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>

Author: walken-google

lib/rwsem.c

@@ -216,22 +216,23 @@ 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);
 
 /* Block until there are no active lockers. */
 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);
 }