linux-kernel - Re: [PATCH v2] cgroup: fix cgroup

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Date:	Mon, 18 Feb 2013 12:36:13 +0200
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	Li Zefan <lizefan@...wei.com>
Cc:	Tejun Heo <tj@...nel.org>, Cgroups <cgroups@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] cgroup: fix cgroup_rmdir() vs close(eventfd) race

On Mon, Feb 18, 2013 at 02:12:23PM +0800, Li Zefan wrote:
> commit 205a872bd6f9a9a09ef035ef1e90185a8245cc58 ("cgroup: fix lockdep
> warning for event_control") solved a deadlock by introducing a new
> bug.
> 
> Move cgrp->event_list to a temporary list doesn't mean you can traverse
> this list locklessly, because at the same time cgroup_event_wake() can
> be called and remove the event from the list. The result of this race
> is disastrous.
> 
> We adopt the way how kvm irqfd code implements race-free event removal,
> which is now described in the comments in cgroup_event_wake().
> 
> Signed-off-by: Li Zefan <lizefan@...wei.com>
> ---
>  kernel/cgroup.c | 50 ++++++++++++++++++++++++++++++++++----------------
>  1 file changed, 34 insertions(+), 16 deletions(-)
> 
> diff --git a/kernel/cgroup.c b/kernel/cgroup.c
> index 26c071c..65c8101 100644
> --- a/kernel/cgroup.c
> +++ b/kernel/cgroup.c
> @@ -217,6 +217,10 @@ struct cgroup_event {
>  	 */
>  	struct list_head list;
>  	/*
> +	 * Need to notify userspace when this event is removed?
> +	 */
> +	bool signal_on_remove;
> +	/*
>  	 * All fields below needed to unregister event when
>  	 * userspace closes eventfd.
>  	 */
> @@ -3833,8 +3837,17 @@ static void cgroup_event_remove(struct work_struct *work)
>  			remove);
>  	struct cgroup *cgrp = event->cgrp;
>  
> +	remove_wait_queue(event->wqh, &event->wait);
> +
>  	event->cft->unregister_event(cgrp, event->cft, event->eventfd);
>  
> +	/*
> +	 * If this event is to be removed due to cgroup removal,
> +	 * we notify userspace.
> +	 */
> +	if (event->signal_on_remove)
> +		eventfd_signal(event->eventfd, 1);

It's safe to notify anyway, isn't it? Let's just drop signal_on_remove.

Otherwise, look good.

Acked-by: Kirill A. Shutemov <kirill@...temov.name>

-- 
 Kirill A. Shutemov
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