fsnotify: allow fsnotify_{peek,remove}_first_event with empty queue

Current code has an assumtion that fsnotify_notify_queue_is_empty() is
called to verify that queue is not empty before trying to peek or remove
an event from queue.

Remove this assumption by moving the fsnotify_notify_queue_is_empty()
into the functions, allow them to return NULL value and check return
value by all callers.

This is a prep patch for multi event queues.

Link: https://lore.kernel.org/r/20210304104826.3993892-2-amir73il@gmail.com
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>

fs/notify/fanotify/fanotify_user.c

@@ -100,24 +100,30 @@ static struct fanotify_event *get_one_event(struct fsnotify_group *group,
 {
 	size_t event_size = FAN_EVENT_METADATA_LEN;
 	struct fanotify_event *event = NULL;
+	struct fsnotify_event *fsn_event;
 	unsigned int fid_mode = FAN_GROUP_FLAG(group, FANOTIFY_FID_BITS);
 
 	pr_debug("%s: group=%p count=%zd\n", __func__, group, count);
 
 	spin_lock(&group->notification_lock);
-	if (fsnotify_notify_queue_is_empty(group))
+	fsn_event = fsnotify_peek_first_event(group);
+	if (!fsn_event)
 		goto out;
 
-	if (fid_mode) {
-		event_size += fanotify_event_info_len(fid_mode,
-			FANOTIFY_E(fsnotify_peek_first_event(group)));
-	}
+	event = FANOTIFY_E(fsn_event);
+	if (fid_mode)
+		event_size += fanotify_event_info_len(fid_mode, event);
 
 	if (event_size > count) {
 		event = ERR_PTR(-EINVAL);
 		goto out;
 	}
-	event = FANOTIFY_E(fsnotify_remove_first_event(group));
+
+	/*
+	 * Held the notification_lock the whole time, so this is the
+	 * same event we peeked above.
+	 */
+	fsnotify_remove_first_event(group);
 	if (fanotify_is_perm_event(event->mask))
 		FANOTIFY_PERM(event)->state = FAN_EVENT_REPORTED;
 out:
@@ -573,6 +579,7 @@ static ssize_t fanotify_write(struct file *file, const char __user *buf, size_t
 static int fanotify_release(struct inode *ignored, struct file *file)
 {
 	struct fsnotify_group *group = file->private_data;
+	struct fsnotify_event *fsn_event;
 
 	/*
 	 * Stop new events from arriving in the notification queue. since
@@ -601,13 +608,12 @@ static int fanotify_release(struct inode *ignored, struct file *file)
 	 * dequeue them and set the response. They will be freed once the
 	 * response is consumed and fanotify_get_response() returns.
 	 */
-	while (!fsnotify_notify_queue_is_empty(group)) {
-		struct fanotify_event *event;
+	while ((fsn_event = fsnotify_remove_first_event(group))) {
+		struct fanotify_event *event = FANOTIFY_E(fsn_event);
 
-		event = FANOTIFY_E(fsnotify_remove_first_event(group));
 		if (!(event->mask & FANOTIFY_PERM_EVENTS)) {
 			spin_unlock(&group->notification_lock);
-			fsnotify_destroy_event(group, &event->fse);
+			fsnotify_destroy_event(group, fsn_event);
 		} else {
 			finish_permission_event(group, FANOTIFY_PERM(event),
 						FAN_ALLOW);

fs/notify/inotify/inotify_user.c

@@ -146,10 +146,9 @@ static struct fsnotify_event *get_one_event(struct fsnotify_group *group,
 	size_t event_size = sizeof(struct inotify_event);
 	struct fsnotify_event *event;
 
-	if (fsnotify_notify_queue_is_empty(group))
-		return NULL;
-
 	event = fsnotify_peek_first_event(group);
+	if (!event)
+		return NULL;
 
 	pr_debug("%s: group=%p event=%p\n", __func__, group, event);
 

fs/notify/notification.c

@@ -47,13 +47,6 @@ u32 fsnotify_get_cookie(void)
 }
 EXPORT_SYMBOL_GPL(fsnotify_get_cookie);
 
-/* return true if the notify queue is empty, false otherwise */
-bool fsnotify_notify_queue_is_empty(struct fsnotify_group *group)
-{
-	assert_spin_locked(&group->notification_lock);
-	return list_empty(&group->notification_list) ? true : false;
-}
-
 void fsnotify_destroy_event(struct fsnotify_group *group,
 			    struct fsnotify_event *event)
 {
@@ -141,33 +134,36 @@ void fsnotify_remove_queued_event(struct fsnotify_group *group,
 }
 
 /*
- * Remove and return the first event from the notification list.  It is the
- * responsibility of the caller to destroy the obtained event
+ * Return the first event on the notification list without removing it.
+ * Returns NULL if the list is empty.
  */
-struct fsnotify_event *fsnotify_remove_first_event(struct fsnotify_group *group)
+struct fsnotify_event *fsnotify_peek_first_event(struct fsnotify_group *group)
 {
-	struct fsnotify_event *event;
-
 	assert_spin_locked(&group->notification_lock);
 
-	pr_debug("%s: group=%p\n", __func__, group);
+	if (fsnotify_notify_queue_is_empty(group))
+		return NULL;
 
-	event = list_first_entry(&group->notification_list,
-				 struct fsnotify_event, list);
-	fsnotify_remove_queued_event(group, event);
-	return event;
+	return list_first_entry(&group->notification_list,
+				struct fsnotify_event, list);
 }
 
 /*
- * This will not remove the event, that must be done with
- * fsnotify_remove_first_event()
+ * Remove and return the first event from the notification list.  It is the
+ * responsibility of the caller to destroy the obtained event
  */
-struct fsnotify_event *fsnotify_peek_first_event(struct fsnotify_group *group)
+struct fsnotify_event *fsnotify_remove_first_event(struct fsnotify_group *group)
 {
-	assert_spin_locked(&group->notification_lock);
+	struct fsnotify_event *event = fsnotify_peek_first_event(group);
 
-	return list_first_entry(&group->notification_list,
-				struct fsnotify_event, list);
+	if (!event)
+		return NULL;
+
+	pr_debug("%s: group=%p event=%p\n", __func__, group, event);
+
+	fsnotify_remove_queued_event(group, event);
+
+	return event;
 }
 
 /*

include/linux/fsnotify_backend.h

@@ -495,7 +495,13 @@ static inline void fsnotify_queue_overflow(struct fsnotify_group *group)
 	fsnotify_add_event(group, group->overflow_event, NULL);
 }
 
-/* true if the group notification queue is empty */
+static inline bool fsnotify_notify_queue_is_empty(struct fsnotify_group *group)
+{
+	assert_spin_locked(&group->notification_lock);
+
+	return list_empty(&group->notification_list);
+}
+
 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_first_event(struct fsnotify_group *group);