[PATCH tip/core/rcu 2/4] srcu: Prevent __call_srcu() counter wrap with read-side critical section

["Paul E. McKenney" <paulmck@linux.ibm.com>]

From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>

Ever since cdf7abc4610a ("srcu: Allow use of Tiny/Tree SRCU from
both process and interrupt context"), it has been permissible
to use SRCU read-side critical sections in interrupt context.
This allows __call_srcu() to use SRCU read-side critical sections to
prevent a new SRCU grace period from ending before the call to either
srcu_funnel_gp_start() or srcu_funnel_exp_start completes, thus preventing
SRCU grace-period counter overflow during that time.

Note that this does not permit removal of the counter-wrap checks in
srcu_gp_end().  These check are necessary to handle the case where
a given CPU does not interact at all with SRCU for an extended time
period.

This commit therefore adds an SRCU read-side critical section to
__call_srcu() in order to prevent grace period counter wrap during
the funnel-locking process.

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

diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index a8846ed7f352..60f3236beaf7 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -858,6 +858,7 @@ void __call_srcu(struct srcu_struct *sp, struct rcu_head *rhp,
 		 rcu_callback_t func, bool do_norm)
 {
 	unsigned long flags;
+	int idx;
 	bool needexp = false;
 	bool needgp = false;
 	unsigned long s;
@@ -871,6 +872,7 @@ void __call_srcu(struct srcu_struct *sp, struct rcu_head *rhp,
 		return;
 	}
 	rhp->func = func;
+	idx = srcu_read_lock(sp);
 	local_irq_save(flags);
 	sdp = this_cpu_ptr(sp->sda);
 	spin_lock_rcu_node(sdp);
@@ -892,6 +894,7 @@ void __call_srcu(struct srcu_struct *sp, struct rcu_head *rhp,
 		srcu_funnel_gp_start(sp, sdp, s, do_norm);
 	else if (needexp)
 		srcu_funnel_exp_start(sp, sdp->mynode, s);
+	srcu_read_unlock(sp, idx);
 }
 
 /**
-- 
2.17.1