btrfs: comment waitqueue_active implied by locks

Suggested-by: Chris Mason <clm@fb.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/raid56.c

@@ -810,6 +810,10 @@ static noinline void unlock_stripe(struct btrfs_raid_bio *rbio)
 			}
 
 			goto done_nolock;
+			/*
+			 * The barrier for this waitqueue_active is not needed,
+			 * we're protected by h->lock and can't miss a wakeup.
+			 */
 		} else if (waitqueue_active(&h->wait)) {
 			spin_unlock(&rbio->bio_list_lock);
 			spin_unlock_irqrestore(&h->lock, flags);

fs/btrfs/tree-log.c

@@ -2950,6 +2950,9 @@ int btrfs_sync_log(struct btrfs_trans_handle *trans,
 	atomic_set(&log_root_tree->log_commit[index2], 0);
 	mutex_unlock(&log_root_tree->log_mutex);
 
+	/*
+	 * The barrier before waitqueue_active is implied by mutex_unlock
+	 */
 	if (waitqueue_active(&log_root_tree->log_commit_wait[index2]))
 		wake_up(&log_root_tree->log_commit_wait[index2]);
 out:
@@ -2961,6 +2964,9 @@ int btrfs_sync_log(struct btrfs_trans_handle *trans,
 	atomic_set(&root->log_commit[index1], 0);
 	mutex_unlock(&root->log_mutex);
 
+	/*
+	 * The barrier before waitqueue_active is implied by mutex_unlock
+	 */
 	if (waitqueue_active(&root->log_commit_wait[index1]))
 		wake_up(&root->log_commit_wait[index1]);
 	return ret;