fsnotify: lock annotation for event replacement

fsnotify_replace_event need to lock both the old and the new event.  This
causes lockdep to get all pissed off since it dosn't know this is safe.
It's safe in this case since the new event is impossible to be reached from
other places in the kernel.
Signed-off-by: NEric Paris <eparis@redhat.com>

fs/notify/notification.c

@@ -289,43 +289,28 @@ static void initialize_event(struct fsnotify_event *event)
 
 /*
  * Caller damn well better be holding whatever mutex is protecting the
- * old_holder->event_list.
+ * old_holder->event_list and the new_event must be a clean event which
+ * cannot be found anywhere else in the kernel.
  */
 int fsnotify_replace_event(struct fsnotify_event_holder *old_holder,
 			   struct fsnotify_event *new_event)
 {
 	struct fsnotify_event *old_event = old_holder->event;
-	struct fsnotify_event_holder *new_holder = NULL;
+	struct fsnotify_event_holder *new_holder = &new_event->holder;
+
+	enum event_spinlock_class {
+		SPINLOCK_OLD,
+		SPINLOCK_NEW,
+	};
 
 	/*
-	 * There is one fsnotify_event_holder embedded inside each fsnotify_event.
-	 * Check if we expect to be able to use that holder.  If not alloc a new
-	 * holder.
-	 * For the overflow event it's possible that something will use the in
-	 * event holder before we get the lock so we may need to jump back and
-	 * alloc a new holder, this can't happen for most events...
+	 * if the new_event's embedded holder is in use someone
+	 * screwed up and didn't give us a clean new event.
 	 */
-	if (!list_empty(&new_event->holder.event_list)) {
-alloc_holder:
-		new_holder = fsnotify_alloc_event_holder();
-		if (!new_holder)
-			return -ENOMEM;
-	}
+	BUG_ON(!list_empty(&new_holder->event_list));
 
-	spin_lock(&old_event->lock);
-	spin_lock(&new_event->lock);
-
-	if (list_empty(&new_event->holder.event_list)) {
-		if (unlikely(new_holder))
-			fsnotify_destroy_event_holder(new_holder);
-		new_holder = &new_event->holder;
-	} else if (unlikely(!new_holder)) {
-		/* between the time we checked above and got the lock the in
-		 * event holder was used, go back and get a new one */
-		spin_unlock(&new_event->lock);
-		spin_unlock(&old_event->lock);
-		goto alloc_holder;
-	}
+	spin_lock_nested(&old_event->lock, SPINLOCK_OLD);
+	spin_lock_nested(&new_event->lock, SPINLOCK_NEW);
 
 	new_holder->event = new_event;
 	list_replace_init(&old_holder->event_list, &new_holder->event_list);