 
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 2036dc7426ac..f4a88e3c388d 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -1755,12 +1755,12 @@ static void print_cpu_stall_info(struct rcu_state *rsp, int cpu)
 	 */
 	touch_nmi_watchdog();
 
-	if (rsp->gpnum == rdp->gpnum) {
+	ticks_value = rcu_seq_ctr(rsp->gp_seq - rdp->gp_seq);
+	if (ticks_value) {
+		ticks_title = "GPs behind";
+	} else {
 		ticks_title = "ticks this GP";
 		ticks_value = rdp->ticks_this_gp;
-	} else {
-		ticks_title = "GPs behind";
-		ticks_value = rsp->gpnum - rdp->gpnum;
 	}
 	print_cpu_stall_fast_no_hz(fast_no_hz, cpu);
 	delta = rcu_seq_ctr(rdp->mynode->gp_seq - rdp->rcu_iw_gp_seq);
-- 
2.17.1

[PATCH tip/core/rcu 17/40] rcutorture: Convert rcutorture_get_gp_data() to ->gp_seq

[Paul E. McKenney]

SRCU has long used ->srcu_gp_seq, and now RCU uses ->gp_seq.  This
commit therefore moves the rcutorture_get_gp_data() function from
a ->gpnum / ->completed pair to ->gp_seq.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 kernel/rcu/rcu.h        | 17 ++++++-----------
 kernel/rcu/rcutorture.c | 24 ++++++++++--------------
 kernel/rcu/srcutree.c   |  5 ++---
 kernel/rcu/tree.c       |  5 ++---
 4 files changed, 20 insertions(+), 31 deletions(-)

diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
index 7568a3fd0815..003671825d62 100644
--- a/kernel/rcu/rcu.h
+++ b/kernel/rcu/rcu.h
@@ -425,7 +425,7 @@ enum rcutorture_type {
 
 #if defined(CONFIG_TREE_RCU) || defined(CONFIG_PREEMPT_RCU)
 void rcutorture_get_gp_data(enum rcutorture_type test_type, int *flags,
-			    unsigned long *gpnum, unsigned long *completed);
+			    unsigned long *gp_seq);
 void rcutorture_record_test_transition(void);
 void rcutorture_record_progress(unsigned long vernum);
 void do_trace_rcu_torture_read(const char *rcutorturename,
@@ -435,13 +435,10 @@ void do_trace_rcu_torture_read(const char *rcutorturename,
 			       unsigned long c);
 #else
 static inline void rcutorture_get_gp_data(enum rcutorture_type test_type,
-					  int *flags,
-					  unsigned long *gpnum,
-					  unsigned long *completed)
+					  int *flags, unsigned long *gp_seq)
 {
 	*flags = 0;
-	*gpnum = 0;
-	*completed = 0;
+	*gp_seq = 0;
 }
 static inline void rcutorture_record_test_transition(void) { }
 static inline void rcutorture_record_progress(unsigned long vernum) { }
@@ -461,21 +458,19 @@ void do_trace_rcu_torture_read(const char *rcutorturename,
 
 static inline void srcutorture_get_gp_data(enum rcutorture_type test_type,
 					   struct srcu_struct *sp, int *flags,
-					   unsigned long *gpnum,
-					   unsigned long *completed)
+					   unsigned long *gp_seq)
 {
 	if (test_type != SRCU_FLAVOR)
 		return;
 	*flags = 0;
-	*completed = sp->srcu_idx;
-	*gpnum = *completed;
+	*gp_seq = sp->srcu_idx;
 }
 
 #elif defined(CONFIG_TREE_SRCU)
 
 void srcutorture_get_gp_data(enum rcutorture_type test_type,
 			     struct srcu_struct *sp, int *flags,
-			     unsigned long *gpnum, unsigned long *completed);
+			     unsigned long *gp_seq);
 
 #endif
 
diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
index 1f66597c7783..81fb43530d64 100644
--- a/kernel/rcu/rcutorture.c
+++ b/kernel/rcu/rcutorture.c
@@ -1313,18 +1313,16 @@ rcu_torture_stats_print(void)
 	if (rtcv_snap == rcu_torture_current_version &&
 	    rcu_torture_current != NULL) {
 		int __maybe_unused flags = 0;
-		unsigned long __maybe_unused gpnum = 0;
-		unsigned long __maybe_unused completed = 0;
+		unsigned long __maybe_unused gp_seq = 0;
 
 		rcutorture_get_gp_data(cur_ops->ttype,
-				       &flags, &gpnum, &completed);
+				       &flags, &gp_seq);
 		srcutorture_get_gp_data(cur_ops->ttype, srcu_ctlp,
-					&flags, &gpnum, &completed);
+					&flags, &gp_seq);
 		wtp = READ_ONCE(writer_task);
-		pr_alert("??? Writer stall state %s(%d) g%lu c%lu f%#x ->state %#lx cpu %d\n",
+		pr_alert("??? Writer stall state %s(%d) g%lu f%#x ->state %#lx cpu %d\n",
 			 rcu_torture_writer_state_getname(),
-			 rcu_torture_writer_state,
-			 gpnum, completed, flags,
+			 rcu_torture_writer_state, gp_seq, flags,
 			 wtp == NULL ? ~0UL : wtp->state,
 			 wtp == NULL ? -1 : (int)task_cpu(wtp));
 		if (!splatted && wtp) {
@@ -1605,8 +1603,7 @@ static void
 rcu_torture_cleanup(void)
 {
 	int flags = 0;
-	unsigned long gpnum = 0;
-	unsigned long completed = 0;
+	unsigned long gp_seq = 0;
 	int i;
 
 	rcutorture_record_test_transition();
@@ -1637,11 +1634,10 @@ rcu_torture_cleanup(void)
 		fakewriter_tasks = NULL;
 	}
 
-	rcutorture_get_gp_data(cur_ops->ttype, &flags, &gpnum, &completed);
-	srcutorture_get_gp_data(cur_ops->ttype, srcu_ctlp,
-				&flags, &gpnum, &completed);
-	pr_alert("%s:  End-test grace-period state: g%lu c%lu f%#x\n",
-		 cur_ops->name, gpnum, completed, flags);
+	rcutorture_get_gp_data(cur_ops->ttype, &flags, &gp_seq);
+	srcutorture_get_gp_data(cur_ops->ttype, srcu_ctlp, &flags, &gp_seq);
+	pr_alert("%s:  End-test grace-period state: g%lu f%#x\n",
+		 cur_ops->name, gp_seq, flags);
 	torture_stop_kthread(rcu_torture_stats, stats_task);
 	torture_stop_kthread(rcu_torture_fqs, fqs_task);
 	for (i = 0; i < ncbflooders; i++)
diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index 5a1a9a07b407..d6d6ea9738c0 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -1249,13 +1249,12 @@ static void process_srcu(struct work_struct *work)
 
 void srcutorture_get_gp_data(enum rcutorture_type test_type,
 			     struct srcu_struct *sp, int *flags,
-			     unsigned long *gpnum, unsigned long *completed)
+			     unsigned long *gp_seq)
 {
 	if (test_type != SRCU_FLAVOR)
 		return;
 	*flags = 0;
-	*completed = rcu_seq_ctr(sp->srcu_gp_seq);
-	*gpnum = rcu_seq_ctr(sp->srcu_gp_seq_needed);
+	*gp_seq = rcu_seq_current(&sp->srcu_gp_seq);
 }
 EXPORT_SYMBOL_GPL(srcutorture_get_gp_data);
 
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 9452842e0590..779db9644177 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -638,7 +638,7 @@ EXPORT_SYMBOL_GPL(rcutorture_record_test_transition);
  * Send along grace-period-related data for rcutorture diagnostics.
  */
 void rcutorture_get_gp_data(enum rcutorture_type test_type, int *flags,
-			    unsigned long *gpnum, unsigned long *completed)
+			    unsigned long *gp_seq)
 {
 	struct rcu_state *rsp = NULL;
 
@@ -658,8 +658,7 @@ void rcutorture_get_gp_data(enum rcutorture_type test_type, int *flags,
 	if (rsp == NULL)
 		return;
 	*flags = READ_ONCE(rsp->gp_flags);
-	*gpnum = READ_ONCE(rsp->gpnum);
-	*completed = READ_ONCE(rsp->completed);
+	*gp_seq = rcu_seq_current(&rsp->gp_seq);
 }
 EXPORT_SYMBOL_GPL(rcutorture_get_gp_data);
 
-- 
2.17.1

[PATCH tip/core/rcu 18/40] rcu: Move from ->need_future_gp[] to ->gp_seq_needed

[Paul E. McKenney]

One problem with the ->need_future_gp[] array is that the grace-period
assignment of each element changes as the grace periods complete.
This means that it is necessary to hold a lock when checking this
array to learn if a given grace period has already been requested.
This increase lock contention, which is the opposite of helpful.
This commit therefore replaces the ->need_future_gp[] with a single
->gp_seq_needed value and keeps it updated in the rcu_data structure.

This will enable reliable lockless checking of whether or not a given
grace period has already been requested.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 kernel/rcu/tree.c | 33 ++++++++++++++++++---------------
 kernel/rcu/tree.h | 19 ++-----------------
 2 files changed, 20 insertions(+), 32 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 779db9644177..e5d1369e099f 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1560,7 +1560,7 @@ static void trace_rcu_this_gp(struct rcu_node *rnp, struct rcu_data *rdp,
 /*
  * Start the specified grace period, as needed to handle newly arrived
  * callbacks.  The required future grace periods are recorded in each
- * rcu_node structure's ->need_future_gp[] field.  Returns true if there
+ * rcu_node structure's ->gp_seq_needed field.  Returns true if there
  * is reason to awaken the grace-period kthread.
  *
  * The caller must hold the specified rcu_node structure's ->lock, which
@@ -1589,14 +1589,14 @@ static bool rcu_start_this_gp(struct rcu_node *rnp, struct rcu_data *rdp,
 	for (rnp_root = rnp; 1; rnp_root = rnp_root->parent) {
 		if (rnp_root != rnp)
 			raw_spin_lock_rcu_node(rnp_root);
-		if (need_future_gp_element(rnp_root, c) ||
+		if (ULONG_CMP_GE(rnp_root->gp_seq_needed, c) ||
 		    rcu_seq_done(&rnp_root->gp_seq, c) ||
 		    (rnp != rnp_root &&
 		     rcu_seq_state(rcu_seq_current(&rnp_root->gp_seq)))) {
 			trace_rcu_this_gp(rnp_root, rdp, c, TPS("Prestarted"));
 			goto unlock_out;
 		}
-		need_future_gp_element(rnp_root, c) = true;
+		rnp_root->gp_seq_needed = c;
 		if (rnp_root != rnp && rnp_root->parent != NULL)
 			raw_spin_unlock_rcu_node(rnp_root);
 		if (!rnp_root->parent)
@@ -1633,8 +1633,9 @@ static bool rcu_future_gp_cleanup(struct rcu_state *rsp, struct rcu_node *rnp)
 	bool needmore;
 	struct rcu_data *rdp = this_cpu_ptr(rsp->rda);
 
-	need_future_gp_element(rnp, c) = false;
-	needmore = need_any_future_gp(rnp);
+	needmore = ULONG_CMP_LT(rnp->gp_seq, rnp->gp_seq_needed);
+	if (!needmore)
+		rnp->gp_seq_needed = rnp->gp_seq; /* Avoid counter wrap. */
 	trace_rcu_this_gp(rnp, rdp, c,
 			  needmore ? TPS("CleanupMore") : TPS("Cleanup"));
 	return needmore;
@@ -2046,7 +2047,7 @@ static void rcu_gp_cleanup(struct rcu_state *rsp)
 	rsp->gp_state = RCU_GP_IDLE;
 	/* Check for GP requests since above loop. */
 	rdp = this_cpu_ptr(rsp->rda);
-	if (need_any_future_gp(rnp)) {
+	if (ULONG_CMP_LT(rnp->gp_seq, rnp->gp_seq_needed)) {
 		trace_rcu_this_gp(rnp, rdp, rsp->completed - 1,
 				  TPS("CleanupMore"));
 		needgp = true;
@@ -2700,7 +2701,8 @@ rcu_check_gp_start_stall(struct rcu_state *rsp, struct rcu_node *rnp,
 	struct rcu_node *rnp_root = rcu_get_root(rsp);
 	static atomic_t warned = ATOMIC_INIT(0);
 
-	if (rcu_gp_in_progress(rsp) || !need_any_future_gp(rcu_get_root(rsp)))
+	if (rcu_gp_in_progress(rsp) ||
+	    ULONG_CMP_GE(rnp_root->gp_seq, rnp_root->gp_seq_needed))
 		return;
 	j = jiffies; /* Expensive access, and in common case don't get here. */
 	if (time_before(j, READ_ONCE(rsp->gp_req_activity) + HZ) ||
@@ -2710,7 +2712,8 @@ rcu_check_gp_start_stall(struct rcu_state *rsp, struct rcu_node *rnp,
 
 	raw_spin_lock_irqsave_rcu_node(rnp, flags);
 	j = jiffies;
-	if (rcu_gp_in_progress(rsp) || !need_any_future_gp(rcu_get_root(rsp)) ||
+	if (rcu_gp_in_progress(rsp) ||
+	    ULONG_CMP_GE(rnp_root->gp_seq, rnp_root->gp_seq_needed) ||
 	    time_before(j, READ_ONCE(rsp->gp_req_activity) + HZ) ||
 	    time_before(j, READ_ONCE(rsp->gp_activity) + HZ) ||
 	    atomic_read(&warned)) {
@@ -2721,7 +2724,8 @@ rcu_check_gp_start_stall(struct rcu_state *rsp, struct rcu_node *rnp,
 
 	raw_spin_lock_rcu_node(rnp_root); /* irqs already disabled. */
 	j = jiffies;
-	if (rcu_gp_in_progress(rsp) || !need_any_future_gp(rcu_get_root(rsp)) ||
+	if (rcu_gp_in_progress(rsp) ||
+	    ULONG_CMP_GE(rnp_root->gp_seq, rnp_root->gp_seq_needed) ||
 	    time_before(j, rsp->gp_req_activity + HZ) ||
 	    time_before(j, rsp->gp_activity + HZ) ||
 	    atomic_xchg(&warned, 1)) {
@@ -2729,12 +2733,9 @@ rcu_check_gp_start_stall(struct rcu_state *rsp, struct rcu_node *rnp,
 		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 		return;
 	}
-	pr_alert("%s: g%lu %d%d%d%d gar:%lu ga:%lu f%#x %s->state:%#lx\n",
-		 __func__, READ_ONCE(rsp->gpnum),
-		 need_future_gp_element(rcu_get_root(rsp), 0),
-		 need_future_gp_element(rcu_get_root(rsp), 1),
-		 need_future_gp_element(rcu_get_root(rsp), 2),
-		 need_future_gp_element(rcu_get_root(rsp), 3),
+	pr_alert("%s: g%ld->%ld gar:%lu ga:%lu f%#x %s->state:%#lx\n",
+		 __func__, (long)READ_ONCE(rsp->gp_seq),
+		 (long)READ_ONCE(rnp_root->gp_seq_needed),
 		 j - rsp->gp_req_activity, j - rsp->gp_activity,
 		 rsp->gp_flags, rsp->name,
 		 rsp->gp_kthread ? rsp->gp_kthread->state : 0x1ffffL);
@@ -3524,6 +3525,7 @@ rcu_init_percpu_data(int cpu, struct rcu_state *rsp)
 	rdp->gpnum = rnp->completed; /* Make CPU later note any new GP. */
 	rdp->completed = rnp->completed;
 	rdp->gp_seq = rnp->gp_seq;
+	rdp->gp_seq_needed = rnp->gp_seq;
 	rdp->cpu_no_qs.b.norm = true;
 	rdp->rcu_qs_ctr_snap = per_cpu(rcu_dynticks.rcu_qs_ctr, cpu);
 	rdp->core_needs_qs = false;
@@ -3904,6 +3906,7 @@ static void __init rcu_init_one(struct rcu_state *rsp)
 			rnp->gpnum = rsp->gpnum;
 			rnp->completed = rsp->completed;
 			rnp->gp_seq = rsp->gp_seq;
+			rnp->gp_seq_needed = rsp->gp_seq;
 			rnp->completedqs = rsp->gp_seq;
 			rnp->qsmask = 0;
 			rnp->qsmaskinit = 0;
diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index a21d403a6010..9329c1ff695f 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -88,6 +88,7 @@ struct rcu_node {
 				/*  This will either be equal to or one */
 				/*  behind the root rcu_node's gpnum. */
 	unsigned long gp_seq;	/* Track rsp->rcu_gp_seq. */
+	unsigned long gp_seq_needed; /* Track rsp->rcu_gp_seq_needed. */
 	unsigned long completedqs; /* All QSes done for this node. */
 	unsigned long qsmask;	/* CPUs or groups that need to switch in */
 				/*  order for current grace period to proceed.*/
@@ -160,7 +161,6 @@ struct rcu_node {
 	struct swait_queue_head nocb_gp_wq[2];
 				/* Place for rcu_nocb_kthread() to wait GP. */
 #endif /* #ifdef CONFIG_RCU_NOCB_CPU */
-	u8 need_future_gp[4];	/* Counts of upcoming GP requests. */
 	raw_spinlock_t fqslock ____cacheline_internodealigned_in_smp;
 
 	spinlock_t exp_lock ____cacheline_internodealigned_in_smp;
@@ -170,22 +170,6 @@ struct rcu_node {
 	bool exp_need_flush;	/* Need to flush workitem? */
 } ____cacheline_internodealigned_in_smp;
 
