rwsem: use cmpxchg for trying to steal write lock
Using rwsem_atomic_update to try stealing the write lock forced us to
undo the adjustment in the failure path. We can have simpler and faster
code by using cmpxchg instead.
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 2360bf20..64c2dc0 100644
--- a/lib/rwsem.c
+++ b/lib/rwsem.c
@@ -142,25 +142,6 @@
return sem;
}
-/* Try to get write sem, caller holds sem->wait_lock: */
-static int try_get_writer_sem(struct rw_semaphore *sem)
-{
- long oldcount, adjustment;
-
- adjustment = RWSEM_ACTIVE_WRITE_BIAS;
- if (list_is_singular(&sem->wait_list))
- adjustment -= RWSEM_WAITING_BIAS;
-
-try_again_write:
- oldcount = rwsem_atomic_update(adjustment, sem) - adjustment;
- if (!(oldcount & RWSEM_ACTIVE_MASK))
- return 1;
- /* some one grabbed the sem already */
- if (rwsem_atomic_update(-adjustment, sem) & RWSEM_ACTIVE_MASK)
- return 0;
- goto try_again_write;
-}
-
/*
* wait for the read lock to be granted
*/
@@ -236,7 +217,12 @@
while (true) {
set_task_state(tsk, TASK_UNINTERRUPTIBLE);
- if (try_get_writer_sem(sem))
+ /* 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;
raw_spin_unlock_irq(&sem->wait_lock);