[PATCH v4 2/5] locking/rwsem: Limit # of null owner retries for handoff writer

[Waiman Long <longman@redhat.com>]

Commit 91d2a812dfb9 ("locking/rwsem: Make handoff writer optimistically
spin on owner") assumes that when the owner field is changed to NULL,
the lock will become free soon. Commit 48dfb5d2560d ("locking/rwsem:
Disable preemption while trying for rwsem lock") disable preemption
when acquiring rwsem for write. However, preemption has not yet been
disabled when acquiring a read lock on a rwsem. So a reader can add a
RWSEM_READER_BIAS to count without setting owner to signal a reader,
got preempted out by a RT task which then spins in the writer slowpath
as owner remains NULL leading to live lock.

One way to fix that is to disable preemption before the read lock attempt
and then immediately remove RWSEM_READER_BIAS when the trylock fails
before reenabling preemption. This will remove some optimizations that
can be done by delaying the RWSEM_READER_BIAS backoff. Alternatively
we could delay the preempt_enable() into the rwsem_down_read_slowpath()
and even after acquiring and releasing the wait_lock. Another possible
alternative is to limit the number of trylock attempts without sleeping.
The last alternative seems to be less messy and is being implemented
in this patch.

The current limit is now set to 8 to allow enough time for the other
task to hopefully complete its action.

By adding new lock events to track the number of NULL owner retries with
handoff flag set before a successful trylock when running a 96 threads
locking microbenchmark with equal number of readers and writers running
on a 2-core 96-thread system for 15 seconds, the following stats are
obtained. Note that none of locking threads are RT tasks.

  Retries of successful trylock    Count
  -----------------------------    -----
             1                     1738
             2                       19
             3                       11
             4                        2
             5                        1
             6                        1
             7                        1
             8                        0
             X                        1

The last row is the one failed attempt that needs more than 8 retries.
So a retry count maximum of 8 should capture most of them if no RT task
is in the mix.

Fixes: 91d2a812dfb9 ("locking/rwsem: Make handoff writer optimistically spin on owner")
Reported-by: Mukesh Ojha <quic_mojha@quicinc.com>
Signed-off-by: Waiman Long <longman@redhat.com>
Reviewed-and-tested-by: Mukesh Ojha <quic_mojha@quicinc.com>
Cc: stable@vger.kernel.org
---
 kernel/locking/rwsem.c | 18 ++++++++++++++++--
 1 file changed, 16 insertions(+), 2 deletions(-)

diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
index be2df9ea7c30..c68d76fc8c68 100644
--- a/kernel/locking/rwsem.c
+++ b/kernel/locking/rwsem.c
@@ -1115,6 +1115,7 @@ static struct rw_semaphore __sched *
 rwsem_down_write_slowpath(struct rw_semaphore *sem, int state)
 {
 	struct rwsem_waiter waiter;
+	int null_owner_retries;
 	DEFINE_WAKE_Q(wake_q);
 
 	/* do optimistic spinning and steal lock if possible */
@@ -1156,7 +1157,7 @@ rwsem_down_write_slowpath(struct rw_semaphore *sem, int state)
 	set_current_state(state);
 	trace_contention_begin(sem, LCB_F_WRITE);
 
-	for (;;) {
+	for (null_owner_retries = 0;;) {
 		if (rwsem_try_write_lock(sem, &waiter)) {
 			/* rwsem_try_write_lock() implies ACQUIRE on success */
 			break;
@@ -1182,8 +1183,21 @@ rwsem_down_write_slowpath(struct rw_semaphore *sem, int state)
 			owner_state = rwsem_spin_on_owner(sem);
 			preempt_enable();
 
-			if (owner_state == OWNER_NULL)
+			/*
+			 * owner is NULL doesn't guarantee the lock is free.
+			 * An incoming reader will temporarily increment the
+			 * reader count without changing owner and the
+			 * rwsem_try_write_lock() will fails if the reader
+			 * is not able to decrement it in time. Allow 8
+			 * trylock attempts when hitting a NULL owner before
+			 * going to sleep.
+			 */
+			if ((owner_state == OWNER_NULL) &&
+			    (null_owner_retries < 8)) {
+				null_owner_retries++;
 				goto trylock_again;
+			}
+			null_owner_retries = 0;
 		}
 
 		schedule();
-- 
2.31.1