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authorMichel Lespinasse <walken@google.com>2013-05-07 06:45:57 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2013-05-07 07:20:16 -0700
commit9b0fc9c09f1b262b7fe697eba6b05095d78850e5 (patch)
tree5fb25ae32c9aed77f604858a77a27cd4b37bbfc6 /lib
parenta7d2c573ae7fad1b2c877d1a1342fa5bb0d6478c (diff)
downloadlinux-linaro-stable-9b0fc9c09f1b262b7fe697eba6b05095d78850e5.tar.gz
rwsem: skip initial trylock in rwsem_down_write_failed
We can skip the initial trylock in rwsem_down_write_failed() if there are known active lockers already, thus saving one likely-to-fail cmpxchg. Signed-off-by: Michel Lespinasse <walken@google.com> Reviewed-by: Peter Hurley <peter@hurleysoftware.com> Acked-by: Davidlohr Bueso <davidlohr.bueso@hp.com> Acked-by: Rik van Riel <riel@redhat.com> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'lib')
-rw-r--r--lib/rwsem.c17
1 files changed, 9 insertions, 8 deletions
diff --git a/lib/rwsem.c b/lib/rwsem.c
index edf3d9ca670e..0d50e46d5b0c 100644
--- a/lib/rwsem.c
+++ b/lib/rwsem.c
@@ -216,14 +216,15 @@ struct rw_semaphore __sched *rwsem_down_write_failed(struct rw_semaphore *sem)
/* wait until we successfully acquire the lock */
set_task_state(tsk, TASK_UNINTERRUPTIBLE);
while (true) {
-
- /* Try acquiring the write lock. */
- count = RWSEM_ACTIVE_WRITE_BIAS;
- if (!list_is_singular(&sem->wait_list))
- count += RWSEM_WAITING_BIAS;
- if (cmpxchg(&sem->count, RWSEM_WAITING_BIAS, count) ==
+ if (!(count & RWSEM_ACTIVE_MASK)) {
+ /* Try acquiring the write lock. */
+ count = RWSEM_ACTIVE_WRITE_BIAS;
+ if (!list_is_singular(&sem->wait_list))
+ count += RWSEM_WAITING_BIAS;
+ if (cmpxchg(&sem->count, RWSEM_WAITING_BIAS, count) ==
RWSEM_WAITING_BIAS)
- break;
+ break;
+ }
raw_spin_unlock_irq(&sem->wait_lock);
@@ -231,7 +232,7 @@ struct rw_semaphore __sched *rwsem_down_write_failed(struct rw_semaphore *sem)
do {
schedule();
set_task_state(tsk, TASK_UNINTERRUPTIBLE);
- } while (sem->count & RWSEM_ACTIVE_MASK);
+ } while ((count = sem->count) & RWSEM_ACTIVE_MASK);
raw_spin_lock_irq(&sem->wait_lock);
}