[patch 004/127] fanotify: fix possible false warning when freeing events

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From: Jan Kara <jack@xxxxxxx>
Subject: fanotify: fix possible false warning when freeing events

When freeing permission events by fsnotify_destroy_event(), the warning
WARN_ON(!list_empty(&event->list)); may falsely hit.  This is because
although fanotify_get_response() saw event->response set, there is nothing
to make sure the current CPU also sees the removal of the event from the
list.  Add proper locking around the WARN_ON() to avoid the false warning.

Link: http://lkml.kernel.org/r/1473797711-14111-7-git-send-email-jack@xxxxxxx
Reported-by: Miklos Szeredi <mszeredi@xxxxxxxxxx>
Signed-off-by: Jan Kara <jack@xxxxxxx>
Reviewed-by: Lino Sanfilippo <LinoSanfilippo@xxxxxx>
Cc: Eric Paris <eparis@xxxxxxxxxx>
Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---

 fs/notify/notification.c |   13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff -puN fs/notify/notification.c~fanotify-fix-possible-false-warning-when-freeing-events fs/notify/notification.c
--- a/fs/notify/notification.c~fanotify-fix-possible-false-warning-when-freeing-events
+++ a/fs/notify/notification.c
@@ -74,8 +74,17 @@ void fsnotify_destroy_event(struct fsnot
 	/* Overflow events are per-group and we don't want to free them */
 	if (!event || event->mask == FS_Q_OVERFLOW)
 		return;
-	/* If the event is still queued, we have a problem... */
-	WARN_ON(!list_empty(&event->list));
+	/*
+	 * If the event is still queued, we have a problem... Do an unreliable
+	 * lockless check first to avoid locking in the common case. The
+	 * locking may be necessary for permission events which got removed
+	 * from the list by a different CPU than the one freeing the event.
+	 */
+	if (!list_empty(&event->list)) {
+		spin_lock(&group->notification_lock);
+		WARN_ON(!list_empty(&event->list));
+		spin_unlock(&group->notification_lock);
+	}
 	group->ops->free_event(event);
 }
 
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