From: Waiman Long Date: Wed, 18 May 2016 01:26:22 +0000 (-0400) Subject: locking/rwsem: Improve reader wakeup code X-Git-Url: https://git.karo-electronics.de/?a=commitdiff_plain;h=bf7b4c472db44413251bcef79ca1f6bf1ec81475;p=linux-beck.git 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 Signed-off-by: Peter Zijlstra (Intel) Reviewed-by: Peter Hurley Cc: Andrew Morton Cc: Dave Chinner Cc: Davidlohr Bueso Cc: Douglas Hatch Cc: Jason Low Cc: Linus Torvalds Cc: Paul E. McKenney Cc: Peter Zijlstra Cc: Scott J Norton Cc: Thomas Gleixner Link: http://lkml.kernel.org/r/1463534783-38814-5-git-send-email-Waiman.Long@hpe.com Signed-off-by: Ingo Molnar --- 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;