diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index 295795a..379526f 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -802,6 +802,9 @@
 	limit = btrfs_async_submit_limit(fs_info);
 	limit = limit * 2 / 3;
 
+	/*
+	 * atomic_dec_return implies a barrier for waitqueue_active
+	 */
 	if (atomic_dec_return(&fs_info->nr_async_submits) < limit &&
 	    waitqueue_active(&fs_info->async_submit_wait))
 		wake_up(&fs_info->async_submit_wait);