fsnotify: allow groups to add private data to events

inotify needs per group information attached to events.  This patch allows
groups to attach private information and implements a callback so that
information can be freed when an event is being destroyed.

Signed-off-by: Eric Paris <eparis@redhat.com>
Acked-by: Al Viro <viro@zeniv.linux.org.uk>
Cc: Christoph Hellwig <hch@lst.de>

fs/notify/dnotify/dnotify.c

@@ -183,6 +183,7 @@ static struct fsnotify_ops dnotify_fsnotify_ops = {
 	.should_send_event = dnotify_should_send_event,
 	.free_group_priv = NULL,
 	.freeing_mark = dnotify_freeing_mark,
+	.free_event_priv = NULL,
 };
 
 /*

fs/notify/notification.c

@@ -90,6 +90,8 @@ void fsnotify_put_event(struct fsnotify_event *event)
 		if (event->data_type == FSNOTIFY_EVENT_PATH)
 			path_put(&event->path);
 
+		BUG_ON(!list_empty(&event->private_data_list));
+
 		kfree(event->file_name);
 		kmem_cache_free(fsnotify_event_cachep, event);
 	}
@@ -106,7 +108,29 @@ void fsnotify_destroy_event_holder(struct fsnotify_event_holder *holder)
 }
 
 /*
- * check if 2 events contain the same information.
+ * Find the private data that the group previously attached to this event when
+ * the group added the event to the notification queue (fsnotify_add_notify_event)
+ */
+struct fsnotify_event_private_data *fsnotify_remove_priv_from_event(struct fsnotify_group *group, struct fsnotify_event *event)
+{
+	struct fsnotify_event_private_data *lpriv;
+	struct fsnotify_event_private_data *priv = NULL;
+
+	assert_spin_locked(&event->lock);
+
+	list_for_each_entry(lpriv, &event->private_data_list, event_list) {
+		if (lpriv->group == group) {
+			priv = lpriv;
+			list_del(&priv->event_list);
+			break;
+		}
+	}
+	return priv;
+}
+
+/*
+ * Check if 2 events contain the same information.  We do not compare private data
+ * but at this moment that isn't a problem for any know fsnotify listeners.
  */
 static bool event_compare(struct fsnotify_event *old, struct fsnotify_event *new)
 {
@@ -134,13 +158,17 @@ static bool event_compare(struct fsnotify_event *old, struct fsnotify_event *new
  * event off the queue to deal with.  If the event is successfully added to the
  * group's notification queue, a reference is taken on event.
  */
-int fsnotify_add_notify_event(struct fsnotify_group *group, struct fsnotify_event *event)
+int fsnotify_add_notify_event(struct fsnotify_group *group, struct fsnotify_event *event,
+			      struct fsnotify_event_private_data *priv)
 {
 	struct fsnotify_event_holder *holder = NULL;
 	struct list_head *list = &group->notification_list;
 	struct fsnotify_event_holder *last_holder;
 	struct fsnotify_event *last_event;
 
+	/* easy to tell if priv was attached to the event */
+	INIT_LIST_HEAD(&priv->event_list);
+
 	/*
 	 * 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
@@ -158,8 +186,11 @@ alloc_holder:
 
 	mutex_lock(&group->notification_mutex);
 
-	if (group->q_len >= group->max_events)
+	if (group->q_len >= group->max_events) {
 		event = &q_overflow_event;
+		/* sorry, no private data on the overflow event */
+		priv = NULL;
+	}
 
 	spin_lock(&event->lock);
 
@@ -183,7 +214,7 @@ alloc_holder:
 			mutex_unlock(&group->notification_mutex);
 			if (holder != &event->holder)
 				fsnotify_destroy_event_holder(holder);
-			return 0;
+			return -EEXIST;
 		}
 	}
 
@@ -192,6 +223,8 @@ alloc_holder:
 
 	fsnotify_get_event(event);
 	list_add_tail(&holder->event_list, list);
+	if (priv)
+		list_add_tail(&priv->event_list, &event->private_data_list);
 	spin_unlock(&event->lock);
 	mutex_unlock(&group->notification_mutex);
 
