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>

1 files changed, 6 insertions, 20 deletions

diff --git a/lib/rwsem.c b/lib/rwsem.c
index 2360bf204098..64c2dc007be2 100644
--- a/lib/rwsem.c
+++ b/lib/rwsem.c
@@ -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);