sbitmap: use smp_mb__after_atomic() in sbq_wake_up()

We always do an atomic clear_bit() right before we call sbq_wake_up(),
so we can use smp_mb__after_atomic(). While we're here, comment the
memory barriers in here a little more.

Signed-off-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

diff --git a/lib/sbitmap.c b/lib/sbitmap.c
index 2cecf05..df4e472 100644 (file)
--- a/lib/sbitmap.c
+++ b/lib/sbitmap.c
@@ -299,8 +299,14 @@ static void sbq_wake_up(struct sbitmap_queue *sbq)
        struct sbq_wait_state *ws;
        int wait_cnt;
 
-       /* Ensure that the wait list checks occur after clear_bit(). */
-       smp_mb();
+       /*
+        * Pairs with the memory barrier in set_current_state() to ensure the
+        * proper ordering of clear_bit()/waitqueue_active() in the waker and
+        * test_and_set_bit()/prepare_to_wait()/finish_wait() in the waiter. See
+        * the comment on waitqueue_active(). This is __after_atomic because we
+        * just did clear_bit() in the caller.
+        */
+       smp_mb__after_atomic();
 
        ws = sbq_wake_ptr(sbq);
        if (!ws)
@@ -331,7 +337,8 @@ void sbitmap_queue_wake_all(struct sbitmap_queue *sbq)
        int i, wake_index;
 
        /*
-        * Make sure all changes prior to this are visible from other CPUs.
+        * Pairs with the memory barrier in set_current_state() like in
+        * sbq_wake_up().
         */
        smp_mb();
        wake_index = atomic_read(&sbq->wake_index);