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[PATCH v3 3/4] locking/rwsem: Improve reader wakeup code

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  [PATCH v3 3/4] locking/rwsem: Improve reader wakeup code Waiman Long <Waiman.Long@hpe.com> - 2016-05-13 01:00 +0200
    Re: [PATCH v3 3/4] locking/rwsem: Improve reader wakeup code Peter Hurley <peter@hurleysoftware.com> - 2016-05-17 19:40 +0200

#1400408 — [PATCH v3 3/4] locking/rwsem: Improve reader wakeup code

In __rwsem_do_wake(), the reader wakeup code will assume a writer
has stolen the lock if the active reader/writer count is not 0.
However, this is not as reliable an indicator as the original
"< RWSEM_WAITING_BIAS" check. If another reader is present, the code
will still break out and exit even if the writer is gone. This patch
changes it to check the same "< RWSEM_WAITING_BIAS" condition to
reduce the chance of false positive.

Signed-off-by: Waiman Long <Waiman.Long@hpe.com>
---
 kernel/locking/rwsem-xadd.c |   11 ++++++++---
 1 files changed, 8 insertions(+), 3 deletions(-)

diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index 22f7d58..6c08ad9 100644
--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -148,9 +148,14 @@ __rwsem_do_wake(struct rw_semaphore *sem, enum rwsem_wake_type wake_type)
  try_reader_grant:
 		oldcount = rwsem_atomic_update(adjustment, sem) - adjustment;
 		if (unlikely(oldcount < RWSEM_WAITING_BIAS)) {
-			/* A writer stole the lock. Undo our reader grant. */
-			if (rwsem_atomic_update(-adjustment, sem) &
-						RWSEM_ACTIVE_MASK)
+			/*
+			 * If the count is still less than RWSEM_WAITING_BIAS
+			 * after removing the adjustment, it is assumed that
+			 * a writer has stolen the lock. We have to undo our
+			 * reader grant.
+			 */
+			if (rwsem_atomic_update(-adjustment, sem)
+			    < RWSEM_WAITING_BIAS)
 				goto out;
 			/* Last active locker left. Retry waking readers. */
 			goto try_reader_grant;
-- 
1.7.1

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#1402493

On 05/12/2016 03:56 PM, Waiman Long wrote:
> In __rwsem_do_wake(), the reader wakeup code will assume a writer
> has stolen the lock if the active reader/writer count is not 0.
> However, this is not as reliable an indicator as the original
> "< RWSEM_WAITING_BIAS" check. If another reader is present, the code
> will still break out and exit even if the writer is gone. This patch
> changes it to check the same "< RWSEM_WAITING_BIAS" condition to
> reduce the chance of false positive.

Nice.

Reviewed-by: Peter Hurley <peter@hurleysoftware.com>

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