@@ -252,10 +285,19 @@ struct fsnotify_event *fsnotify_peek_notify_event(struct fsnotify_group *group)
 void fsnotify_flush_notify(struct fsnotify_group *group)
 {
 	struct fsnotify_event *event;
+	struct fsnotify_event_private_data *priv;
 
 	mutex_lock(&group->notification_mutex);
 	while (!fsnotify_notify_queue_is_empty(group)) {
 		event = fsnotify_remove_notify_event(group);
+		/* if they don't implement free_event_priv they better not have attached any */
+		if (group->ops->free_event_priv) {
+			spin_lock(&event->lock);
+			priv = fsnotify_remove_priv_from_event(group, event);
+			spin_unlock(&event->lock);
+			if (priv)
+				group->ops->free_event_priv(priv);
+		}
 		fsnotify_put_event(event); /* matches fsnotify_add_notify_event */
 	}
 	mutex_unlock(&group->notification_mutex);
@@ -274,6 +316,8 @@ static void initialize_event(struct fsnotify_event *event)
 	event->inode = NULL;
 	event->data_type = FSNOTIFY_EVENT_NONE;
 
+	INIT_LIST_HEAD(&event->private_data_list);
+
 	event->to_tell = NULL;
 
 	event->file_name = NULL;

include/linux/fsnotify_backend.h

@@ -63,6 +63,7 @@
 struct fsnotify_group;
 struct fsnotify_event;
 struct fsnotify_mark_entry;
+struct fsnotify_event_private_data;
 
 /*
  * Each group much define these ops.  The fsnotify infrastructure will call
@@ -81,6 +82,7 @@ struct fsnotify_ops {
 	int (*handle_event)(struct fsnotify_group *group, struct fsnotify_event *event);
 	void (*free_group_priv)(struct fsnotify_group *group);
 	void (*freeing_mark)(struct fsnotify_mark_entry *entry, struct fsnotify_group *group);
+	void (*free_event_priv)(struct fsnotify_event_private_data *priv);
 };
 
 /*
@@ -157,6 +159,15 @@ struct fsnotify_event_holder {
 	struct list_head event_list;
 };
 
+/*
+ * Inotify needs to tack data onto an event.  This struct lets us later find the
+ * correct private data of the correct group.
+ */
+struct fsnotify_event_private_data {
+	struct fsnotify_group *group;
+	struct list_head event_list;
+};
+
 /*
  * all of the information about the original object we want to now send to
  * a group.  If you want to carry more info from the accessing task to the
@@ -196,6 +207,8 @@ struct fsnotify_event {
 	u32 sync_cookie;	/* used to corrolate events, namely inotify mv events */
 	char *file_name;
 	size_t name_len;
+
+	struct list_head private_data_list;	/* groups can store private data here */
 };
 
 /*
@@ -294,17 +307,18 @@ extern void fsnotify_put_group(struct fsnotify_group *group);
 /* take a reference to an event */
 extern void fsnotify_get_event(struct fsnotify_event *event);
 extern void fsnotify_put_event(struct fsnotify_event *event);
-/* find private data previously attached to an event */
-extern struct fsnotify_event_private_data *fsnotify_get_priv_from_event(struct fsnotify_group *group,
-									struct fsnotify_event *event);
+/* find private data previously attached to an event and unlink it */
+extern struct fsnotify_event_private_data *fsnotify_remove_priv_from_event(struct fsnotify_group *group,
+									   struct fsnotify_event *event);
 
 /* attach the event to the group notification queue */
-extern int fsnotify_add_notify_event(struct fsnotify_group *group, struct fsnotify_event *event);
+extern int fsnotify_add_notify_event(struct fsnotify_group *group, struct fsnotify_event *event,
+				     struct fsnotify_event_private_data *priv);
 /* true if the group notification queue is empty */
 extern bool fsnotify_notify_queue_is_empty(struct fsnotify_group *group);
 /* return, but do not dequeue the first event on the notification queue */
 extern struct fsnotify_event *fsnotify_peek_notify_event(struct fsnotify_group *group);
-/* reutnr AND dequeue the first event on the notification queue */
+/* return AND dequeue the first event on the notification queue */
 extern struct fsnotify_event *fsnotify_remove_notify_event(struct fsnotify_group *group);
 
 /* functions used to manipulate the marks attached to inodes */