-/* Accessors for ->need_future_gp[] array. */
-#define need_future_gp_mask() \
-	(ARRAY_SIZE(((struct rcu_node *)NULL)->need_future_gp) - 1)
-#define need_future_gp_element(rnp, c) \
-	((rnp)->need_future_gp[(c >> RCU_SEQ_CTR_SHIFT) & need_future_gp_mask()])
-#define need_any_future_gp(rnp)						\
-({									\
-	int __i;							\
-	bool __nonzero = false;						\
-									\
-	for (__i = 0; __i < ARRAY_SIZE((rnp)->need_future_gp); __i++)	\
-		__nonzero = __nonzero ||				\
-			    READ_ONCE((rnp)->need_future_gp[__i]);	\
-	__nonzero;							\
-})
-
 /*
  * Bitmasks in an rcu_node cover the interval [grplo, grphi] of CPU IDs, and
  * are indexed relative to this interval rather than the global CPU ID space.
@@ -213,6 +197,7 @@ struct rcu_data {
 	unsigned long	gpnum;		/* Highest gp number that this CPU */
 					/*  is aware of having started. */
 	unsigned long	gp_seq;		/* Track rsp->rcu_gp_seq counter. */
+	unsigned long	gp_seq_needed;	/* Track rsp->rcu_gp_seq_needed ctr. */
 	unsigned long	rcu_qs_ctr_snap;/* Snapshot of rcu_qs_ctr to check */
 					/*  for rcu_all_qs() invocations. */
 	union rcu_noqs	cpu_no_qs;	/* No QSes yet for this CPU. */
-- 
2.17.1

[PATCH tip/core/rcu 19/40] rcu: Make rcu_nocb_wait_gp() check if GP already requested

[Paul E. McKenney]

This commit makes rcu_nocb_wait_gp() check rdp->gp_seq_needed to see
if the current CPU already knows about the needed grace period having
already been requested.  If so, it avoids acquiring the corresponding
leaf rcu_node structure's ->lock, thus decreasing contention.  This
optimization is intended for cases where either multiple leader rcuo
kthreads are running on the same CPU or these kthreads are running on
a non-offloaded (e.g., housekeeping) CPU.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
[ paulmck: Move lock release past "if" as suggested by Joel Fernandes. ]
[ paulmck: Fix caching of furthest-future requested grace period. ]
---
 kernel/rcu/tree.c        |  5 +++++
 kernel/rcu/tree_plugin.h | 15 ++++++++++-----
 2 files changed, 15 insertions(+), 5 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index e5d1369e099f..0da0db1128d0 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1618,6 +1618,11 @@ static bool rcu_start_this_gp(struct rcu_node *rnp, struct rcu_data *rdp,
 	trace_rcu_grace_period(rsp->name, READ_ONCE(rsp->gpnum), TPS("newreq"));
 	ret = true;  /* Caller must wake GP kthread. */
 unlock_out:
+	/* Push furthest requested GP to leaf node and rcu_data structure. */
+	if (ULONG_CMP_LT(c, rnp_root->gp_seq_needed)) {
+		rnp->gp_seq_needed = rnp_root->gp_seq_needed;
+		rdp->gp_seq_needed = rnp_root->gp_seq_needed;
+	}
 	if (rnp != rnp_root)
 		raw_spin_unlock_rcu_node(rnp_root);
 	return ret;
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index f4a88e3c388d..ca73931f7b30 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -2104,12 +2104,17 @@ static void rcu_nocb_wait_gp(struct rcu_data *rdp)
 	bool needwake;
 	struct rcu_node *rnp = rdp->mynode;
 
-	raw_spin_lock_irqsave_rcu_node(rnp, flags);
+	local_irq_save(flags);
 	c = rcu_seq_snap(&rdp->rsp->gp_seq);
-	needwake = rcu_start_this_gp(rnp, rdp, c);
-	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
-	if (needwake)
-		rcu_gp_kthread_wake(rdp->rsp);
+	if (!rdp->gpwrap && ULONG_CMP_GE(rdp->gp_seq_needed, c)) {
+		local_irq_restore(flags);
+	} else {
+		raw_spin_lock_rcu_node(rnp); /* irqs already disabled. */
+		needwake = rcu_start_this_gp(rnp, rdp, c);
+		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
+		if (needwake)
+			rcu_gp_kthread_wake(rdp->rsp);
+	}
 
 	/*
 	 * Wait for the grace period.  Do so interruptibly to avoid messing
-- 
2.17.1

[PATCH tip/core/rcu 20/40] rcu: Convert rcu_grace_period tracepoint to gp_seq

[Paul E. McKenney]

This commit makes the rcu_grace_period tracepoint use gp_seq instead
of ->gpnum or ->completed.  It also introduces a "cpuofl-bgp" string to
less obscurely indicate when a CPU has gone offline while a grace period
is waiting on it.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 include/trace/events/rcu.h | 13 ++++++------
 kernel/rcu/tree.c          | 43 +++++++++++++++++++-------------------
 kernel/rcu/tree_plugin.h   |  2 +-
 3 files changed, 29 insertions(+), 29 deletions(-)

diff --git a/include/trace/events/rcu.h b/include/trace/events/rcu.h
index 5936aac357ab..cd229e82ec8b 100644
--- a/include/trace/events/rcu.h
+++ b/include/trace/events/rcu.h
@@ -52,6 +52,7 @@ TRACE_EVENT(rcu_utilization,
  *	"cpuqs": CPU passes through a quiescent state.
  *	"cpuonl": CPU comes online.
  *	"cpuofl": CPU goes offline.
+ *	"cpuofl-bgp": CPU goes offline while blocking a grace period.
  *	"reqwait": GP kthread sleeps waiting for grace-period request.
  *	"reqwaitsig": GP kthread awakened by signal from reqwait state.
  *	"fqswait": GP kthread waiting until time to force quiescent states.
@@ -63,24 +64,24 @@ TRACE_EVENT(rcu_utilization,
  */
 TRACE_EVENT(rcu_grace_period,
 
-	TP_PROTO(const char *rcuname, unsigned long gpnum, const char *gpevent),
+	TP_PROTO(const char *rcuname, unsigned long gp_seq, const char *gpevent),
 
-	TP_ARGS(rcuname, gpnum, gpevent),
+	TP_ARGS(rcuname, gp_seq, gpevent),
 
 	TP_STRUCT__entry(
 		__field(const char *, rcuname)
-		__field(unsigned long, gpnum)
+		__field(unsigned long, gp_seq)
 		__field(const char *, gpevent)
 	),
 
 	TP_fast_assign(
 		__entry->rcuname = rcuname;
-		__entry->gpnum = gpnum;
+		__entry->gp_seq = gp_seq;
 		__entry->gpevent = gpevent;
 	),
 
 	TP_printk("%s %lu %s",
-		  __entry->rcuname, __entry->gpnum, __entry->gpevent)
+		  __entry->rcuname, __entry->gp_seq, __entry->gpevent)
 );
 
 /*
@@ -753,7 +754,7 @@ TRACE_EVENT(rcu_barrier,
 
 #else /* #ifdef CONFIG_RCU_TRACE */
 
-#define trace_rcu_grace_period(rcuname, gpnum, gpevent) do { } while (0)
+#define trace_rcu_grace_period(rcuname, gp_seq, gpevent) do { } while (0)
 #define trace_rcu_future_grace_period(rcuname, gpnum, completed, c, \
 				      level, grplo, grphi, event) \
 				      do { } while (0)
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 0da0db1128d0..70e5d155042c 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -234,7 +234,7 @@ void rcu_sched_qs(void)
 	if (!__this_cpu_read(rcu_sched_data.cpu_no_qs.s))
 		return;
 	trace_rcu_grace_period(TPS("rcu_sched"),
-			       __this_cpu_read(rcu_sched_data.gpnum),
+			       __this_cpu_read(rcu_sched_data.gp_seq),
 			       TPS("cpuqs"));
 	__this_cpu_write(rcu_sched_data.cpu_no_qs.b.norm, false);
 	if (!__this_cpu_read(rcu_sched_data.cpu_no_qs.b.exp))
@@ -249,7 +249,7 @@ void rcu_bh_qs(void)
 	RCU_LOCKDEP_WARN(preemptible(), "rcu_bh_qs() invoked with preemption enabled!!!");
 	if (__this_cpu_read(rcu_bh_data.cpu_no_qs.s)) {
 		trace_rcu_grace_period(TPS("rcu_bh"),
-				       __this_cpu_read(rcu_bh_data.gpnum),
+				       __this_cpu_read(rcu_bh_data.gp_seq),
 				       TPS("cpuqs"));
 		__this_cpu_write(rcu_bh_data.cpu_no_qs.b.norm, false);
 	}
@@ -1615,7 +1615,7 @@ static bool rcu_start_this_gp(struct rcu_node *rnp, struct rcu_data *rdp,
 		trace_rcu_this_gp(rnp_root, rdp, c, TPS("NoGPkthread"));
 		goto unlock_out;
 	}
-	trace_rcu_grace_period(rsp->name, READ_ONCE(rsp->gpnum), TPS("newreq"));
+	trace_rcu_grace_period(rsp->name, READ_ONCE(rsp->gp_seq), TPS("newreq"));
 	ret = true;  /* Caller must wake GP kthread. */
 unlock_out:
 	/* Push furthest requested GP to leaf node and rcu_data structure. */
@@ -1702,9 +1702,9 @@ static bool rcu_accelerate_cbs(struct rcu_state *rsp, struct rcu_node *rnp,
 
 	/* Trace depending on how much we were able to accelerate. */
 	if (rcu_segcblist_restempty(&rdp->cblist, RCU_WAIT_TAIL))
-		trace_rcu_grace_period(rsp->name, rdp->gpnum, TPS("AccWaitCB"));
+		trace_rcu_grace_period(rsp->name, rdp->gp_seq, TPS("AccWaitCB"));
 	else
-		trace_rcu_grace_period(rsp->name, rdp->gpnum, TPS("AccReadyCB"));
+		trace_rcu_grace_period(rsp->name, rdp->gp_seq, TPS("AccReadyCB"));
 	return ret;
 }
 
@@ -1774,7 +1774,7 @@ static bool __note_gp_changes(struct rcu_state *rsp, struct rcu_node *rnp,
 		 * go looking for one.
 		 */
 		rdp->gpnum = rnp->gpnum;
-		trace_rcu_grace_period(rsp->name, rdp->gpnum, TPS("cpustart"));
+		trace_rcu_grace_period(rsp->name, rdp->gp_seq, TPS("cpustart"));
 		need_gp = !!(rnp->qsmask & rdp->grpmask);
 		rdp->cpu_no_qs.b.norm = need_gp;
 		rdp->rcu_qs_ctr_snap = __this_cpu_read(rcu_dynticks.rcu_qs_ctr);
@@ -1851,7 +1851,7 @@ static bool rcu_gp_init(struct rcu_state *rsp)
 	rcu_seq_start(&rsp->gp_seq);
 	if (WARN_ON_ONCE(((rnp->completed << RCU_SEQ_CTR_SHIFT) >> RCU_SEQ_CTR_SHIFT) != rcu_seq_ctr(rnp->gp_seq))) /* Catch any ->completed/->gp_seq mismatches. */
 		pr_info("%s ->completed: %#lx (%#lx) ->gp_seq %#lx (%#lx)\n", __func__, rnp->completed, (rnp->completed << RCU_SEQ_CTR_SHIFT) >> RCU_SEQ_CTR_SHIFT, rnp->gp_seq, rcu_seq_ctr(rnp->gp_seq));
-	trace_rcu_grace_period(rsp->name, rsp->gpnum, TPS("start"));
+	trace_rcu_grace_period(rsp->name, rsp->gp_seq, TPS("start"));
 	raw_spin_unlock_irq_rcu_node(rnp);
 
 	/*
@@ -1928,7 +1928,7 @@ static bool rcu_gp_init(struct rcu_state *rsp)
 		if (rnp == rdp->mynode)
 			(void)__note_gp_changes(rsp, rnp, rdp);
 		rcu_preempt_boost_start_gp(rnp);
-		trace_rcu_grace_period_init(rsp->name, rnp->gpnum,
+		trace_rcu_grace_period_init(rsp->name, rnp->gp_seq,
 					    rnp->level, rnp->grplo,
 					    rnp->grphi, rnp->qsmask);
 		raw_spin_unlock_irq_rcu_node(rnp);
@@ -2048,7 +2048,7 @@ static void rcu_gp_cleanup(struct rcu_state *rsp)
 	/* Declare grace period done. */
 	WRITE_ONCE(rsp->completed, rsp->gpnum);
 	rcu_seq_end(&rsp->gp_seq);
-	trace_rcu_grace_period(rsp->name, rsp->completed, TPS("end"));
+	trace_rcu_grace_period(rsp->name, rsp->gp_seq, TPS("end"));
 	rsp->gp_state = RCU_GP_IDLE;
 	/* Check for GP requests since above loop. */
 	rdp = this_cpu_ptr(rsp->rda);
@@ -2061,7 +2061,7 @@ static void rcu_gp_cleanup(struct rcu_state *rsp)
 	if (!rcu_accelerate_cbs(rsp, rnp, rdp) && needgp) {
 		WRITE_ONCE(rsp->gp_flags, RCU_GP_FLAG_INIT);
 		rsp->gp_req_activity = jiffies;
-		trace_rcu_grace_period(rsp->name, READ_ONCE(rsp->gpnum),
+		trace_rcu_grace_period(rsp->name, READ_ONCE(rsp->gp_seq),
 				       TPS("newreq"));
 	} else {
 		WRITE_ONCE(rsp->gp_flags, rsp->gp_flags & RCU_GP_FLAG_INIT);
@@ -2087,7 +2087,7 @@ static int __noreturn rcu_gp_kthread(void *arg)
 		/* Handle grace-period start. */
 		for (;;) {
 			trace_rcu_grace_period(rsp->name,
-					       READ_ONCE(rsp->gpnum),
+					       READ_ONCE(rsp->gp_seq),
 					       TPS("reqwait"));
 			rsp->gp_state = RCU_GP_WAIT_GPS;
 			swait_event_idle(rsp->gp_wq, READ_ONCE(rsp->gp_flags) &
@@ -2100,7 +2100,7 @@ static int __noreturn rcu_gp_kthread(void *arg)
 			WRITE_ONCE(rsp->gp_activity, jiffies);
 			WARN_ON(signal_pending(current));
 			trace_rcu_grace_period(rsp->name,
-					       READ_ONCE(rsp->gpnum),
+					       READ_ONCE(rsp->gp_seq),
 					       TPS("reqwaitsig"));
 		}
 
@@ -2119,7 +2119,7 @@ static int __noreturn rcu_gp_kthread(void *arg)
 					   jiffies + 3 * j);
 			}
 			trace_rcu_grace_period(rsp->name,
-					       READ_ONCE(rsp->gpnum),
+					       READ_ONCE(rsp->gp_seq),
 					       TPS("fqswait"));
 			rsp->gp_state = RCU_GP_WAIT_FQS;
 			ret = swait_event_idle_timeout(rsp->gp_wq,
@@ -2134,12 +2134,12 @@ static int __noreturn rcu_gp_kthread(void *arg)
 			if (ULONG_CMP_GE(jiffies, rsp->jiffies_force_qs) ||
 			    (gf & RCU_GP_FLAG_FQS)) {
 				trace_rcu_grace_period(rsp->name,
-						       READ_ONCE(rsp->gpnum),
+						       READ_ONCE(rsp->gp_seq),
 						       TPS("fqsstart"));
 				rcu_gp_fqs(rsp, first_gp_fqs);
 				first_gp_fqs = false;
 				trace_rcu_grace_period(rsp->name,
-						       READ_ONCE(rsp->gpnum),
+						       READ_ONCE(rsp->gp_seq),
 						       TPS("fqsend"));
 				cond_resched_tasks_rcu_qs();
 				WRITE_ONCE(rsp->gp_activity, jiffies);
@@ -2158,7 +2158,7 @@ static int __noreturn rcu_gp_kthread(void *arg)
 				WRITE_ONCE(rsp->gp_activity, jiffies);
 				WARN_ON(signal_pending(current));
 				trace_rcu_grace_period(rsp->name,
-						       READ_ONCE(rsp->gpnum),
+						       READ_ONCE(rsp->gp_seq),
 						       TPS("fqswaitsig"));
 				ret = 1; /* Keep old FQS timing. */
 				j = jiffies;
@@ -2388,17 +2388,16 @@ rcu_check_quiescent_state(struct rcu_state *rsp, struct rcu_data *rdp)
  */
 static void rcu_cleanup_dying_cpu(struct rcu_state *rsp)
 {
-	RCU_TRACE(unsigned long mask;)
+	RCU_TRACE(bool blkd;)
 	RCU_TRACE(struct rcu_data *rdp = this_cpu_ptr(rsp->rda);)
 	RCU_TRACE(struct rcu_node *rnp = rdp->mynode;)
 
 	if (!IS_ENABLED(CONFIG_HOTPLUG_CPU))
 		return;
 
-	RCU_TRACE(mask = rdp->grpmask;)
-	trace_rcu_grace_period(rsp->name,
-			       rnp->gpnum + 1 - !!(rnp->qsmask & mask),
-			       TPS("cpuofl"));
+	RCU_TRACE(blkd = !!(rnp->qsmask & rdp->grpmask);)
+	trace_rcu_grace_period(rsp->name, rnp->gp_seq,
+			       blkd ? TPS("cpuofl") : TPS("cpuofl-bgp"));
 }
 
 /*
@@ -3536,7 +3535,7 @@ rcu_init_percpu_data(int cpu, struct rcu_state *rsp)
 	rdp->core_needs_qs = false;
 	rdp->rcu_iw_pending = false;
 	rdp->rcu_iw_gp_seq = rnp->gp_seq - 1;
-	trace_rcu_grace_period(rsp->name, rdp->gpnum, TPS("cpuonl"));
+	trace_rcu_grace_period(rsp->name, rdp->gp_seq, TPS("cpuonl"));
 	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 }
 
