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 <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Reviewed-by: Peter Hurley <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Dave Chinner <[email protected]>
Cc: Davidlohr Bueso <[email protected]>
Cc: Douglas Hatch <[email protected]>
Cc: Jason Low <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Paul E. McKenney <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Scott J Norton <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

1 files changed, 8 insertions, 3 deletions

diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index 6b0d0605910e..4f1daf5a472d 100644
--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -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;