fsnotify: replace an event on a list

fanotify would like to clone events already on its notification list, make
changes to the new event, and then replace the old event on the list with
the new event.  This patch implements the replace functionality of that
process.
Signed-off-by: Eric Paris <eparis@redhat.com>

fs/notify/notification.c

@@ -287,6 +287,62 @@ static void initialize_event(struct fsnotify_event *event)
 	INIT_LIST_HEAD(&event->private_data_list);
 }
 
+/*
+ * Caller damn well better be holding whatever mutex is protecting the
+ * old_holder->event_list.
+ */
+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;
+
+	/*
+	 * 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 (!list_empty(&new_event->holder.event_list)) {
+alloc_holder:
+		new_holder = fsnotify_alloc_event_holder();
+		if (!new_holder)
+			return -ENOMEM;
+	}
+
+	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;
+	}
+
+	new_holder->event = new_event;
+	list_replace_init(&old_holder->event_list, &new_holder->event_list);
+
+	spin_unlock(&new_event->lock);
+	spin_unlock(&old_event->lock);
+
+	/* event == holder means we are referenced through the in event holder */
+	if (old_holder != &old_event->holder)
+		fsnotify_destroy_event_holder(old_holder);
+
+	fsnotify_get_event(new_event); /* on the list take reference */
+	fsnotify_put_event(old_event); /* off the list, drop reference */
+
+	return 0;
+}
+
 struct fsnotify_event *fsnotify_clone_event(struct fsnotify_event *old_event)
 {
 	struct fsnotify_event *event;

include/linux/fsnotify_backend.h

@@ -365,6 +365,8 @@ extern struct fsnotify_event *fsnotify_create_event(struct inode *to_tell, __u32
 
 /* fanotify likes to change events after they are on lists... */
 extern struct fsnotify_event *fsnotify_clone_event(struct fsnotify_event *old_event);
+extern int fsnotify_replace_event(struct fsnotify_event_holder *old_holder,
+				  struct fsnotify_event *new_event);
 
 #else