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index ca73931f7b30..aca9d187c509 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -301,7 +301,7 @@ static void rcu_preempt_qs(void)
 	RCU_LOCKDEP_WARN(preemptible(), "rcu_preempt_qs() invoked with preemption enabled!!!\n");
 	if (__this_cpu_read(rcu_data_p->cpu_no_qs.s)) {
 		trace_rcu_grace_period(TPS("rcu_preempt"),
-				       __this_cpu_read(rcu_data_p->gpnum),
+				       __this_cpu_read(rcu_data_p->gp_seq),
 				       TPS("cpuqs"));
 		__this_cpu_write(rcu_data_p->cpu_no_qs.b.norm, false);
 		barrier(); /* Coordinate with rcu_preempt_check_callbacks(). */
-- 
2.17.1

[PATCH tip/core/rcu 21/40] rcu: Convert rcu_future_grace_period tracepoint to gp_seq

[Paul E. McKenney]

This commit makes the rcu_future_grace_period tracepoint use gp_seq
instead of ->gpnum and ->completed.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 include/trace/events/rcu.h | 22 +++++++++-------------
 kernel/rcu/tree.c          |  7 +++----
 2 files changed, 12 insertions(+), 17 deletions(-)

diff --git a/include/trace/events/rcu.h b/include/trace/events/rcu.h
index cd229e82ec8b..286047d22314 100644
--- a/include/trace/events/rcu.h
+++ b/include/trace/events/rcu.h
@@ -103,16 +103,14 @@ TRACE_EVENT(rcu_grace_period,
  */
 TRACE_EVENT(rcu_future_grace_period,
 
-	TP_PROTO(const char *rcuname, unsigned long gpnum, unsigned long completed,
-		 unsigned long c, u8 level, int grplo, int grphi,
-		 const char *gpevent),
+	TP_PROTO(const char *rcuname, unsigned long gp_seq, unsigned long c,
+		 u8 level, int grplo, int grphi, const char *gpevent),
 
-	TP_ARGS(rcuname, gpnum, completed, c, level, grplo, grphi, gpevent),
+	TP_ARGS(rcuname, gp_seq, c, level, grplo, grphi, gpevent),
 
 	TP_STRUCT__entry(
 		__field(const char *, rcuname)
-		__field(unsigned long, gpnum)
-		__field(unsigned long, completed)
+		__field(unsigned long, gp_seq)
 		__field(unsigned long, c)
 		__field(u8, level)
 		__field(int, grplo)
@@ -122,8 +120,7 @@ TRACE_EVENT(rcu_future_grace_period,
 
 	TP_fast_assign(
 		__entry->rcuname = rcuname;
-		__entry->gpnum = gpnum;
-		__entry->completed = completed;
+		__entry->gp_seq = gp_seq;
 		__entry->c = c;
 		__entry->level = level;
 		__entry->grplo = grplo;
@@ -131,10 +128,9 @@ TRACE_EVENT(rcu_future_grace_period,
 		__entry->gpevent = gpevent;
 	),
 
-	TP_printk("%s %lu %lu %lu %u %d %d %s",
-		  __entry->rcuname, __entry->gpnum, __entry->completed,
-		  __entry->c, __entry->level, __entry->grplo, __entry->grphi,
-		  __entry->gpevent)
+	TP_printk("%s %lu %lu %u %d %d %s",
+		  __entry->rcuname, __entry->gp_seq, __entry->c, __entry->level,
+		  __entry->grplo, __entry->grphi, __entry->gpevent)
 );
 
 /*
@@ -755,7 +751,7 @@ TRACE_EVENT(rcu_barrier,
 #else /* #ifdef CONFIG_RCU_TRACE */
 
 #define trace_rcu_grace_period(rcuname, gp_seq, gpevent) do { } while (0)
-#define trace_rcu_future_grace_period(rcuname, gpnum, completed, c, \
+#define trace_rcu_future_grace_period(rcuname, gp_seq, c, \
 				      level, grplo, grphi, event) \
 				      do { } while (0)
 #define trace_rcu_grace_period_init(rcuname, gpnum, level, grplo, grphi, \
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 70e5d155042c..de4b7994ffc4 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1552,9 +1552,8 @@ void rcu_cpu_stall_reset(void)
 static void trace_rcu_this_gp(struct rcu_node *rnp, struct rcu_data *rdp,
 			      unsigned long c, const char *s)
 {
-	trace_rcu_future_grace_period(rdp->rsp->name, rnp->gpnum,
-				      rnp->completed, c, rnp->level,
-				      rnp->grplo, rnp->grphi, s);
+	trace_rcu_future_grace_period(rdp->rsp->name, rnp->gp_seq, c,
+				      rnp->level, rnp->grplo, rnp->grphi, s);
 }
 
 /*
@@ -2053,7 +2052,7 @@ static void rcu_gp_cleanup(struct rcu_state *rsp)
 	/* Check for GP requests since above loop. */
 	rdp = this_cpu_ptr(rsp->rda);
 	if (ULONG_CMP_LT(rnp->gp_seq, rnp->gp_seq_needed)) {
-		trace_rcu_this_gp(rnp, rdp, rsp->completed - 1,
+		trace_rcu_this_gp(rnp, rdp, rnp->gp_seq_needed,
 				  TPS("CleanupMore"));
 		needgp = true;
 	}
-- 
2.17.1

[PATCH tip/core/rcu 22/40] rcu: Convert rcu_grace_period_init tracepoint to gp_seq

[Paul E. McKenney]

This commit makes the rcu_grace_period_init tracepoint use gp_seq instead
of ->gpnum.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 include/trace/events/rcu.h | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/include/trace/events/rcu.h b/include/trace/events/rcu.h
index 286047d22314..892016ad0647 100644
--- a/include/trace/events/rcu.h
+++ b/include/trace/events/rcu.h
@@ -142,14 +142,14 @@ TRACE_EVENT(rcu_future_grace_period,
  */
 TRACE_EVENT(rcu_grace_period_init,
 
-	TP_PROTO(const char *rcuname, unsigned long gpnum, u8 level,
+	TP_PROTO(const char *rcuname, unsigned long gp_seq, u8 level,
 		 int grplo, int grphi, unsigned long qsmask),
 
-	TP_ARGS(rcuname, gpnum, level, grplo, grphi, qsmask),
+	TP_ARGS(rcuname, gp_seq, level, grplo, grphi, qsmask),
 
 	TP_STRUCT__entry(
 		__field(const char *, rcuname)
-		__field(unsigned long, gpnum)
+		__field(unsigned long, gp_seq)
 		__field(u8, level)
 		__field(int, grplo)
 		__field(int, grphi)
@@ -158,7 +158,7 @@ TRACE_EVENT(rcu_grace_period_init,
 
 	TP_fast_assign(
 		__entry->rcuname = rcuname;
-		__entry->gpnum = gpnum;
+		__entry->gp_seq = gp_seq;
 		__entry->level = level;
 		__entry->grplo = grplo;
 		__entry->grphi = grphi;
@@ -166,7 +166,7 @@ TRACE_EVENT(rcu_grace_period_init,
 	),
 
 	TP_printk("%s %lu %u %d %d %lx",
-		  __entry->rcuname, __entry->gpnum, __entry->level,
+		  __entry->rcuname, __entry->gp_seq, __entry->level,
 		  __entry->grplo, __entry->grphi, __entry->qsmask)
 );
 
@@ -754,7 +754,7 @@ TRACE_EVENT(rcu_barrier,
 #define trace_rcu_future_grace_period(rcuname, gp_seq, c, \
 				      level, grplo, grphi, event) \
 				      do { } while (0)
-#define trace_rcu_grace_period_init(rcuname, gpnum, level, grplo, grphi, \
+#define trace_rcu_grace_period_init(rcuname, gp_seq, level, grplo, grphi, \
 				    qsmask) do { } while (0)
 #define trace_rcu_exp_grace_period(rcuname, gqseq, gpevent) \
 	do { } while (0)
-- 
2.17.1

[PATCH tip/core/rcu 23/40] rcu: Convert rcu_preempt_task tracepoint to ->gp_seq

[Paul E. McKenney]

This commit makes the rcu_preempt_task tracepoint use ->gp_seq instead
of ->gpnum.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 include/trace/events/rcu.h | 12 ++++++------
 kernel/rcu/tree_plugin.h   |  4 ++--
 2 files changed, 8 insertions(+), 8 deletions(-)

diff --git a/include/trace/events/rcu.h b/include/trace/events/rcu.h
index 892016ad0647..38dbd97d65a3 100644
--- a/include/trace/events/rcu.h
+++ b/include/trace/events/rcu.h
@@ -298,24 +298,24 @@ TRACE_EVENT(rcu_nocb_wake,
  */
 TRACE_EVENT(rcu_preempt_task,
 
-	TP_PROTO(const char *rcuname, int pid, unsigned long gpnum),
+	TP_PROTO(const char *rcuname, int pid, unsigned long gp_seq),
 
-	TP_ARGS(rcuname, pid, gpnum),
+	TP_ARGS(rcuname, pid, gp_seq),
 
 	TP_STRUCT__entry(
 		__field(const char *, rcuname)
-		__field(unsigned long, gpnum)
+		__field(unsigned long, gp_seq)
 		__field(int, pid)
 	),
 
 	TP_fast_assign(
 		__entry->rcuname = rcuname;
-		__entry->gpnum = gpnum;
+		__entry->gp_seq = gp_seq;
 		__entry->pid = pid;
 	),
 
 	TP_printk("%s %lu %d",
-		  __entry->rcuname, __entry->gpnum, __entry->pid)
+		  __entry->rcuname, __entry->gp_seq, __entry->pid)
 );
 
 /*
@@ -761,7 +761,7 @@ TRACE_EVENT(rcu_barrier,
 #define trace_rcu_exp_funnel_lock(rcuname, level, grplo, grphi, gpevent) \
 	do { } while (0)
 #define trace_rcu_nocb_wake(rcuname, cpu, reason) do { } while (0)
-#define trace_rcu_preempt_task(rcuname, pid, gpnum) do { } while (0)
+#define trace_rcu_preempt_task(rcuname, pid, gp_seq) do { } while (0)
 #define trace_rcu_unlock_preempted_task(rcuname, gpnum, pid) do { } while (0)
 #define trace_rcu_quiescent_state_report(rcuname, gpnum, mask, qsmask, level, \
 					 grplo, grphi, gp_tasks) do { } \
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index aca9d187c509..02ca3b4e6a8f 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -350,8 +350,8 @@ static void rcu_preempt_note_context_switch(bool preempt)
 		trace_rcu_preempt_task(rdp->rsp->name,
 				       t->pid,
 				       (rnp->qsmask & rdp->grpmask)
-				       ? rnp->gpnum
-				       : rnp->gpnum + 1);
+				       ? rnp->gp_seq
+				       : rcu_seq_snap(&rnp->gp_seq));
 		rcu_preempt_ctxt_queue(rnp, rdp);
 	} else if (t->rcu_read_lock_nesting < 0 &&
 		   t->rcu_read_unlock_special.s) {
-- 
2.17.1

[PATCH tip/core/rcu 24/40] rcu: Convert rcu_unlock_preempted_task tracepoint to ->gp_seq

[Paul E. McKenney]

This commit makes the rcu_unlock_preempted_task tracepoint use ->gp_seq
instead of ->gpnum.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 include/trace/events/rcu.h | 12 ++++++------
 kernel/rcu/tree_plugin.h   |  4 ++--
 2 files changed, 8 insertions(+), 8 deletions(-)

diff --git a/include/trace/events/rcu.h b/include/trace/events/rcu.h
index 38dbd97d65a3..95b7491196aa 100644
--- a/include/trace/events/rcu.h
+++ b/include/trace/events/rcu.h
@@ -325,23 +325,23 @@ TRACE_EVENT(rcu_preempt_task,
  */
 TRACE_EVENT(rcu_unlock_preempted_task,
 
-	TP_PROTO(const char *rcuname, unsigned long gpnum, int pid),
+	TP_PROTO(const char *rcuname, unsigned long gp_seq, int pid),
 
-	TP_ARGS(rcuname, gpnum, pid),
+	TP_ARGS(rcuname, gp_seq, pid),
 
 	TP_STRUCT__entry(
 		__field(const char *, rcuname)
-		__field(unsigned long, gpnum)
+		__field(unsigned long, gp_seq)
 		__field(int, pid)
 	),
 
 	TP_fast_assign(
 		__entry->rcuname = rcuname;
-		__entry->gpnum = gpnum;
+		__entry->gp_seq = gp_seq;
 		__entry->pid = pid;
 	),
 
-	TP_printk("%s %lu %d", __entry->rcuname, __entry->gpnum, __entry->pid)
+	TP_printk("%s %lu %d", __entry->rcuname, __entry->gp_seq, __entry->pid)
 );
 
 /*
@@ -762,7 +762,7 @@ TRACE_EVENT(rcu_barrier,
 	do { } while (0)
 #define trace_rcu_nocb_wake(rcuname, cpu, reason) do { } while (0)
 #define trace_rcu_preempt_task(rcuname, pid, gp_seq) do { } while (0)
-#define trace_rcu_unlock_preempted_task(rcuname, gpnum, pid) do { } while (0)
+#define trace_rcu_unlock_preempted_task(rcuname, gp_seq, pid) do { } while (0)
 #define trace_rcu_quiescent_state_report(rcuname, gpnum, mask, qsmask, level, \
 					 grplo, grphi, gp_tasks) do { } \
 	while (0)
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 02ca3b4e6a8f..a10b0e26ce19 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -545,7 +545,7 @@ void rcu_read_unlock_special(struct task_struct *t)
 		list_del_init(&t->rcu_node_entry);
 		t->rcu_blocked_node = NULL;
 		trace_rcu_unlock_preempted_task(TPS("rcu_preempt"),
-						rnp->gpnum, t->pid);
+						rnp->gp_seq, t->pid);
 		if (&t->rcu_node_entry == rnp->gp_tasks)
 			rnp->gp_tasks = np;
 		if (&t->rcu_node_entry == rnp->exp_tasks)
@@ -708,7 +708,7 @@ static void rcu_preempt_check_blocked_tasks(struct rcu_node *rnp)
 		t = container_of(rnp->gp_tasks, struct task_struct,
 				 rcu_node_entry);
 		trace_rcu_unlock_preempted_task(TPS("rcu_preempt-GPS"),
-						rnp->gpnum, t->pid);
+						rnp->gp_seq, t->pid);
 	}
 	WARN_ON_ONCE(rnp->qsmask);
 }
-- 
2.17.1

[PATCH tip/core/rcu 25/40] rcu: Convert rcu_quiescent_state_report tracepoint to ->gp_seq

[Paul E. McKenney]

This commit makes the rcu_quiescent_state_report tracepoint use ->gp_seq
instead of ->gpnum.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 include/trace/events/rcu.h | 12 ++++++------
 kernel/rcu/tree.c          |  2 +-
 kernel/rcu/tree_plugin.h   |  2 +-
 3 files changed, 8 insertions(+), 8 deletions(-)

diff --git a/include/trace/events/rcu.h b/include/trace/events/rcu.h
index 95b7491196aa..ac4d9d4a1ebf 100644
--- a/include/trace/events/rcu.h
+++ b/include/trace/events/rcu.h
@@ -354,15 +354,15 @@ TRACE_EVENT(rcu_unlock_preempted_task,
  */
 TRACE_EVENT(rcu_quiescent_state_report,
 
-	TP_PROTO(const char *rcuname, unsigned long gpnum,
+	TP_PROTO(const char *rcuname, unsigned long gp_seq,
 		 unsigned long mask, unsigned long qsmask,
 		 u8 level, int grplo, int grphi, int gp_tasks),
 
-	TP_ARGS(rcuname, gpnum, mask, qsmask, level, grplo, grphi, gp_tasks),
+	TP_ARGS(rcuname, gp_seq, mask, qsmask, level, grplo, grphi, gp_tasks),
 
 	TP_STRUCT__entry(
 		__field(const char *, rcuname)
-		__field(unsigned long, gpnum)
+		__field(unsigned long, gp_seq)
 		__field(unsigned long, mask)
 		__field(unsigned long, qsmask)
 		__field(u8, level)
@@ -373,7 +373,7 @@ TRACE_EVENT(rcu_quiescent_state_report,
 
 	TP_fast_assign(
 		__entry->rcuname = rcuname;
-		__entry->gpnum = gpnum;
+		__entry->gp_seq = gp_seq;
 		__entry->mask = mask;
 		__entry->qsmask = qsmask;
 		__entry->level = level;
@@ -383,7 +383,7 @@ TRACE_EVENT(rcu_quiescent_state_report,
 	),
 
 	TP_printk("%s %lu %lx>%lx %u %d %d %u",
-		  __entry->rcuname, __entry->gpnum,
+		  __entry->rcuname, __entry->gp_seq,
 		  __entry->mask, __entry->qsmask, __entry->level,
 		  __entry->grplo, __entry->grphi, __entry->gp_tasks)
 );
@@ -763,7 +763,7 @@ TRACE_EVENT(rcu_barrier,
 #define trace_rcu_nocb_wake(rcuname, cpu, reason) do { } while (0)
 #define trace_rcu_preempt_task(rcuname, pid, gp_seq) do { } while (0)
 #define trace_rcu_unlock_preempted_task(rcuname, gp_seq, pid) do { } while (0)
-#define trace_rcu_quiescent_state_report(rcuname, gpnum, mask, qsmask, level, \
+#define trace_rcu_quiescent_state_report(rcuname, gp_seq, mask, qsmask, level, \
 					 grplo, grphi, gp_tasks) do { } \
 	while (0)
 #define trace_rcu_fqs(rcuname, gpnum, cpu, qsevent) do { } while (0)
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index de4b7994ffc4..dcd185078132 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2229,7 +2229,7 @@ rcu_report_qs_rnp(unsigned long mask, struct rcu_state *rsp,
 		WARN_ON_ONCE(!rcu_is_leaf_node(rnp) &&
 			     rcu_preempt_blocked_readers_cgp(rnp));
 		rnp->qsmask &= ~mask;
-		trace_rcu_quiescent_state_report(rsp->name, rnp->gpnum,
+		trace_rcu_quiescent_state_report(rsp->name, rnp->gp_seq,
 						 mask, rnp->qsmask, rnp->level,
 						 rnp->grplo, rnp->grphi,
 						 !!rnp->gp_tasks);
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index a10b0e26ce19..c8a2c7760121 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -566,7 +566,7 @@ void rcu_read_unlock_special(struct task_struct *t)
 		empty_exp_now = sync_rcu_preempt_exp_done(rnp);
 		if (!empty_norm && !rcu_preempt_blocked_readers_cgp(rnp)) {
 			trace_rcu_quiescent_state_report(TPS("preempt_rcu"),
-							 rnp->gpnum,
+							 rnp->gp_seq,
 							 0, rnp->qsmask,
 							 rnp->level,
 							 rnp->grplo,
-- 
2.17.1

