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locking/rwsem: Improve reader wakeup code

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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>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Peter Hurley <peter@hurleysoftware.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Douglas Hatch <doug.hatch@hpe.com>
Cc: Jason Low <jason.low2@hp.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Scott J Norton <scott.norton@hpe.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1463534783-38814-5-git-send-email-Waiman.Long@hpe.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Waiman Long 2016-05-17 21:26:22 -04:00 committed by Ingo Molnar
parent fb6a44f33b
commit bf7b4c472d
1 changed files with 8 additions and 3 deletions
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11
kernel/locking/rwsem-xadd.c
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@ -156,9 +156,14 @@ __rwsem_mark_wake(struct rw_semaphore *sem,
oldcount = atomic_long_add_return(adjustment, &sem->count) - adjustment;
if (unlikely(oldcount < RWSEM_WAITING_BIAS)) {
/* A writer stole the lock. Undo our reader grant. */
if (atomic_long_sub_return(adjustment, &sem->count) &
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 (atomic_long_add_return(-adjustment, &sem->count) <
RWSEM_WAITING_BIAS)
goto out;
/* Last active locker left. Retry waking readers. */
goto try_reader_grant;