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authorWaiman Long <Waiman.Long@hpe.com>2016-05-17 21:26:22 -0400
committerIngo Molnar <mingo@kernel.org>2016-06-08 15:17:00 +0200
commitbf7b4c472db44413251bcef79ca1f6bf1ec81475 (patch)
tree0de51a7219f1aa5060ee230a655d2fb82b369a17 /kernel/locking
parentfb6a44f33be542fd81575ff93a4e8118d6a58592 (diff)
downloadlinux-bf7b4c472db44413251bcef79ca1f6bf1ec81475.tar.bz2
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> 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>
Diffstat (limited to 'kernel/locking')
-rw-r--r--kernel/locking/rwsem-xadd.c11
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;