[PATCH tip/core/rcu 26/40] rcu: Convert rcu_fqs tracepoint to ->gp_seq

[Paul E. McKenney]

This commit makes the rcu_fqs tracepoint use ->gp_seq instead of ->gpnum.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 include/trace/events/rcu.h | 12 ++++++------
 kernel/rcu/tree.c          |  8 ++++----
 2 files changed, 10 insertions(+), 10 deletions(-)

diff --git a/include/trace/events/rcu.h b/include/trace/events/rcu.h
index ac4d9d4a1ebf..7d3650cc9d30 100644
--- a/include/trace/events/rcu.h
+++ b/include/trace/events/rcu.h
@@ -398,26 +398,26 @@ TRACE_EVENT(rcu_quiescent_state_report,
  */
 TRACE_EVENT(rcu_fqs,
 
-	TP_PROTO(const char *rcuname, unsigned long gpnum, int cpu, const char *qsevent),
+	TP_PROTO(const char *rcuname, unsigned long gp_seq, int cpu, const char *qsevent),
 
-	TP_ARGS(rcuname, gpnum, cpu, qsevent),
+	TP_ARGS(rcuname, gp_seq, cpu, qsevent),
 
 	TP_STRUCT__entry(
 		__field(const char *, rcuname)
-		__field(unsigned long, gpnum)
+		__field(unsigned long, gp_seq)
 		__field(int, cpu)
 		__field(const char *, qsevent)
 	),
 
 	TP_fast_assign(
 		__entry->rcuname = rcuname;
-		__entry->gpnum = gpnum;
+		__entry->gp_seq = gp_seq;
 		__entry->cpu = cpu;
 		__entry->qsevent = qsevent;
 	),
 
 	TP_printk("%s %lu %d %s",
-		  __entry->rcuname, __entry->gpnum,
+		  __entry->rcuname, __entry->gp_seq,
 		  __entry->cpu, __entry->qsevent)
 );
 
@@ -766,7 +766,7 @@ TRACE_EVENT(rcu_barrier,
 #define trace_rcu_quiescent_state_report(rcuname, gp_seq, mask, qsmask, level, \
 					 grplo, grphi, gp_tasks) do { } \
 	while (0)
-#define trace_rcu_fqs(rcuname, gpnum, cpu, qsevent) do { } while (0)
+#define trace_rcu_fqs(rcuname, gp_seq, cpu, qsevent) do { } while (0)
 #define trace_rcu_dyntick(polarity, oldnesting, newnesting, dyntick) do { } while (0)
 #define trace_rcu_callback(rcuname, rhp, qlen_lazy, qlen) do { } while (0)
 #define trace_rcu_kfree_callback(rcuname, rhp, offset, qlen_lazy, qlen) \
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index dcd185078132..43ba06abf081 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1111,7 +1111,7 @@ static int dyntick_save_progress_counter(struct rcu_data *rdp)
 {
 	rdp->dynticks_snap = rcu_dynticks_snap(rdp->dynticks);
 	if (rcu_dynticks_in_eqs(rdp->dynticks_snap)) {
-		trace_rcu_fqs(rdp->rsp->name, rdp->gpnum, rdp->cpu, TPS("dti"));
+		trace_rcu_fqs(rdp->rsp->name, rdp->gp_seq, rdp->cpu, TPS("dti"));
 		rcu_gpnum_ovf(rdp->mynode, rdp);
 		return 1;
 	}
@@ -1161,7 +1161,7 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
 	 * of the current RCU grace period.
 	 */
 	if (rcu_dynticks_in_eqs_since(rdp->dynticks, rdp->dynticks_snap)) {
-		trace_rcu_fqs(rdp->rsp->name, rdp->gpnum, rdp->cpu, TPS("dti"));
+		trace_rcu_fqs(rdp->rsp->name, rdp->gp_seq, rdp->cpu, TPS("dti"));
 		rdp->dynticks_fqs++;
 		rcu_gpnum_ovf(rnp, rdp);
 		return 1;
@@ -1178,7 +1178,7 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
 	if (time_after(jiffies, rdp->rsp->gp_start + jtsq) &&
 	    READ_ONCE(rdp->rcu_qs_ctr_snap) != per_cpu(rcu_dynticks.rcu_qs_ctr, rdp->cpu) &&
 	    rcu_seq_current(&rdp->gp_seq) == rnp->gp_seq && !rdp->gpwrap) {
-		trace_rcu_fqs(rdp->rsp->name, rdp->gpnum, rdp->cpu, TPS("rqc"));
+		trace_rcu_fqs(rdp->rsp->name, rdp->gp_seq, rdp->cpu, TPS("rqc"));
 		rcu_gpnum_ovf(rnp, rdp);
 		return 1;
 	} else if (time_after(jiffies, rdp->rsp->gp_start + jtsq)) {
@@ -1188,7 +1188,7 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
 
 	/* Check for the CPU being offline. */
 	if (!(rdp->grpmask & rcu_rnp_online_cpus(rnp))) {
-		trace_rcu_fqs(rdp->rsp->name, rdp->gpnum, rdp->cpu, TPS("ofl"));
+		trace_rcu_fqs(rdp->rsp->name, rdp->gp_seq, rdp->cpu, TPS("ofl"));
 		rdp->offline_fqs++;
 		rcu_gpnum_ovf(rnp, rdp);
 		return 1;
-- 
2.17.1

[PATCH tip/core/rcu 27/40] rcu: Remove ->gpnum and ->completed

[Paul E. McKenney]

Now that everything has been converted to use ->gp_seq instead of
->gpnum and ->completed, this commit removes ->gpnum and ->completed.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 kernel/rcu/tree.c        | 45 ++++++++++------------------------------
 kernel/rcu/tree.h        | 14 +------------
 kernel/rcu/tree_plugin.h |  2 +-
 3 files changed, 13 insertions(+), 48 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 43ba06abf081..2b66da93cf5b 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -95,8 +95,6 @@ struct rcu_state sname##_state = { \
 	.rda = &sname##_data, \
 	.call = cr, \
 	.gp_state = RCU_GP_IDLE, \
-	.gpnum = 0UL - 300UL, \
-	.completed = 0UL - 300UL, \
 	.gp_seq = (0UL - 300UL) << RCU_SEQ_CTR_SHIFT, \
 	.barrier_mutex = __MUTEX_INITIALIZER(sname##_state.barrier_mutex), \
 	.name = RCU_STATE_NAME(sname), \
@@ -1349,8 +1347,6 @@ static void print_other_cpu_stall(struct rcu_state *rsp, unsigned long gp_seq)
 	struct rcu_node *rnp = rcu_get_root(rsp);
 	long totqlen = 0;
 
-	WARN_ON_ONCE(gp_seq & 0x2); /* Remove when ->gpnum removed. */
-
 	/* Kick and suppress, if so configured. */
 	rcu_stall_kick_kthreads(rsp);
 	if (rcu_cpu_stall_suppress)
@@ -1582,8 +1578,6 @@ static bool rcu_start_this_gp(struct rcu_node *rnp, struct rcu_data *rdp,
 	 * not be released.
 	 */
 	raw_lockdep_assert_held_rcu_node(rnp);
-	WARN_ON_ONCE(c & 0x2); /* Catch any lingering use of ->gpnum. */
-	WARN_ON_ONCE(((rnp->completed << RCU_SEQ_CTR_SHIFT) >> RCU_SEQ_CTR_SHIFT) != rcu_seq_ctr(rnp->gp_seq)); /* Catch any ->completed/->gp_seq mismatches. */
 	trace_rcu_this_gp(rnp, rdp, c, TPS("Startleaf"));
 	for (rnp_root = rnp; 1; rnp_root = rnp_root->parent) {
 		if (rnp_root != rnp)
@@ -1757,8 +1751,6 @@ static bool __note_gp_changes(struct rcu_state *rsp, struct rcu_node *rnp,
 	if (rcu_seq_completed_gp(rdp->gp_seq, rnp->gp_seq) ||
 	    unlikely(READ_ONCE(rdp->gpwrap))) {
 		ret = rcu_advance_cbs(rsp, rnp, rdp); /* Advance callbacks. */
-		/* Remember that we saw this grace-period completion. */
-		rdp->completed = rnp->completed;
 		trace_rcu_grace_period(rsp->name, rdp->gp_seq, TPS("cpuend"));
 	} else {
 		ret = rcu_accelerate_cbs(rsp, rnp, rdp); /* Recent callbacks. */
@@ -1772,7 +1764,6 @@ static bool __note_gp_changes(struct rcu_state *rsp, struct rcu_node *rnp,
 		 * set up to detect a quiescent state, otherwise don't
 		 * go looking for one.
 		 */
-		rdp->gpnum = rnp->gpnum;
 		trace_rcu_grace_period(rsp->name, rdp->gp_seq, TPS("cpustart"));
 		need_gp = !!(rnp->qsmask & rdp->grpmask);
 		rdp->cpu_no_qs.b.norm = need_gp;
@@ -1843,13 +1834,8 @@ static bool rcu_gp_init(struct rcu_state *rsp)
 
 	/* Advance to a new grace period and initialize state. */
 	record_gp_stall_check_time(rsp);
-	/* Record GP times before starting GP, hence smp_store_release(). */
-	WARN_ON_ONCE(rsp->gpnum << RCU_SEQ_CTR_SHIFT != rsp->gp_seq);
-	smp_store_release(&rsp->gpnum, rsp->gpnum + 1);
-	smp_mb(); /* Pairs with barriers in stall-warning code. */
+	/* Record GP times before starting GP, hence rcu_seq_start(). */
 	rcu_seq_start(&rsp->gp_seq);
-	if (WARN_ON_ONCE(((rnp->completed << RCU_SEQ_CTR_SHIFT) >> RCU_SEQ_CTR_SHIFT) != rcu_seq_ctr(rnp->gp_seq))) /* Catch any ->completed/->gp_seq mismatches. */
-		pr_info("%s ->completed: %#lx (%#lx) ->gp_seq %#lx (%#lx)\n", __func__, rnp->completed, (rnp->completed << RCU_SEQ_CTR_SHIFT) >> RCU_SEQ_CTR_SHIFT, rnp->gp_seq, rcu_seq_ctr(rnp->gp_seq));
 	trace_rcu_grace_period(rsp->name, rsp->gp_seq, TPS("start"));
 	raw_spin_unlock_irq_rcu_node(rnp);
 
@@ -1920,9 +1906,6 @@ static bool rcu_gp_init(struct rcu_state *rsp)
 		rdp = this_cpu_ptr(rsp->rda);
 		rcu_preempt_check_blocked_tasks(rnp);
 		rnp->qsmask = rnp->qsmaskinit;
-		WRITE_ONCE(rnp->gpnum, rsp->gpnum);
-		if (WARN_ON_ONCE(rnp->completed != rsp->completed))
-			WRITE_ONCE(rnp->completed, rsp->completed);
 		WRITE_ONCE(rnp->gp_seq, rsp->gp_seq);
 		if (rnp == rdp->mynode)
 			(void)__note_gp_changes(rsp, rnp, rdp);
@@ -2012,13 +1995,13 @@ static void rcu_gp_cleanup(struct rcu_state *rsp)
 	raw_spin_unlock_irq_rcu_node(rnp);
 
 	/*
-	 * Propagate new ->completed value to rcu_node structures so
-	 * that other CPUs don't have to wait until the start of the next
-	 * grace period to process their callbacks.  This also avoids
-	 * some nasty RCU grace-period initialization races by forcing
-	 * the end of the current grace period to be completely recorded in
-	 * all of the rcu_node structures before the beginning of the next
-	 * grace period is recorded in any of the rcu_node structures.
+	 * Propagate new ->gp_seq value to rcu_node structures so that
+	 * other CPUs don't have to wait until the start of the next grace
+	 * period to process their callbacks.  This also avoids some nasty
+	 * RCU grace-period initialization races by forcing the end of
+	 * the current grace period to be completely recorded in all of
+	 * the rcu_node structures before the beginning of the next grace
+	 * period is recorded in any of the rcu_node structures.
 	 */
 	new_gp_seq = rsp->gp_seq;
 	rcu_seq_end(&new_gp_seq);
@@ -2027,7 +2010,6 @@ static void rcu_gp_cleanup(struct rcu_state *rsp)
 		if (WARN_ON_ONCE(rcu_preempt_blocked_readers_cgp(rnp)))
 			dump_blkd_tasks(rnp, 10);
 		WARN_ON_ONCE(rnp->qsmask);
-		WRITE_ONCE(rnp->completed, rsp->gpnum);
 		WRITE_ONCE(rnp->gp_seq, new_gp_seq);
 		rdp = this_cpu_ptr(rsp->rda);
 		if (rnp == rdp->mynode)
@@ -2045,7 +2027,6 @@ static void rcu_gp_cleanup(struct rcu_state *rsp)
 	raw_spin_lock_irq_rcu_node(rnp); /* GP before rsp->gp_seq update. */
 
 	/* Declare grace period done. */
-	WRITE_ONCE(rsp->completed, rsp->gpnum);
 	rcu_seq_end(&rsp->gp_seq);
 	trace_rcu_grace_period(rsp->name, rsp->gp_seq, TPS("end"));
 	rsp->gp_state = RCU_GP_IDLE;
@@ -3494,9 +3475,9 @@ rcu_boot_init_percpu_data(int cpu, struct rcu_state *rsp)
 
 /*
  * Initialize a CPU's per-CPU RCU data.  Note that only one online or
- * offline event can be happening at a given time.  Note also that we
- * can accept some slop in the rsp->completed access due to the fact
- * that this CPU cannot possibly have any RCU callbacks in flight yet.
+ * offline event can be happening at a given time.  Note also that we can
+ * accept some slop in the rsp->gp_seq access due to the fact that this
+ * CPU cannot possibly have any RCU callbacks in flight yet.
  */
 static void
 rcu_init_percpu_data(int cpu, struct rcu_state *rsp)
@@ -3525,8 +3506,6 @@ rcu_init_percpu_data(int cpu, struct rcu_state *rsp)
 	rnp = rdp->mynode;
 	raw_spin_lock_rcu_node(rnp);		/* irqs already disabled. */
 	rdp->beenonline = true;	 /* We have now been online. */
-	rdp->gpnum = rnp->completed; /* Make CPU later note any new GP. */
-	rdp->completed = rnp->completed;
 	rdp->gp_seq = rnp->gp_seq;
 	rdp->gp_seq_needed = rnp->gp_seq;
 	rdp->cpu_no_qs.b.norm = true;
@@ -3906,8 +3885,6 @@ static void __init rcu_init_one(struct rcu_state *rsp)
 			raw_spin_lock_init(&rnp->fqslock);
 			lockdep_set_class_and_name(&rnp->fqslock,
 						   &rcu_fqs_class[i], fqs[i]);
-			rnp->gpnum = rsp->gpnum;
-			rnp->completed = rsp->completed;
 			rnp->gp_seq = rsp->gp_seq;
 			rnp->gp_seq_needed = rsp->gp_seq;
 			rnp->completedqs = rsp->gp_seq;
diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index 9329c1ff695f..3def94fc9c74 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -81,12 +81,6 @@ struct rcu_node {
 	raw_spinlock_t __private lock;	/* Root rcu_node's lock protects */
 					/*  some rcu_state fields as well as */
 					/*  following. */
-	unsigned long gpnum;	/* Current grace period for this node. */
-				/*  This will either be equal to or one */
-				/*  behind the root rcu_node's gpnum. */
-	unsigned long completed; /* Last GP completed for this node. */
-				/*  This will either be equal to or one */
-				/*  behind the root rcu_node's gpnum. */
 	unsigned long gp_seq;	/* Track rsp->rcu_gp_seq. */
 	unsigned long gp_seq_needed; /* Track rsp->rcu_gp_seq_needed. */
 	unsigned long completedqs; /* All QSes done for this node. */
@@ -192,10 +186,6 @@ union rcu_noqs {
 /* Per-CPU data for read-copy update. */
 struct rcu_data {
 	/* 1) quiescent-state and grace-period handling : */
-	unsigned long	completed;	/* Track rsp->completed gp number */
-					/*  in order to detect GP end. */
-	unsigned long	gpnum;		/* Highest gp number that this CPU */
-					/*  is aware of having started. */
 	unsigned long	gp_seq;		/* Track rsp->rcu_gp_seq counter. */
 	unsigned long	gp_seq_needed;	/* Track rsp->rcu_gp_seq_needed ctr. */
 	unsigned long	rcu_qs_ctr_snap;/* Snapshot of rcu_qs_ctr to check */
@@ -203,7 +193,7 @@ struct rcu_data {
 	union rcu_noqs	cpu_no_qs;	/* No QSes yet for this CPU. */
 	bool		core_needs_qs;	/* Core waits for quiesc state. */
 	bool		beenonline;	/* CPU online at least once. */
-	bool		gpwrap;		/* Possible gpnum/completed wrap. */
+	bool		gpwrap;		/* Possible ->gp_seq wrap. */
 	struct rcu_node *mynode;	/* This CPU's leaf of hierarchy */
 	unsigned long grpmask;		/* Mask to apply to leaf qsmask. */
 	unsigned long	ticks_this_gp;	/* The number of scheduling-clock */
@@ -328,8 +318,6 @@ struct rcu_state {
 
 	u8	boost ____cacheline_internodealigned_in_smp;
 						/* Subject to priority boost. */
-	unsigned long gpnum;			/* Current gp number. */
-	unsigned long completed;		/* # of last completed gp. */
 	unsigned long gp_seq;			/* Grace-period sequence #. */
 	struct task_struct *gp_kthread;		/* Task for grace periods. */
 	struct swait_queue_head gp_wq;		/* Where GP task waits. */
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index c8a2c7760121..3a6e04103de1 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -690,7 +690,7 @@ static int rcu_print_task_exp_stall(struct rcu_node *rnp)
  * Check that the list of blocked tasks for the newly completed grace
  * period is in fact empty.  It is a serious bug to complete a grace
  * period that still has RCU readers blocked!  This function must be
- * invoked -before- updating this rnp's ->gpnum, and the rnp's ->lock
+ * invoked -before- updating this rnp's ->gp_seq, and the rnp's ->lock
  * must be held by the caller.
  *
  * Also, if there are blocked tasks on the list, they automatically
-- 
2.17.1

[PATCH tip/core/rcu 28/40] rcu: Make simple callback acceleration refer to rdp->gp_seq_needed

[Paul E. McKenney]

Now that the rcu_data structure contains ->gp_seq_needed, create an
rcu_accelerate_cbs_unlocked() helper function that locklessly checks to
see if new callbacks' required grace period has already been requested.
If so, update the callback list locally and again locklessly.  (Though
interrupts must be and are disabled to avoid racing with conflicting
updates in interrupt handlers.)

