linux-kernel - Re: [PATCH] sched: Fix race between task_group and sched_task

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Message-ID: <20141027122146.GC3337@twins.programming.kicks-ass.net>
Date:	Mon, 27 Oct 2014 13:21:46 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Kirill Tkhai <ktkhai@...allels.com>
Cc:	linux-kernel@...r.kernel.org, Oleg Nesterov <oleg@...hat.com>,
	Ingo Molnar <mingo@...hat.com>,
	Burke Libbey <burke.libbey@...pify.com>,
	Vladimir Davydov <vdavydov@...allels.com>,
	Kirill Tkhai <tkhai@...dex.ru>
Subject: Re: [PATCH] sched: Fix race between task_group and sched_task_group

On Mon, Oct 27, 2014 at 02:18:25PM +0400, Kirill Tkhai wrote:
> 
> The race may happen when somebody is changing task_group of a forking task.
> Child's cgroup is the same as parent's after dup_task_struct() (there just
> memory copying). Also, cfs_rq and rt_rq are the same as parent's.
> 
> But if parent changes its task_group before it's called cgroup_post_fork(),
> we do not reflect this situation on child. Child's cfs_rq and rt_rq remain
> the same, while child's task_group changes in cgroup_post_fork().
> 
> To fix this we introduce fork() method, which calls sched_move_task() directly.
> This function changes sched_task_group on appropriate (also its logic has
> no problem with freshly created tasks, so we shouldn't introduce something
> special; we are able just to use it).

Right, I read some of that cgroup.c stuff and this is indeed possible,
yucky. Applied, thanks!
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