linux-kernel - Re: [PATCH] cgroup_freezer: fix freezing groups with stopped tasks

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Message-ID: <20111122022118.GT25776@google.com>
Date:	Mon, 21 Nov 2011 18:21:18 -0800
From:	Tejun Heo <htejun@...il.com>
To:	Li Zefan <lizf@...fujitsu.com>
Cc:	Michal Hocko <mhocko@...e.cz>, linux-kernel@...r.kernel.org,
	cgroups@...r.kernel.org, containers@...ts.linux-foundation.org,
	Tomasz Buchert <tomasz.buchert@...ia.fr>,
	Paul Menage <paul@...lmenage.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] cgroup_freezer: fix freezing groups with stopped tasks

On Tue, Nov 22, 2011 at 10:20:02AM +0800, Li Zefan wrote:
> Tejun Heo wrote:
> > Hello, Michal.
> > 
> > On Wed, Nov 16, 2011 at 10:50:34PM +0100, Michal Hocko wrote:
> >> +/* Task is frozen or will freeze immediately when next it gets woken */
> >> +static bool is_task_frozen_enough(struct task_struct *task)
> >> +{
> >> +	return frozen(task) ||
> >> +		(task_is_stopped_or_traced(task) && freezing(task));
> >> +}
> > 
> > Hmmm... w/ pending freezer updates, the above would always return
> > %true if there's freezing in progress, which can't be right.  Maybe
> 
> Only if the task is stopped/trace.

You're right, missed parantheses.

> If we try to freeze a stopped task, it will be kept in freezing state.
> 
> > just test stopped/traced?
> 
> This can trigger a BUG_ON in update_if_frozen(), because we always count a
> stopped task as frozen.

So, yeah, the patch looks good to me, but it still isn't difficult to
trigger BUG_ON() there.  We need a lot of fixes in cgroup_freezer.

Thanks.

-- 
tejun
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