Otherwise, call rcu_accelerate_cbs() as before.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 kernel/rcu/tree.c | 51 +++++++++++++++++++++++++++++------------------
 1 file changed, 32 insertions(+), 19 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 2b66da93cf5b..fe986771c5cf 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1701,6 +1701,34 @@ static bool rcu_accelerate_cbs(struct rcu_state *rsp, struct rcu_node *rnp,
 	return ret;
 }
 
+/*
+ * Similar to rcu_accelerate_cbs(), but does not require that the leaf
+ * rcu_node structure's ->lock be held.  It consults the cached value
+ * of ->gp_seq_needed in the rcu_data structure, and if that indicates
+ * that a new grace-period request be made, invokes rcu_accelerate_cbs()
+ * while holding the leaf rcu_node structure's ->lock.
+ */
+static void rcu_accelerate_cbs_unlocked(struct rcu_state *rsp,
+					struct rcu_node *rnp,
+					struct rcu_data *rdp)
+{
+	unsigned long c;
+	bool needwake;
+
+	lockdep_assert_irqs_disabled();
+	c = rcu_seq_snap(&rsp->gp_seq);
+	if (!rdp->gpwrap && ULONG_CMP_GE(rdp->gp_seq_needed, c)) {
+		/* Old request still live, so mark recent callbacks. */
+		(void)rcu_segcblist_accelerate(&rdp->cblist, c);
+		return;
+	}
+	raw_spin_lock_rcu_node(rnp); /* irqs already disabled. */
+	needwake = rcu_accelerate_cbs(rsp, rnp, rdp);
+	raw_spin_unlock_rcu_node(rnp); /* irqs remain disabled. */
+	if (needwake)
+		rcu_gp_kthread_wake(rsp);
+}
+
 /*
  * Move any callbacks whose grace period has completed to the
  * RCU_DONE_TAIL sublist, then compact the remaining sublists and
@@ -2737,7 +2765,6 @@ static void
 __rcu_process_callbacks(struct rcu_state *rsp)
 {
 	unsigned long flags;
-	bool needwake;
 	struct rcu_data *rdp = raw_cpu_ptr(rsp->rda);
 	struct rcu_node *rnp = rdp->mynode;
 
@@ -2750,15 +2777,9 @@ __rcu_process_callbacks(struct rcu_state *rsp)
 	if (!rcu_gp_in_progress(rsp) &&
 	    rcu_segcblist_is_enabled(&rdp->cblist)) {
 		local_irq_save(flags);
-		if (rcu_segcblist_restempty(&rdp->cblist, RCU_NEXT_READY_TAIL)) {
-			local_irq_restore(flags);
-		} else {
-			raw_spin_lock_rcu_node(rnp); /* irqs disabled. */
-			needwake = rcu_accelerate_cbs(rsp, rnp, rdp);
-			raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
-			if (needwake)
-				rcu_gp_kthread_wake(rsp);
-		}
+		if (!rcu_segcblist_restempty(&rdp->cblist, RCU_NEXT_READY_TAIL))
+			rcu_accelerate_cbs_unlocked(rsp, rnp, rdp);
+		local_irq_restore(flags);
 	}
 
 	rcu_check_gp_start_stall(rsp, rnp, rdp);
@@ -2816,8 +2837,6 @@ static void invoke_rcu_core(void)
 static void __call_rcu_core(struct rcu_state *rsp, struct rcu_data *rdp,
 			    struct rcu_head *head, unsigned long flags)
 {
-	bool needwake;
-
 	/*
 	 * If called from an extended quiescent state, invoke the RCU
 	 * core in order to force a re-evaluation of RCU's idleness.
@@ -2844,13 +2863,7 @@ static void __call_rcu_core(struct rcu_state *rsp, struct rcu_data *rdp,
 
 		/* Start a new grace period if one not already started. */
 		if (!rcu_gp_in_progress(rsp)) {
-			struct rcu_node *rnp = rdp->mynode;
-
-			raw_spin_lock_rcu_node(rnp);
-			needwake = rcu_accelerate_cbs(rsp, rnp, rdp);
-			raw_spin_unlock_rcu_node(rnp);
-			if (needwake)
-				rcu_gp_kthread_wake(rsp);
+			rcu_accelerate_cbs_unlocked(rsp, rdp->mynode, rdp);
 		} else {
 			/* Give the grace period a kick. */
 			rdp->blimit = LONG_MAX;
-- 
2.17.1

[PATCH tip/core/rcu 29/40] doc: Update data-structure documentation for ->gp_seq

[Paul E. McKenney]

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 .../Data-Structures/Data-Structures.html      | 118 ++++++++++--------
 1 file changed, 63 insertions(+), 55 deletions(-)

diff --git a/Documentation/RCU/Design/Data-Structures/Data-Structures.html b/Documentation/RCU/Design/Data-Structures/Data-Structures.html
index 6c06e10bd04b..f5120a00f511 100644
--- a/Documentation/RCU/Design/Data-Structures/Data-Structures.html
+++ b/Documentation/RCU/Design/Data-Structures/Data-Structures.html
@@ -380,31 +380,26 @@ and therefore need no protection.
 as follows:
 
 <pre>
-  1   unsigned long gpnum;
-  2   unsigned long completed;
+  1   unsigned long gp_seq;
 </pre>
 
 <p>RCU grace periods are numbered, and
-the <tt>-&gt;gpnum</tt> field contains the number of the grace
-period that started most recently.
-The <tt>-&gt;completed</tt> field contains the number of the
-grace period that completed most recently.
-If the two fields are equal, the RCU grace period that most recently
-started has already completed, and therefore the corresponding
-flavor of RCU is idle.
-If <tt>-&gt;gpnum</tt> is one greater than <tt>-&gt;completed</tt>,
-then <tt>-&gt;gpnum</tt> gives the number of the current RCU
-grace period, which has not yet completed.
-Any other combination of values indicates that something is broken.
-These two fields are protected by the root <tt>rcu_node</tt>'s
+the <tt>-&gt;gp_seq</tt> field contains the current grace-period
+sequence number.
+The bottom two bits are the state of the current grace period,
+which can be zero for not yet started or one for in progress.
+In other words, if the bottom two bits of <tt>-&gt;gp_seq</tt> are
+zero, the corresponding flavor of RCU is idle.
+Any other value in the bottom two bits indicates that something is broken.
+This field is protected by the root <tt>rcu_node</tt> structure's
 <tt>-&gt;lock</tt> field.
 
-</p><p>There are <tt>-&gt;gpnum</tt> and <tt>-&gt;completed</tt> fields
+</p><p>There are <tt>-&gt;gp_seq</tt> fields
 in the <tt>rcu_node</tt> and <tt>rcu_data</tt> structures
 as well.
 The fields in the <tt>rcu_state</tt> structure represent the
-most current values, and those of the other structures are compared
-in order to detect the start of a new grace period in a distributed
+most current value, and those of the other structures are compared
+in order to detect the beginnings and ends of grace periods in a distributed
 fashion.
 The values flow from <tt>rcu_state</tt> to <tt>rcu_node</tt>
 (down the tree from the root to the leaves) to <tt>rcu_data</tt>.
@@ -512,27 +507,47 @@ than to be heisenbugged out of existence.
 as follows:
 
 <pre>
-  1   unsigned long gpnum;
-  2   unsigned long completed;
+  1   unsigned long gp_seq;
+  2   unsigned long gp_seq_needed;
 </pre>
 
-<p>These fields are the counterparts of the fields of the same name in
-the <tt>rcu_state</tt> structure.
-They each may lag up to one behind their <tt>rcu_state</tt>
-counterparts.
-If a given <tt>rcu_node</tt> structure's <tt>-&gt;gpnum</tt> and
-<tt>-&gt;complete</tt> fields are equal, then this <tt>rcu_node</tt>
+<p>The <tt>rcu_node</tt> structures' <tt>-&gt;gp_seq</tt> fields are
+the counterparts of the field of the same name in the <tt>rcu_state</tt>
+structure.
+They each may lag up to one step behind their <tt>rcu_state</tt>
+counterpart.
+If the bottom two bits of a given <tt>rcu_node</tt> structure's
+<tt>-&gt;gp_seq</tt> field is zero, then this <tt>rcu_node</tt>
 structure believes that RCU is idle.
-Otherwise, as with the <tt>rcu_state</tt> structure,
-the <tt>-&gt;gpnum</tt> field will be one greater than the
-<tt>-&gt;complete</tt> fields, with <tt>-&gt;gpnum</tt>
-indicating which grace period this <tt>rcu_node</tt> believes
-is still being waited for.
+</p><p>The <tt>&gt;gp_seq</tt> field of each <tt>rcu_node</tt>
+structure is updated at the beginning and the end
+of each grace period.
+
+<p>The <tt>-&gt;gp_seq_needed</tt> fields record the
+furthest-in-the-future grace period request seen by the corresponding
+<tt>rcu_node</tt> structure.  The request is considered fulfilled when
+the value of the <tt>-&gt;gp_seq</tt> field equals or exceeds that of
+the <tt>-&gt;gp_seq_needed</tt> field.
 
-</p><p>The <tt>&gt;gpnum</tt> field of each <tt>rcu_node</tt>
-structure is updated at the beginning
-of each grace period, and the <tt>-&gt;completed</tt> fields are
-updated at the end of each grace period.
+<table>
+<tr><th>&nbsp;</th></tr>
+<tr><th align="left">Quick Quiz:</th></tr>
+<tr><td>
+	Suppose that this <tt>rcu_node</tt> structure doesn't see
+	a request for a very long time.
+	Won't wrapping of the <tt>-&gt;gp_seq</tt> field cause
+	problems?
+</td></tr>
+<tr><th align="left">Answer:</th></tr>
+<tr><td bgcolor="#ffffff"><font color="ffffff">
+	No, because if the <tt>-&gt;gp_seq_needed</tt> field lags behind the
+	<tt>-&gt;gp_seq</tt> field, the <tt>-&gt;gp_seq_needed</tt> field
+	will be updated at the end of the grace period.
+	Modulo-arithmetic comparisons therefore will always get the
+	correct answer, even with wrapping.
+</font></td></tr>
+<tr><td>&nbsp;</td></tr>
+</table>
 
 <h5>Quiescent-State Tracking</h5>
 
@@ -626,9 +641,8 @@ normal and expedited grace periods, respectively.
 	</ol>
 
 	<p><font color="ffffff">So the locking is absolutely required in
-	order to coordinate
-	clearing of the bits with the grace-period numbers in
-	<tt>-&gt;gpnum</tt> and <tt>-&gt;completed</tt>.
+	order to coordinate clearing of the bits with updating of the
+	grace-period sequence number in <tt>-&gt;gp_seq</tt>.
 </font></td></tr>
 <tr><td>&nbsp;</td></tr>
 </table>
@@ -1038,15 +1052,15 @@ out any <tt>rcu_data</tt> structure for which this flag is not set.
 as follows:
 
 <pre>
-  1   unsigned long completed;
-  2   unsigned long gpnum;
+  1   unsigned long gp_seq;
+  2   unsigned long gp_seq_needed;
   3   bool cpu_no_qs;
   4   bool core_needs_qs;
   5   bool gpwrap;
   6   unsigned long rcu_qs_ctr_snap;
 </pre>
 
-<p>The <tt>completed</tt> and <tt>gpnum</tt>
+<p>The <tt>-&gt;gp_seq</tt> and <tt>-&gt;gp_seq_needed</tt>
 fields are the counterparts of the fields of the same name
 in the <tt>rcu_state</tt> and <tt>rcu_node</tt> structures.
 They may each lag up to one behind their <tt>rcu_node</tt>
@@ -1054,15 +1068,9 @@ counterparts, but in <tt>CONFIG_NO_HZ_IDLE</tt> and
 <tt>CONFIG_NO_HZ_FULL</tt> kernels can lag
 arbitrarily far behind for CPUs in dyntick-idle mode (but these counters
 will catch up upon exit from dyntick-idle mode).
-If a given <tt>rcu_data</tt> structure's <tt>-&gt;gpnum</tt> and
-<tt>-&gt;complete</tt> fields are equal, then this <tt>rcu_data</tt>
+If the lower two bits of a given <tt>rcu_data</tt> structure's
+<tt>-&gt;gp_seq</tt> are zero, then this <tt>rcu_data</tt>
 structure believes that RCU is idle.
-Otherwise, as with the <tt>rcu_state</tt> and <tt>rcu_node</tt>
-structure,
-the <tt>-&gt;gpnum</tt> field will be one greater than the
-<tt>-&gt;complete</tt> fields, with <tt>-&gt;gpnum</tt>
-indicating which grace period this <tt>rcu_data</tt> believes
-is still being waited for.
 
 <table>
 <tr><th>&nbsp;</th></tr>
@@ -1070,13 +1078,13 @@ is still being waited for.
 <tr><td>
 	All this replication of the grace period numbers can only cause
 	massive confusion.
-	Why not just keep a global pair of counters and be done with it???
+	Why not just keep a global sequence number and be done with it???
 </td></tr>
 <tr><th align="left">Answer:</th></tr>
 <tr><td bgcolor="#ffffff"><font color="ffffff">
-	Because if there was only a single global pair of grace-period
+	Because if there was only a single global sequence
 	numbers, there would need to be a single global lock to allow
-	safely accessing and updating them.
+	safely accessing and updating it.
 	And if we are not going to have a single global lock, we need
 	to carefully manage the numbers on a per-node basis.
 	Recall from the answer to a previous Quick Quiz that the consequences
@@ -1091,8 +1099,8 @@ CPU has not yet passed through a quiescent state,
 while the <tt>-&gt;core_needs_qs</tt> flag indicates that the
 RCU core needs a quiescent state from the corresponding CPU.
 The <tt>-&gt;gpwrap</tt> field indicates that the corresponding
-CPU has remained idle for so long that the <tt>completed</tt>
-and <tt>gpnum</tt> counters are in danger of overflow, which
+CPU has remained idle for so long that the
+<tt>gp_seq</tt> counter is in danger of overflow, which
 will cause the CPU to disregard the values of its counters on
 its next exit from idle.
 Finally, the <tt>rcu_qs_ctr_snap</tt> field is used to detect
@@ -1130,10 +1138,10 @@ The CPU advances the callbacks in its <tt>rcu_data</tt> structure
 whenever it notices that another RCU grace period has completed.
 The CPU detects the completion of an RCU grace period by noticing
 that the value of its <tt>rcu_data</tt> structure's
-<tt>-&gt;completed</tt> field differs from that of its leaf
+<tt>-&gt;gp_seq</tt> field differs from that of its leaf
 <tt>rcu_node</tt> structure.
 Recall that each <tt>rcu_node</tt> structure's
-<tt>-&gt;completed</tt> field is updated at the end of each
+<tt>-&gt;gp_seq</tt> field is updated at the beginnings and ends of each
 grace period.
 
 <p>
-- 
2.17.1

[PATCH tip/core/rcu 30/40] doc: Update memory-ordering documentation for ->gp-seq

[Paul E. McKenney]

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 .../Tree-RCU-Memory-Ordering.html             |  22 +-
 .../Memory-Ordering/TreeRCU-gp-cleanup.svg    | 123 ++++-----
 .../Memory-Ordering/TreeRCU-gp-init-1.svg     |  16 +-
 .../Memory-Ordering/TreeRCU-gp-init-3.svg     |  56 +++--
 .../RCU/Design/Memory-Ordering/TreeRCU-gp.svg | 237 +++++++++---------
 .../RCU/Design/Memory-Ordering/TreeRCU-qs.svg |  12 +-
 6 files changed, 243 insertions(+), 223 deletions(-)

diff --git a/Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Memory-Ordering.html b/Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Memory-Ordering.html
index 8651b0b4fd79..a346ce0116eb 100644
--- a/Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Memory-Ordering.html
+++ b/Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Memory-Ordering.html
@@ -357,7 +357,7 @@ parts, starting in this section with the various phases of
 grace-period initialization.
 
