Re: [PATCH 6/6] fanotify: Fix possible false warning when freeing events

[Lino Sanfilippo <LinoSanfilippo@gmx.de>]

On 13.09.2016 22:15, Jan Kara wrote:
> 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.
> 
> Reported-by: Miklos Szeredi <mszeredi@redhat.com>
> Signed-off-by: Jan Kara <jack@suse.cz>
> ---
>  fs/notify/notification.c | 13 +++++++++++--
>  1 file changed, 11 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/notify/notification.c b/fs/notify/notification.c
> index 070d255b24a2..6b7f430bb2de 100644
> --- a/fs/notify/notification.c
> +++ b/fs/notify/notification.c
> @@ -73,8 +73,17 @@ void fsnotify_destroy_event(struct fsnotify_group *group,
>  	/* 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);
>  }
>  
> 

Reviewed-by: Lino Sanfilippo <LinoSanfilippo@gmx.de>