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From: Lino Sanfilippo <LinoSanfilippo@gmx.de>
To: Jan Kara <jack@suse.cz>, Andrew Morton <akpm@linux-foundation.org>
Cc: linux-fsdevel@vger.kernel.org,
	Miklos Szeredi <mszeredi@redhat.com>,
	Eric Paris <eparis@redhat.com>, Al Viro <viro@ZenIV.linux.org.uk>
Subject: Re: [PATCH 6/6] fanotify: Fix possible false warning when freeing events
Date: Wed, 14 Sep 2016 19:14:31 +0200
Message-ID: <9f2f7c99-3994-4e7c-247c-9b2ce7bc8f46@gmx.de> (raw)
In-Reply-To: <1473797711-14111-7-git-send-email-jack@suse.cz>

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>

      reply index

Thread overview: 11+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-09-13 20:15 [PATCH 0/6 v2] fsnotify: Fix list corruption for permission events Jan Kara
2016-09-13 20:15 ` [PATCH 1/6] fsnotify: Add a way to stop queueing events on group shutdown Jan Kara
2016-09-13 20:15 ` [PATCH 2/6] fanotify: Fix list corruption in fanotify_get_response() Jan Kara
2016-09-13 20:15 ` [PATCH 3/6] fsnotify: Drop notification_mutex before destroying event Jan Kara
2016-09-14 17:11   ` Lino Sanfilippo
2016-09-13 20:15 ` [PATCH 4/6] fsnotify: Convert notification_mutex to a spinlock Jan Kara
2016-09-14 17:13   ` Lino Sanfilippo
2016-09-13 20:15 ` [PATCH 5/6] fanotify: Use notification_lock instead of access_lock Jan Kara
2016-09-14 17:14   ` Lino Sanfilippo
2016-09-13 20:15 ` [PATCH 6/6] fanotify: Fix possible false warning when freeing events Jan Kara
2016-09-14 17:14   ` Lino Sanfilippo [this message]

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