rwsem: use cmpxchg for trying to steal write lock
Michel Lespinasse [Tue, 7 May 2013 13:45:55 +0000 (06:45 -0700)]
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>

lib/rwsem.c

index 2360bf2..64c2dc0 100644 (file)
@@ -142,25 +142,6 @@ __rwsem_do_wake(struct rw_semaphore *sem, int wake_type)
        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 @@ struct rw_semaphore __sched *rwsem_down_write_failed(struct rw_semaphore *sem)
        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);