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Commit bf7b4c47 authored by Waiman Long's avatar Waiman Long Committed by Ingo Molnar
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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: default avatarWaiman Long <Waiman.Long@hpe.com>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: default avatarPeter 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: default avatarIngo Molnar <mingo@kernel.org>
parent fb6a44f3

kernel/locking/rwsem-xadd.c

+8 −3
Original line number Diff line number Diff line
@@ -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;