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>

1 files changed, 9 insertions, 8 deletions

diff --git a/lib/rwsem.c b/lib/rwsem.c
index edf3d9ca670e..0d50e46d5b0c 100644
--- a/lib/rwsem.c
+++ b/lib/rwsem.c
@@ -216,14 +216,15 @@ 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);
 
@@ -231,7 +232,7 @@ struct rw_semaphore __sched *rwsem_down_write_failed(struct rw_semaphore *sem)
                 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);
         }