 <p>The first ordering-related grace-period initialization action is to
-increment the <tt>rcu_state</tt> structure's <tt>-&gt;gpnum</tt>
+advance the <tt>rcu_state</tt> structure's <tt>-&gt;gp_seq</tt>
 grace-period-number counter, as shown below:
 
 </p><p><img src="TreeRCU-gp-init-1.svg" alt="TreeRCU-gp-init-1.svg" width="75%">
@@ -388,7 +388,7 @@ its last CPU and if the next <tt>rcu_node</tt> structure has no online CPUs).
 
 <p>The final <tt>rcu_gp_init()</tt> pass through the <tt>rcu_node</tt>
 tree traverses breadth-first, setting each <tt>rcu_node</tt> structure's
-<tt>-&gt;gpnum</tt> field to the newly incremented value from the
+<tt>-&gt;gp_seq</tt> field to the newly advanced value from the
 <tt>rcu_state</tt> structure, as shown in the following diagram.
 
 </p><p><img src="TreeRCU-gp-init-3.svg" alt="TreeRCU-gp-init-1.svg" width="75%">
@@ -398,9 +398,9 @@ tree traverses breadth-first, setting each <tt>rcu_node</tt> structure's
 to notice that a new grace period has started, as described in the next
 section.
 But because the grace-period kthread started the grace period at the
-root (with the increment of the <tt>rcu_state</tt> structure's
-<tt>-&gt;gpnum</tt> field) before setting each leaf <tt>rcu_node</tt>
-structure's <tt>-&gt;gpnum</tt> field, each CPU's observation of
+root (with the advancing of the <tt>rcu_state</tt> structure's
+<tt>-&gt;gp_seq</tt> field) before setting each leaf <tt>rcu_node</tt>
+structure's <tt>-&gt;gp_seq</tt> field, each CPU's observation of
 the start of the grace period will happen after the actual start
 of the grace period.
 
@@ -466,7 +466,7 @@ section that the grace period must wait on.
 <tr><td>
 	But a RCU read-side critical section might have started
 	after the beginning of the grace period
-	(the <tt>-&gt;gpnum++</tt> from earlier), so why should
+	(the advancing of <tt>-&gt;gp_seq</tt> from earlier), so why should
 	the grace period wait on such a critical section?
 </td></tr>
 <tr><th align="left">Answer:</th></tr>
@@ -609,10 +609,8 @@ states outstanding from other CPUs.
 <h4><a name="Grace-Period Cleanup">Grace-Period Cleanup</a></h4>
 
 <p>Grace-period cleanup first scans the <tt>rcu_node</tt> tree
-breadth-first setting all the <tt>-&gt;completed</tt> fields equal
-to the number of the newly completed grace period, then it sets
-the <tt>rcu_state</tt> structure's <tt>-&gt;completed</tt> field,
-again to the number of the newly completed grace period.
+breadth-first advancing all the <tt>-&gt;gp_seq</tt> fields, then it
+advances the <tt>rcu_state</tt> structure's <tt>-&gt;gp_seq</tt> field.
 The ordering effects are shown below:
 
 </p><p><img src="TreeRCU-gp-cleanup.svg" alt="TreeRCU-gp-cleanup.svg" width="75%">
@@ -634,7 +632,7 @@ grace-period cleanup is complete, the next grace period can begin.
 	CPU has reported its quiescent state, but it may be some
 	milliseconds before RCU becomes aware of this.
 	The latest reasonable candidate is once the <tt>rcu_state</tt>
-	structure's <tt>-&gt;completed</tt> field has been updated,
+	structure's <tt>-&gt;gp_seq</tt> field has been updated,
 	but it is quite possible that some CPUs have already completed
 	phase two of their updates by that time.
 	In short, if you are going to work with RCU, you need to
@@ -647,7 +645,7 @@ grace-period cleanup is complete, the next grace period can begin.
 <h4><a name="Callback Invocation">Callback Invocation</a></h4>
 
 <p>Once a given CPU's leaf <tt>rcu_node</tt> structure's
-<tt>-&gt;completed</tt> field has been updated, that CPU can begin
+<tt>-&gt;gp_seq</tt> field has been updated, that CPU can begin
 invoking its RCU callbacks that were waiting for this grace period
 to end.
 These callbacks are identified by <tt>rcu_advance_cbs()</tt>,
diff --git a/Documentation/RCU/Design/Memory-Ordering/TreeRCU-gp-cleanup.svg b/Documentation/RCU/Design/Memory-Ordering/TreeRCU-gp-cleanup.svg
index 754f426b297a..bf84fbab27ee 100644
--- a/Documentation/RCU/Design/Memory-Ordering/TreeRCU-gp-cleanup.svg
+++ b/Documentation/RCU/Design/Memory-Ordering/TreeRCU-gp-cleanup.svg
@@ -384,11 +384,11 @@
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-       style="font-size:192px;font-style:normal;font-weight:bold;text-anchor:start;fill:#000000;font-family:Courier">-&gt;completed = -&gt;gpnum</text>
+       style="font-size:192px;font-style:normal;font-weight:bold;text-anchor:start;fill:#000000;font-family:Courier"><tspan
+         style="font-size:172.87567139px"
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@@ -502,13 +504,15 @@
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+       id="text202-36-7"
+       style="font-size:192px;font-style:normal;font-weight:bold;text-anchor:start;fill:#000000;stroke-width:0.025in;font-family:Courier"><tspan
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   </g>
   <g
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@@ -547,15 +551,6 @@
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@@ -566,15 +561,6 @@
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@@ -627,7 +604,18 @@
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   <g
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@@ -887,13 +886,15 @@
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+       style="font-size:192px;font-style:normal;font-weight:bold;text-anchor:start;fill:#000000;stroke-width:0.025in;font-family:Courier"><tspan
+         style="font-size:172.87567139px"
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@@ -972,13 +973,15 @@
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+       id="text202-36-2"
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diff --git a/Documentation/RCU/Design/Memory-Ordering/TreeRCU-gp-init-1.svg b/Documentation/RCU/Design/Memory-Ordering/TreeRCU-gp-init-1.svg
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+       style="font-size:192px;font-style:normal;font-weight:bold;text-anchor:start;fill:#000000;stroke-width:0.025in;font-family:Courier">-&gt;gp_seq = rsp-&gt;gp_seq</text>
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index b13b7b01bb3a..acd73c7ad0f4 100644
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+       font-size="192"
+       id="text202-36-2"
+       style="font-size:192px;font-style:normal;font-weight:bold;text-anchor:start;fill:#000000;stroke-width:0.025in;font-family:Courier"><tspan
+         style="font-size:172.87567139px"
+         id="tspan3166-7">rcu_seq_end(&amp;rnp-&gt;gp_seq)</tspan></text>
+    <text
+       xml:space="preserve"
+       x="10023.457"
+       y="70885.234"
+       font-style="normal"
+       font-weight="bold"
+       font-size="192"
+       id="text202-36-0"
+       style="font-size:192px;font-style:normal;font-weight:bold;text-anchor:start;fill:#000000;stroke-width:0.025in;font-family:Courier"><tspan
+         style="font-size:172.87567139px"
+         id="tspan3166-9">rcu_seq_end(&amp;rnp-&gt;gp_seq)</tspan></text>
+    <text
+       xml:space="preserve"
+       x="5023.3389"
+       y="74209.773"
+       font-style="normal"
+       font-weight="bold"
+       font-size="192"
+       id="text202-36-36"
+       style="font-size:192px;font-style:normal;font-weight:bold;text-anchor:start;fill:#000000;stroke-width:0.025in;font-family:Courier"><tspan
+         style="font-size:172.87567139px"
+         id="tspan3166-0">rcu_seq_end(&amp;rsp-&gt;gp_seq)</tspan></text>
+    <text
+       xml:space="preserve"
+       x="6562.5884"
+       y="34870.727"
+       font-style="normal"
+       font-weight="bold"
+       font-size="192"
+       id="text202-3"
+       style="font-size:192px;font-style:normal;font-weight:bold;text-anchor:start;fill:#000000;stroke-width:0.025in;font-family:Courier">-&gt;gp_seq = rsp-&gt;gp_seq</text>
   </g>
 </svg>
diff --git a/Documentation/RCU/Design/Memory-Ordering/TreeRCU-qs.svg b/Documentation/RCU/Design/Memory-Ordering/TreeRCU-qs.svg
index de3992f4cbe1..149bec2a4493 100644
--- a/Documentation/RCU/Design/Memory-Ordering/TreeRCU-qs.svg
+++ b/Documentation/RCU/Design/Memory-Ordering/TreeRCU-qs.svg
@@ -300,13 +300,13 @@
      inkscape:window-height="1144"
      id="namedview208"
      showgrid="true"
-     inkscape:zoom="0.70710678"
-     inkscape:cx="616.47598"
-     inkscape:cy="595.41964"
-     inkscape:window-x="813"
+     inkscape:zoom="0.96484375"
+     inkscape:cx="507.0191"
+     inkscape:cy="885.62207"
+     inkscape:window-x="47"
      inkscape:window-y="28"
      inkscape:window-maximized="0"
-     inkscape:current-layer="g4405"
+     inkscape:current-layer="g3115"
      fit-margin-top="5"
      fit-margin-right="5"
      fit-margin-left="5"
@@ -710,7 +710,7 @@
          font-weight="bold"
          font-size="192"
          id="text202-6-6"
-         style="font-size:192px;font-style:normal;font-weight:bold;text-anchor:start;fill:#000000;stroke-width:0.025in;font-family:Courier">rdp-&gt;gpnum</text>
+         style="font-size:192px;font-style:normal;font-weight:bold;text-anchor:start;fill:#000000;stroke-width:0.025in;font-family:Courier">rdp-&gt;gp_seq</text>
       <text
          xml:space="preserve"
          x="5035.4155"
-- 
2.17.1

[PATCH tip/core/rcu 31/40] doc: Update RCU CPU stall-warning documentation

[Paul E. McKenney]

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 Documentation/RCU/stallwarn.txt | 24 ++++++++++++------------
 1 file changed, 12 insertions(+), 12 deletions(-)

diff --git a/Documentation/RCU/stallwarn.txt b/Documentation/RCU/stallwarn.txt
index 4259f95c3261..f99cf11b314b 100644
--- a/Documentation/RCU/stallwarn.txt
+++ b/Documentation/RCU/stallwarn.txt
@@ -172,7 +172,7 @@ it will print a message similar to the following:
 	INFO: rcu_sched detected stalls on CPUs/tasks:
 	2-...: (3 GPs behind) idle=06c/0/0 softirq=1453/1455 fqs=0
 	16-...: (0 ticks this GP) idle=81c/0/0 softirq=764/764 fqs=0
-	(detected by 32, t=2603 jiffies, g=7073, c=7072, q=625)
+	(detected by 32, t=2603 jiffies, g=7075, q=625)
 
 This message indicates that CPU 32 detected that CPUs 2 and 16 were both
 causing stalls, and that the stall was affecting RCU-sched.  This message
@@ -215,11 +215,10 @@ CPU since the last time that this CPU noted the beginning of a grace
 period.
 
 The "detected by" line indicates which CPU detected the stall (in this
-case, CPU 32), how many jiffies have elapsed since the start of the
-grace period (in this case 2603), the number of the last grace period
-to start and to complete (7073 and 7072, respectively), and an estimate
-of the total number of RCU callbacks queued across all CPUs (625 in
-this case).
+case, CPU 32), how many jiffies have elapsed since the start of the grace
+period (in this case 2603), the grace-period sequence number (7075), and
+an estimate of the total number of RCU callbacks queued across all CPUs
+(625 in this case).
 
 In kernels with CONFIG_RCU_FAST_NO_HZ, more information is printed
 for each CPU:
@@ -266,15 +265,16 @@ If the relevant grace-period kthread has been unable to run prior to
 the stall warning, as was the case in the "All QSes seen" line above,
 the following additional line is printed:
 
-	kthread starved for 23807 jiffies! g7073 c7072 f0x0 RCU_GP_WAIT_FQS(3) ->state=0x1
+	kthread starved for 23807 jiffies! g7075 f0x0 RCU_GP_WAIT_FQS(3) ->state=0x1 ->cpu=5
 
 Starving the grace-period kthreads of CPU time can of course result
 in RCU CPU stall warnings even when all CPUs and tasks have passed
-through the required quiescent states.  The "g" and "c" numbers flag the
-number of the last grace period started and completed, respectively,
-the "f" precedes the ->gp_flags command to the grace-period kthread,
-the "RCU_GP_WAIT_FQS" indicates that the kthread is waiting for a short
-timeout, and the "state" precedes value of the task_struct ->state field.
+through the required quiescent states.  The "g" number shows the current
+grace-period sequence number, the "f" precedes the ->gp_flags command
+to the grace-period kthread, the "RCU_GP_WAIT_FQS" indicates that the
+kthread is waiting for a short timeout, the "state" precedes value of the
+task_struct ->state field, and the "cpu" indicates that the grace-period
+kthread last ran on CPU 5.
 
 
 Multiple Warnings From One Stall
-- 
2.17.1

[PATCH tip/core/rcu 32/40] rcu: Don't funnel-lock above leaf node if GP in progress

[Paul E. McKenney]

The old grace-period start code would acquire only the leaf's rcu_node
structure's ->lock if that structure believed that a grace period was
in progress.  The new code advances to the leaf's parent in this case,
needlessly acquiring then leaf's parent's ->lock.  This commit therefore
checks the grace-period state after marking the leaf with the need for
the specified grace period, and if the leaf believes that a grace period
is in progress, takes an early exit.

Reported-by: Joel Fernandes <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
[ paulmck: Add "Startedleaf" tracing as suggested by Joel Fernandes. ]
---
 include/trace/events/rcu.h | 4 ++--
 kernel/rcu/tree.c          | 9 +++++++++
 2 files changed, 11 insertions(+), 2 deletions(-)

diff --git a/include/trace/events/rcu.h b/include/trace/events/rcu.h
index 7d3650cc9d30..5ab1df0a2801 100644
--- a/include/trace/events/rcu.h
+++ b/include/trace/events/rcu.h
@@ -91,8 +91,8 @@ TRACE_EVENT(rcu_grace_period,
  *
  * "Startleaf": Request a grace period based on leaf-node data.
  * "Prestarted": Someone beat us to the request
- * "Startedleaf": Leaf-node start proved sufficient.
- * "Startedleafroot": Leaf-node start proved sufficient after checking root.
+ * "Startedleaf": Leaf node marked for future GP.
+ * "Startedleafroot": All nodes from leaf to root marked for future GP.
  * "Startedroot": Requested a nocb grace period based on root-node data.
  * "NoGPkthread": The RCU grace-period kthread has not yet started.
  * "StartWait": Start waiting for the requested grace period.
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index fe986771c5cf..1c2aa020688a 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1590,6 +1590,15 @@ static bool rcu_start_this_gp(struct rcu_node *rnp, struct rcu_data *rdp,
 			goto unlock_out;
 		}
 		rnp_root->gp_seq_needed = c;
+		if (rcu_seq_state(rcu_seq_current(&rnp->gp_seq))) {
+			/*
+			 * We just marked the leaf, and a grace period
+			 * is in progress, which means that rcu_gp_cleanup()
+			 * will see the marking.  Bail to reduce contention.
+			 */
+			trace_rcu_this_gp(rnp, rdp, c, TPS("Startedleaf"));
+			goto unlock_out;
+		}
 		if (rnp_root != rnp && rnp_root->parent != NULL)
 			raw_spin_unlock_rcu_node(rnp_root);
 		if (!rnp_root->parent)
-- 
2.17.1

[PATCH tip/core/rcu 33/40] rcu: Produce last "CleanupMore" trace only if late-breaking request

[Paul E. McKenney]

From: "Joel Fernandes (Google)" <joel@joelfernandes.org>

Currently Tree RCU's clean-up code emits a "CleanupMore" trace event in
response to late-arriving grace-period requests even if the grace period
was already requested. This makes "CleanupMore" show up an extra time (in
addition to once for each rcu_node structure that was previously marked
with the request), and for no good reason.  This commit therefore avoids
emitting this trace message unless the the only request for this next
grace period arrived during or after the cleanup scan of the rcu_node
structures.

Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 kernel/rcu/tree.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 1c2aa020688a..8dab564be8e7 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2069,7 +2069,7 @@ static void rcu_gp_cleanup(struct rcu_state *rsp)
 	rsp->gp_state = RCU_GP_IDLE;
 	/* Check for GP requests since above loop. */
 	rdp = this_cpu_ptr(rsp->rda);
-	if (ULONG_CMP_LT(rnp->gp_seq, rnp->gp_seq_needed)) {
+	if (!needgp && ULONG_CMP_LT(rnp->gp_seq, rnp->gp_seq_needed)) {
 		trace_rcu_this_gp(rnp, rdp, rnp->gp_seq_needed,
 				  TPS("CleanupMore"));
 		needgp = true;
-- 
2.17.1

[PATCH tip/core/rcu 34/40] rcu: Fix cpustart tracepoint gp_seq number

[Paul E. McKenney]

From: "Joel Fernandes (Google)" <joel@joelfernandes.org>

The "cpustart" trace event shows a stale gp_seq. This is because it uses
rdp->gp_seq, which is updated only at the end of the __note_gp_changes()
function. This commit therefore instead uses rnp->gp_seq.

An alternative fix would be to update rdp->gp_seq earlier, but this would
break RCU's detection of the beginning of a new-to-this-CPU grace period.

Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 kernel/rcu/tree.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 8dab564be8e7..5c6f59b4fc9c 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1801,7 +1801,7 @@ static bool __note_gp_changes(struct rcu_state *rsp, struct rcu_node *rnp,
 		 * set up to detect a quiescent state, otherwise don't
 		 * go looking for one.
 		 */
-		trace_rcu_grace_period(rsp->name, rdp->gp_seq, TPS("cpustart"));
+		trace_rcu_grace_period(rsp->name, rnp->gp_seq, TPS("cpustart"));
 		need_gp = !!(rnp->qsmask & rdp->grpmask);
 		rdp->cpu_no_qs.b.norm = need_gp;
 		rdp->rcu_qs_ctr_snap = __this_cpu_read(rcu_dynticks.rcu_qs_ctr);
-- 
2.17.1

[PATCH tip/core/rcu 35/40] rcu: Make rcu_start_this_gp() check for grace period already started

[Paul E. McKenney]

In the old days of ->gpnum and ->completed, the code requesting a new
grace period checked to see if that grace period had already started,
bailing early if so.  The new-age ->gp_seq approach instead checks
whether the grace period has already finished.  A compensating change
pushed the requested grace period down to the bottom of the tree, thus
reducing lock contention and even eliminating it in some cases.  But why
not further reduce contention, especially on large systems, by doing both,
especially given that the cost of doing both is extremely small?

This commit therefore adds a new rcu_seq_started() function that checks
whether a specified grace period has already started.  It then uses
this new function in place of rcu_seq_done() in the rcu_start_this_gp()
function's funnel locking code.

Reported-by: Joel Fernandes <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 kernel/rcu/rcu.h  | 9 +++++++++
 kernel/rcu/tree.c | 2 +-
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
index 003671825d62..1c5cbd9d7c97 100644
--- a/kernel/rcu/rcu.h
+++ b/kernel/rcu/rcu.h
@@ -107,6 +107,15 @@ static inline unsigned long rcu_seq_current(unsigned long *sp)
 	return READ_ONCE(*sp);
 }
 
+/*
+ * Given a snapshot from rcu_seq_snap(), determine whether or not the
+ * corresponding update-side operation has started.
+ */
+static inline bool rcu_seq_started(unsigned long *sp, unsigned long s)
+{
+	return ULONG_CMP_LT((s - 1) & ~RCU_SEQ_STATE_MASK, READ_ONCE(*sp));
+}
+
 /*
  * Given a snapshot from rcu_seq_snap(), determine whether or not a
  * full update-side operation has occurred.
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 5c6f59b4fc9c..446163b6feba 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1583,7 +1583,7 @@ static bool rcu_start_this_gp(struct rcu_node *rnp, struct rcu_data *rdp,
 		if (rnp_root != rnp)
 			raw_spin_lock_rcu_node(rnp_root);
 		if (ULONG_CMP_GE(rnp_root->gp_seq_needed, c) ||
-		    rcu_seq_done(&rnp_root->gp_seq, c) ||
+		    rcu_seq_started(&rnp_root->gp_seq, c) ||
 		    (rnp != rnp_root &&
 		     rcu_seq_state(rcu_seq_current(&rnp_root->gp_seq)))) {
 			trace_rcu_this_gp(rnp_root, rdp, c, TPS("Prestarted"));
-- 
2.17.1

[PATCH tip/core/rcu 36/40] rcutorture: Correctly handle grace-period sequence wrap

[Paul E. McKenney]

The new ->gq_seq grace-period sequence numbers must be shifted down,
which give artifacts when these numbers wrap.  This commit therefore
enables rcutorture and rcuperf to handle grace-period sequence numbers
even if they do wrap.  It does this by allowing a special subtraction
function to be specified, and this function subtracts before shifting.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 kernel/rcu/rcu.h        |  9 +++++++++
 kernel/rcu/rcuperf.c    | 18 ++++++++++++++++--
 kernel/rcu/rcutorture.c | 19 +++++++++++++------
 kernel/rcu/tree.c       |  6 +++---
 4 files changed, 41 insertions(+), 11 deletions(-)

diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
index 1c5cbd9d7c97..aa215d6355f8 100644
--- a/kernel/rcu/rcu.h
+++ b/kernel/rcu/rcu.h
@@ -142,6 +142,15 @@ static inline bool rcu_seq_new_gp(unsigned long old, unsigned long new)
 			    new);
 }
 
+/*
+ * Roughly how many full grace periods have elapsed between the collection
+ * of the two specified grace periods?
+ */
+static inline unsigned long rcu_seq_diff(unsigned long new, unsigned long old)
+{
+	return (new - old) >> RCU_SEQ_CTR_SHIFT;
+}
+
 /*
  * debug_rcu_head_queue()/debug_rcu_head_unqueue() are used internally
  * by call_rcu() and rcu callback execution, and are therefore not part of the
diff --git a/kernel/rcu/rcuperf.c b/kernel/rcu/rcuperf.c
index 2b5a613afcf3..b080bc4a4f45 100644
--- a/kernel/rcu/rcuperf.c
+++ b/kernel/rcu/rcuperf.c
@@ -139,6 +139,7 @@ struct rcu_perf_ops {
 	int (*readlock)(void);
 	void (*readunlock)(int idx);
 	unsigned long (*get_gp_seq)(void);
+	unsigned long (*gp_diff)(unsigned long new, unsigned long old);
 	unsigned long (*exp_completed)(void);
 	void (*async)(struct rcu_head *head, rcu_callback_t func);
 	void (*gp_barrier)(void);
@@ -179,6 +180,7 @@ static struct rcu_perf_ops rcu_ops = {
 	.readlock	= rcu_perf_read_lock,
 	.readunlock	= rcu_perf_read_unlock,
 	.get_gp_seq	= rcu_get_gp_seq,
+	.gp_diff	= rcu_seq_diff,
 	.exp_completed	= rcu_exp_batches_completed,
 	.async		= call_rcu,
 	.gp_barrier	= rcu_barrier,
@@ -208,6 +210,7 @@ static struct rcu_perf_ops rcu_bh_ops = {
 	.readlock	= rcu_bh_perf_read_lock,
 	.readunlock	= rcu_bh_perf_read_unlock,
 	.get_gp_seq	= rcu_bh_get_gp_seq,
+	.gp_diff	= rcu_seq_diff,
 	.exp_completed	= rcu_exp_batches_completed_sched,
 	.async		= call_rcu_bh,
 	.gp_barrier	= rcu_barrier_bh,
@@ -264,6 +267,7 @@ static struct rcu_perf_ops srcu_ops = {
 	.readlock	= srcu_perf_read_lock,
 	.readunlock	= srcu_perf_read_unlock,
 	.get_gp_seq	= srcu_perf_completed,
+	.gp_diff	= rcu_seq_diff,
 	.exp_completed	= srcu_perf_completed,
 	.async		= srcu_call_rcu,
 	.gp_barrier	= srcu_rcu_barrier,
@@ -292,6 +296,7 @@ static struct rcu_perf_ops srcud_ops = {
 	.readlock	= srcu_perf_read_lock,
 	.readunlock	= srcu_perf_read_unlock,
 	.get_gp_seq	= srcu_perf_completed,
+	.gp_diff	= rcu_seq_diff,
 	.exp_completed	= srcu_perf_completed,
 	.async		= srcu_call_rcu,
 	.gp_barrier	= srcu_rcu_barrier,
@@ -321,6 +326,7 @@ static struct rcu_perf_ops sched_ops = {
 	.readlock	= sched_perf_read_lock,
 	.readunlock	= sched_perf_read_unlock,
 	.get_gp_seq	= rcu_sched_get_gp_seq,
+	.gp_diff	= rcu_seq_diff,
 	.exp_completed	= rcu_exp_batches_completed_sched,
 	.async		= call_rcu_sched,
 	.gp_barrier	= rcu_barrier_sched,
@@ -348,6 +354,7 @@ static struct rcu_perf_ops tasks_ops = {
 	.readlock	= tasks_perf_read_lock,
 	.readunlock	= tasks_perf_read_unlock,
 	.get_gp_seq	= rcu_no_completed,
+	.gp_diff	= rcu_seq_diff,
 	.async		= call_rcu_tasks,
 	.gp_barrier	= rcu_barrier_tasks,
 	.sync		= synchronize_rcu_tasks,
@@ -355,6 +362,13 @@ static struct rcu_perf_ops tasks_ops = {
 	.name		= "tasks"
 };
 
+static unsigned long rcuperf_seq_diff(unsigned long new, unsigned long old)
+{
+	if (!cur_ops->gp_diff)
+		return new - old;
+	return cur_ops->gp_diff(new, old);
+}
+
 static bool __maybe_unused torturing_tasks(void)
 {
 	return cur_ops == &tasks_ops;
@@ -577,8 +591,8 @@ rcu_perf_cleanup(void)
 			 t_rcu_perf_writer_finished -
 			 t_rcu_perf_writer_started,
 			 ngps,
-			 b_rcu_perf_writer_finished -
-			 b_rcu_perf_writer_started);
+			 rcuperf_seq_diff(b_rcu_perf_writer_finished,
+					  b_rcu_perf_writer_started));
 		for (i = 0; i < nrealwriters; i++) {
 			if (!writer_durations)
 				break;
diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
index 81fb43530d64..0481c7286875 100644
--- a/kernel/rcu/rcutorture.c
+++ b/kernel/rcu/rcutorture.c
@@ -265,6 +265,7 @@ struct rcu_torture_ops {
 	void (*read_delay)(struct torture_random_state *rrsp);
 	void (*readunlock)(int idx);
 	unsigned long (*get_gp_seq)(void);
+	unsigned long (*gp_diff)(unsigned long new, unsigned long old);
 	void (*deferred_free)(struct rcu_torture *p);
 	void (*sync)(void);
 	void (*exp_sync)(void);
@@ -400,6 +401,7 @@ static struct rcu_torture_ops rcu_ops = {
 	.read_delay	= rcu_read_delay,
 	.readunlock	= rcu_torture_read_unlock,
 	.get_gp_seq	= rcu_get_gp_seq,
+	.gp_diff	= rcu_seq_diff,
 	.deferred_free	= rcu_torture_deferred_free,
 	.sync		= synchronize_rcu,
 	.exp_sync	= synchronize_rcu_expedited,
@@ -441,6 +443,7 @@ static struct rcu_torture_ops rcu_bh_ops = {
 	.read_delay	= rcu_read_delay,  /* just reuse rcu's version. */
 	.readunlock	= rcu_bh_torture_read_unlock,
 	.get_gp_seq	= rcu_bh_get_gp_seq,
+	.gp_diff	= rcu_seq_diff,
 	.deferred_free	= rcu_bh_torture_deferred_free,
 	.sync		= synchronize_rcu_bh,
 	.exp_sync	= synchronize_rcu_bh_expedited,
@@ -646,6 +649,7 @@ static struct rcu_torture_ops sched_ops = {
 	.read_delay	= rcu_read_delay,  /* just reuse rcu's version. */
 	.readunlock	= sched_torture_read_unlock,
 	.get_gp_seq	= rcu_sched_get_gp_seq,
+	.gp_diff	= rcu_seq_diff,
 	.deferred_free	= rcu_sched_torture_deferred_free,
 	.sync		= synchronize_sched,
 	.exp_sync	= synchronize_sched_expedited,
@@ -695,6 +699,13 @@ static struct rcu_torture_ops tasks_ops = {
 	.name		= "tasks"
 };
 
+static unsigned long rcutorture_seq_diff(unsigned long new, unsigned long old)
+{
+	if (!cur_ops->gp_diff)
+		return new - old;
+	return cur_ops->gp_diff(new, old);
+}
+
 static bool __maybe_unused torturing_tasks(void)
 {
 	return cur_ops == &tasks_ops;
@@ -1127,9 +1138,7 @@ static void rcu_torture_timer(struct timer_list *unused)
 		rcu_ftrace_dump(DUMP_ALL);
 	}
 	__this_cpu_inc(rcu_torture_count[pipe_count]);
-	completed = completed - started;
-	if (completed > ULONG_MAX >> 1)
-		completed = 0; /* Not all gp_seq have full range. */
+	completed = rcutorture_seq_diff(completed, started);
 	if (completed > RCU_TORTURE_PIPE_LEN) {
 		/* Should not happen, but... */
 		completed = RCU_TORTURE_PIPE_LEN;
@@ -1205,9 +1214,7 @@ rcu_torture_reader(void *arg)
 			rcu_ftrace_dump(DUMP_ALL);
 		}
 		__this_cpu_inc(rcu_torture_count[pipe_count]);
-		completed = completed - started;
-		if (completed > ULONG_MAX >> 1)
-			completed = 0; /* Not all gp_seq have full range. */
+		completed = rcutorture_seq_diff(completed, started);
 		if (completed > RCU_TORTURE_PIPE_LEN) {
 			/* Should not happen, but... */
 			completed = RCU_TORTURE_PIPE_LEN;
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 446163b6feba..fa82f0413d2c 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -532,7 +532,7 @@ static int rcu_pending(void);
  */
 unsigned long rcu_get_gp_seq(void)
 {
-	return rcu_seq_ctr(READ_ONCE(rcu_state_p->gp_seq));
+	return READ_ONCE(rcu_state_p->gp_seq);
 }
 EXPORT_SYMBOL_GPL(rcu_get_gp_seq);
 
@@ -541,7 +541,7 @@ EXPORT_SYMBOL_GPL(rcu_get_gp_seq);
  */
 unsigned long rcu_sched_get_gp_seq(void)
 {
-	return rcu_seq_ctr(READ_ONCE(rcu_sched_state.gp_seq));
+	return READ_ONCE(rcu_sched_state.gp_seq);
 }
 EXPORT_SYMBOL_GPL(rcu_sched_get_gp_seq);
 
@@ -550,7 +550,7 @@ EXPORT_SYMBOL_GPL(rcu_sched_get_gp_seq);
  */
 unsigned long rcu_bh_get_gp_seq(void)
 {
-	return rcu_seq_ctr(READ_ONCE(rcu_bh_state.gp_seq));
+	return READ_ONCE(rcu_bh_state.gp_seq);
 }
 EXPORT_SYMBOL_GPL(rcu_bh_get_gp_seq);
 
-- 
2.17.1

[PATCH tip/core/rcu 37/40] rcu: Regularize resetting of rcu_data wrap indicator

[Paul E. McKenney]

The rcu_data structure's ->gpwrap indicator is currently reset only
when the CPU in question detects a new grace period.  This is in theory
sufficient because any CPU that has been out of action for long enough
that its ->gpwrap indicator is set is guaranteed to see both the end
of an old grace period and the start of a new one.

However, the current code leaves a short window during which the ->gpwrap
indicator has been reset but the corresponding ->gp_seq counter has not
yet been brought up to date.  This is harmless because interrupts are
disabled, but it is likely to (at the very least) cause confusion.

This commit therefore moves the resetting of ->gpwrap to follow the
updating of ->gp_seq.  While in the area, it also resets ->gp_seq_needed.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 kernel/rcu/tree.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index fa82f0413d2c..ad76aeade637 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1807,10 +1807,12 @@ static bool __note_gp_changes(struct rcu_state *rsp, struct rcu_node *rnp,
 		rdp->rcu_qs_ctr_snap = __this_cpu_read(rcu_dynticks.rcu_qs_ctr);
 		rdp->core_needs_qs = need_gp;
 		zero_cpu_stall_ticks(rdp);
-		WRITE_ONCE(rdp->gpwrap, false);
-		rcu_gpnum_ovf(rnp, rdp);
 	}
 	rdp->gp_seq = rnp->gp_seq;  /* Remember new grace-period state. */
+	if (ULONG_CMP_GE(rnp->gp_seq_needed, rdp->gp_seq_needed) || rdp->gpwrap)
+		rdp->gp_seq_needed = rnp->gp_seq_needed;
+	WRITE_ONCE(rdp->gpwrap, false);
+	rcu_gpnum_ovf(rnp, rdp);
 	return ret;
 }
 
-- 
2.17.1

[PATCH tip/core/rcu 38/40] rcu: Rename the grace-period-request variables and parameters

[Paul E. McKenney]

From: Joel Fernandes <joelaf@google.com>

The name 'c' is used for variables and parameters holding the requested
grace-period sequence number.  However it is no longer very meaningful
given the conversions from ->gpnum and (especially) ->completed to
->gp_seq. This commit therefore renames 'c' to 'gp_seq_req'.

