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>
diff --git a/lib/rwsem.c b/lib/rwsem.c
index edf3d9c..0d50e46 100644
--- a/lib/rwsem.c
+++ b/lib/rwsem.c
@@ -216,14 +216,15 @@
/* 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 @@
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);
}