Previous patch discussion is at:
https://patchwork.kernel.org/patch/10396579/

Signed-off-by: Joel Fernandes <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 include/trace/events/rcu.h | 15 +++++++------
 kernel/rcu/tree.c          | 46 ++++++++++++++++++++++----------------
 2 files changed, 35 insertions(+), 26 deletions(-)

diff --git a/include/trace/events/rcu.h b/include/trace/events/rcu.h
index 5ab1df0a2801..759e7e83733d 100644
--- a/include/trace/events/rcu.h
+++ b/include/trace/events/rcu.h
@@ -103,15 +103,16 @@ TRACE_EVENT(rcu_grace_period,
  */
 TRACE_EVENT(rcu_future_grace_period,
 
-	TP_PROTO(const char *rcuname, unsigned long gp_seq, unsigned long c,
-		 u8 level, int grplo, int grphi, const char *gpevent),
+	TP_PROTO(const char *rcuname, unsigned long gp_seq,
+		 unsigned long gp_seq_req, u8 level, int grplo, int grphi,
+		 const char *gpevent),
 
-	TP_ARGS(rcuname, gp_seq, c, level, grplo, grphi, gpevent),
+	TP_ARGS(rcuname, gp_seq, gp_seq_req, level, grplo, grphi, gpevent),
 
 	TP_STRUCT__entry(
 		__field(const char *, rcuname)
 		__field(unsigned long, gp_seq)
-		__field(unsigned long, c)
+		__field(unsigned long, gp_seq_req)
 		__field(u8, level)
 		__field(int, grplo)
 		__field(int, grphi)
@@ -121,7 +122,7 @@ TRACE_EVENT(rcu_future_grace_period,
 	TP_fast_assign(
 		__entry->rcuname = rcuname;
 		__entry->gp_seq = gp_seq;
-		__entry->c = c;
+		__entry->gp_seq_req = gp_seq_req;
 		__entry->level = level;
 		__entry->grplo = grplo;
 		__entry->grphi = grphi;
@@ -129,7 +130,7 @@ TRACE_EVENT(rcu_future_grace_period,
 	),
 
 	TP_printk("%s %lu %lu %u %d %d %s",
-		  __entry->rcuname, __entry->gp_seq, __entry->c, __entry->level,
+		  __entry->rcuname, __entry->gp_seq, __entry->gp_seq_req, __entry->level,
 		  __entry->grplo, __entry->grphi, __entry->gpevent)
 );
 
@@ -751,7 +752,7 @@ TRACE_EVENT(rcu_barrier,
 #else /* #ifdef CONFIG_RCU_TRACE */
 
 #define trace_rcu_grace_period(rcuname, gp_seq, gpevent) do { } while (0)
-#define trace_rcu_future_grace_period(rcuname, gp_seq, c, \
+#define trace_rcu_future_grace_period(rcuname, gp_seq, gp_seq_req, \
 				      level, grplo, grphi, event) \
 				      do { } while (0)
 #define trace_rcu_grace_period_init(rcuname, gp_seq, level, grplo, grphi, \
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index ad76aeade637..89ed285a55c8 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1546,13 +1546,18 @@ void rcu_cpu_stall_reset(void)
 
 /* Trace-event wrapper function for trace_rcu_future_grace_period.  */
 static void trace_rcu_this_gp(struct rcu_node *rnp, struct rcu_data *rdp,
-			      unsigned long c, const char *s)
+			      unsigned long gp_seq_req, const char *s)
 {
-	trace_rcu_future_grace_period(rdp->rsp->name, rnp->gp_seq, c,
+	trace_rcu_future_grace_period(rdp->rsp->name, rnp->gp_seq, gp_seq_req,
 				      rnp->level, rnp->grplo, rnp->grphi, s);
 }
 
 /*
+ * rcu_start_this_gp - Request the start of a particular grace period
+ * @rnp: The leaf node of the CPU from which to start.
+ * @rdp: The rcu_data corresponding to the CPU from which to start.
+ * @gp_seq_req: The gp_seq of the grace period to start.
+ *
  * Start the specified grace period, as needed to handle newly arrived
  * callbacks.  The required future grace periods are recorded in each
  * rcu_node structure's ->gp_seq_needed field.  Returns true if there
@@ -1560,9 +1565,11 @@ static void trace_rcu_this_gp(struct rcu_node *rnp, struct rcu_data *rdp,
  *
  * The caller must hold the specified rcu_node structure's ->lock, which
  * is why the caller is responsible for waking the grace-period kthread.
+ *
+ * Returns true if the GP thread needs to be awakened else false.
  */
 static bool rcu_start_this_gp(struct rcu_node *rnp, struct rcu_data *rdp,
-			      unsigned long c)
+			      unsigned long gp_seq_req)
 {
 	bool ret = false;
 	struct rcu_state *rsp = rdp->rsp;
@@ -1578,25 +1585,27 @@ static bool rcu_start_this_gp(struct rcu_node *rnp, struct rcu_data *rdp,
 	 * not be released.
 	 */
 	raw_lockdep_assert_held_rcu_node(rnp);
-	trace_rcu_this_gp(rnp, rdp, c, TPS("Startleaf"));
+	trace_rcu_this_gp(rnp, rdp, gp_seq_req, TPS("Startleaf"));
 	for (rnp_root = rnp; 1; rnp_root = rnp_root->parent) {
 		if (rnp_root != rnp)
 			raw_spin_lock_rcu_node(rnp_root);
-		if (ULONG_CMP_GE(rnp_root->gp_seq_needed, c) ||
-		    rcu_seq_started(&rnp_root->gp_seq, c) ||
+		if (ULONG_CMP_GE(rnp_root->gp_seq_needed, gp_seq_req) ||
+		    rcu_seq_started(&rnp_root->gp_seq, gp_seq_req) ||
 		    (rnp != rnp_root &&
 		     rcu_seq_state(rcu_seq_current(&rnp_root->gp_seq)))) {
-			trace_rcu_this_gp(rnp_root, rdp, c, TPS("Prestarted"));
+			trace_rcu_this_gp(rnp_root, rdp, gp_seq_req,
+					  TPS("Prestarted"));
 			goto unlock_out;
 		}
-		rnp_root->gp_seq_needed = c;
+		rnp_root->gp_seq_needed = gp_seq_req;
 		if (rcu_seq_state(rcu_seq_current(&rnp->gp_seq))) {
 			/*
 			 * We just marked the leaf, and a grace period
 			 * is in progress, which means that rcu_gp_cleanup()
 			 * will see the marking.  Bail to reduce contention.
 			 */
-			trace_rcu_this_gp(rnp, rdp, c, TPS("Startedleaf"));
+			trace_rcu_this_gp(rnp, rdp, gp_seq_req,
+					  TPS("Startedleaf"));
 			goto unlock_out;
 		}
 		if (rnp_root != rnp && rnp_root->parent != NULL)
@@ -1607,21 +1616,21 @@ static bool rcu_start_this_gp(struct rcu_node *rnp, struct rcu_data *rdp,
 
 	/* If GP already in progress, just leave, otherwise start one. */
 	if (rcu_gp_in_progress(rsp)) {
-		trace_rcu_this_gp(rnp_root, rdp, c, TPS("Startedleafroot"));
+		trace_rcu_this_gp(rnp_root, rdp, gp_seq_req, TPS("Startedleafroot"));
 		goto unlock_out;
 	}
-	trace_rcu_this_gp(rnp_root, rdp, c, TPS("Startedroot"));
+	trace_rcu_this_gp(rnp_root, rdp, gp_seq_req, TPS("Startedroot"));
 	WRITE_ONCE(rsp->gp_flags, rsp->gp_flags | RCU_GP_FLAG_INIT);
 	rsp->gp_req_activity = jiffies;
 	if (!rsp->gp_kthread) {
-		trace_rcu_this_gp(rnp_root, rdp, c, TPS("NoGPkthread"));
+		trace_rcu_this_gp(rnp_root, rdp, gp_seq_req, TPS("NoGPkthread"));
 		goto unlock_out;
 	}
 	trace_rcu_grace_period(rsp->name, READ_ONCE(rsp->gp_seq), TPS("newreq"));
 	ret = true;  /* Caller must wake GP kthread. */
 unlock_out:
 	/* Push furthest requested GP to leaf node and rcu_data structure. */
-	if (ULONG_CMP_LT(c, rnp_root->gp_seq_needed)) {
+	if (ULONG_CMP_LT(gp_seq_req, rnp_root->gp_seq_needed)) {
 		rnp->gp_seq_needed = rnp_root->gp_seq_needed;
 		rdp->gp_seq_needed = rnp_root->gp_seq_needed;
 	}
@@ -1636,14 +1645,13 @@ static bool rcu_start_this_gp(struct rcu_node *rnp, struct rcu_data *rdp,
  */
 static bool rcu_future_gp_cleanup(struct rcu_state *rsp, struct rcu_node *rnp)
 {
-	unsigned long c = rnp->gp_seq;
 	bool needmore;
 	struct rcu_data *rdp = this_cpu_ptr(rsp->rda);
 
 	needmore = ULONG_CMP_LT(rnp->gp_seq, rnp->gp_seq_needed);
 	if (!needmore)
 		rnp->gp_seq_needed = rnp->gp_seq; /* Avoid counter wrap. */
-	trace_rcu_this_gp(rnp, rdp, c,
+	trace_rcu_this_gp(rnp, rdp, rnp->gp_seq,
 			  needmore ? TPS("CleanupMore") : TPS("Cleanup"));
 	return needmore;
 }
@@ -1679,7 +1687,7 @@ static void rcu_gp_kthread_wake(struct rcu_state *rsp)
 static bool rcu_accelerate_cbs(struct rcu_state *rsp, struct rcu_node *rnp,
 			       struct rcu_data *rdp)
 {
-	unsigned long c;
+	unsigned long gp_seq_req;
 	bool ret = false;
 
 	raw_lockdep_assert_held_rcu_node(rnp);
@@ -1698,9 +1706,9 @@ static bool rcu_accelerate_cbs(struct rcu_state *rsp, struct rcu_node *rnp,
 	 * accelerating callback invocation to an earlier grace-period
 	 * number.
 	 */
-	c = rcu_seq_snap(&rsp->gp_seq);
-	if (rcu_segcblist_accelerate(&rdp->cblist, c))
-		ret = rcu_start_this_gp(rnp, rdp, c);
+	gp_seq_req = rcu_seq_snap(&rsp->gp_seq);
+	if (rcu_segcblist_accelerate(&rdp->cblist, gp_seq_req))
+		ret = rcu_start_this_gp(rnp, rdp, gp_seq_req);
 
 	/* Trace depending on how much we were able to accelerate. */
 	if (rcu_segcblist_restempty(&rdp->cblist, RCU_WAIT_TAIL))
-- 
2.17.1

[PATCH tip/core/rcu 39/40] rcu: Use better variable names in funnel locking loop

[Paul E. McKenney]

From: "Joel Fernandes (Google)" <joel@joelfernandes.org>

The funnel locking loop in rcu_start_this_gp uses rcu_root as a
temporary variable while walking the combining tree. This causes a
tiresome exercise of a code reader reminding themselves that rcu_root
may not be root. Lets just call it rnp, and rename other variables as
well to be more appropriate.

Original patch: https://patchwork.kernel.org/patch/10396577/

Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
[ paulmck: Fix name in comment as well. ]
---
 kernel/rcu/tree.c | 64 +++++++++++++++++++++++------------------------
 1 file changed, 32 insertions(+), 32 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 89ed285a55c8..0dac8c716c48 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1554,7 +1554,7 @@ static void trace_rcu_this_gp(struct rcu_node *rnp, struct rcu_data *rdp,
 
 /*
  * rcu_start_this_gp - Request the start of a particular grace period
- * @rnp: The leaf node of the CPU from which to start.
+ * @rnp_start: The leaf node of the CPU from which to start.
  * @rdp: The rcu_data corresponding to the CPU from which to start.
  * @gp_seq_req: The gp_seq of the grace period to start.
  *
@@ -1568,74 +1568,74 @@ static void trace_rcu_this_gp(struct rcu_node *rnp, struct rcu_data *rdp,
  *
  * Returns true if the GP thread needs to be awakened else false.
  */
-static bool rcu_start_this_gp(struct rcu_node *rnp, struct rcu_data *rdp,
+static bool rcu_start_this_gp(struct rcu_node *rnp_start, struct rcu_data *rdp,
 			      unsigned long gp_seq_req)
 {
 	bool ret = false;
 	struct rcu_state *rsp = rdp->rsp;
-	struct rcu_node *rnp_root;
+	struct rcu_node *rnp;
 
 	/*
 	 * Use funnel locking to either acquire the root rcu_node
 	 * structure's lock or bail out if the need for this grace period
-	 * has already been recorded -- or has already started.  If there
-	 * is already a grace period in progress in a non-leaf node, no
-	 * recording is needed because the end of the grace period will
-	 * scan the leaf rcu_node structures.  Note that rnp->lock must
-	 * not be released.
+	 * has already been recorded -- or if that grace period has in
+	 * fact already started.  If there is already a grace period in
+	 * progress in a non-leaf node, no recording is needed because the
+	 * end of the grace period will scan the leaf rcu_node structures.
+	 * Note that rnp_start->lock must not be released.
 	 */
-	raw_lockdep_assert_held_rcu_node(rnp);
-	trace_rcu_this_gp(rnp, rdp, gp_seq_req, TPS("Startleaf"));
-	for (rnp_root = rnp; 1; rnp_root = rnp_root->parent) {
-		if (rnp_root != rnp)
-			raw_spin_lock_rcu_node(rnp_root);
-		if (ULONG_CMP_GE(rnp_root->gp_seq_needed, gp_seq_req) ||
-		    rcu_seq_started(&rnp_root->gp_seq, gp_seq_req) ||
-		    (rnp != rnp_root &&
-		     rcu_seq_state(rcu_seq_current(&rnp_root->gp_seq)))) {
-			trace_rcu_this_gp(rnp_root, rdp, gp_seq_req,
+	raw_lockdep_assert_held_rcu_node(rnp_start);
+	trace_rcu_this_gp(rnp_start, rdp, gp_seq_req, TPS("Startleaf"));
+	for (rnp = rnp_start; 1; rnp = rnp->parent) {
+		if (rnp != rnp_start)
+			raw_spin_lock_rcu_node(rnp);
+		if (ULONG_CMP_GE(rnp->gp_seq_needed, gp_seq_req) ||
+		    rcu_seq_started(&rnp->gp_seq, gp_seq_req) ||
+		    (rnp != rnp_start &&
+		     rcu_seq_state(rcu_seq_current(&rnp->gp_seq)))) {
+			trace_rcu_this_gp(rnp, rdp, gp_seq_req,
 					  TPS("Prestarted"));
 			goto unlock_out;
 		}
-		rnp_root->gp_seq_needed = gp_seq_req;
-		if (rcu_seq_state(rcu_seq_current(&rnp->gp_seq))) {
+		rnp->gp_seq_needed = gp_seq_req;
+		if (rcu_seq_state(rcu_seq_current(&rnp_start->gp_seq))) {
 			/*
 			 * We just marked the leaf, and a grace period
 			 * is in progress, which means that rcu_gp_cleanup()
 			 * will see the marking.  Bail to reduce contention.
 			 */
-			trace_rcu_this_gp(rnp, rdp, gp_seq_req,
+			trace_rcu_this_gp(rnp_start, rdp, gp_seq_req,
 					  TPS("Startedleaf"));
 			goto unlock_out;
 		}
-		if (rnp_root != rnp && rnp_root->parent != NULL)
-			raw_spin_unlock_rcu_node(rnp_root);
-		if (!rnp_root->parent)
+		if (rnp != rnp_start && rnp->parent != NULL)
+			raw_spin_unlock_rcu_node(rnp);
+		if (!rnp->parent)
 			break;  /* At root, and perhaps also leaf. */
 	}
 
 	/* If GP already in progress, just leave, otherwise start one. */
 	if (rcu_gp_in_progress(rsp)) {
-		trace_rcu_this_gp(rnp_root, rdp, gp_seq_req, TPS("Startedleafroot"));
+		trace_rcu_this_gp(rnp, rdp, gp_seq_req, TPS("Startedleafroot"));
 		goto unlock_out;
 	}
-	trace_rcu_this_gp(rnp_root, rdp, gp_seq_req, TPS("Startedroot"));
+	trace_rcu_this_gp(rnp, rdp, gp_seq_req, TPS("Startedroot"));
 	WRITE_ONCE(rsp->gp_flags, rsp->gp_flags | RCU_GP_FLAG_INIT);
 	rsp->gp_req_activity = jiffies;
 	if (!rsp->gp_kthread) {
-		trace_rcu_this_gp(rnp_root, rdp, gp_seq_req, TPS("NoGPkthread"));
+		trace_rcu_this_gp(rnp, rdp, gp_seq_req, TPS("NoGPkthread"));
 		goto unlock_out;
 	}
 	trace_rcu_grace_period(rsp->name, READ_ONCE(rsp->gp_seq), TPS("newreq"));
 	ret = true;  /* Caller must wake GP kthread. */
 unlock_out:
 	/* Push furthest requested GP to leaf node and rcu_data structure. */
-	if (ULONG_CMP_LT(gp_seq_req, rnp_root->gp_seq_needed)) {
-		rnp->gp_seq_needed = rnp_root->gp_seq_needed;
-		rdp->gp_seq_needed = rnp_root->gp_seq_needed;
+	if (ULONG_CMP_LT(gp_seq_req, rnp->gp_seq_needed)) {
+		rnp_start->gp_seq_needed = rnp->gp_seq_needed;
+		rdp->gp_seq_needed = rnp->gp_seq_needed;
 	}
-	if (rnp != rnp_root)
-		raw_spin_unlock_rcu_node(rnp_root);
+	if (rnp != rnp_start)
+		raw_spin_unlock_rcu_node(rnp);
 	return ret;
 }
 
-- 
2.17.1

[PATCH tip/core/rcu 40/40] rcu: Identify grace period is in progress as we advance up the tree

[Paul E. McKenney]

From: "Joel Fernandes (Google)" <joel@joelfernandes.org>

There's no need to keep checking the same starting node for whether a
grace period is in progress as we advance up the funnel lock loop. Its
sufficient if we just checked it in the start, and then subsequently
checked the internal nodes as we advanced up the combining tree. This
also makes sense because the grace-period updates propogate from the
root to the leaf, so there's a chance we may find a grace period has
started as we advance up, lets check for the same.

Reported-by: Paul McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 kernel/rcu/tree.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 0dac8c716c48..73298a27620f 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1598,11 +1598,12 @@ static bool rcu_start_this_gp(struct rcu_node *rnp_start, struct rcu_data *rdp,
 			goto unlock_out;
 		}
 		rnp->gp_seq_needed = gp_seq_req;
-		if (rcu_seq_state(rcu_seq_current(&rnp_start->gp_seq))) {
+		if (rcu_seq_state(rcu_seq_current(&rnp->gp_seq))) {
 			/*
-			 * We just marked the leaf, and a grace period
-			 * is in progress, which means that rcu_gp_cleanup()
-			 * will see the marking.  Bail to reduce contention.
+			 * We just marked the leaf or internal node, and a
+			 * grace period is in progress, which means that
+			 * rcu_gp_cleanup() will see the marking.  Bail to
+			 * reduce contention.
 			 */
 			trace_rcu_this_gp(rnp_start, rdp, gp_seq_req,
 					  TPS("Startedleaf"));
-- 
2.17.1