linux-kernel - Re: sched/autogroup: race if !sysctl_sched_autogroup

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Message-ID: <20161109175005.GS3142@twins.programming.kicks-ass.net>
Date:   Wed, 9 Nov 2016 18:50:05 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Mike Galbraith <efault@....de>, hartsjc@...hat.com,
        vbendel@...hat.com, vlovejoy@...hat.com,
        linux-kernel@...r.kernel.org
Subject: Re: sched/autogroup: race if !sysctl_sched_autogroup_enabled ?

On Wed, Nov 09, 2016 at 05:59:33PM +0100, Oleg Nesterov wrote:

> We need to ensure that autogroup/tg returned by autogroup_task_group()
> can't go away if we race with autogroup_move_group(), and unless the
> caller holds ->siglock we rely on fact that autogroup_move_group()
> will a) see this task and b) do sched_move_task() which needs the same
> same rq->lock.
> 
> However. autogroup_move_group() skips for_each_thread/sched_move_task
> if sysctl_sched_autogroup_enabled == 0.
> 
> So. Doesn't this mean that cgroup migration to the root cgroup can race
> with autogroup_move_group() and use the soon-to-be-freed autogroup->tg?

Argh, its too late for this, also jet-lag. But maybe, I can sort of feel
a hole here but cannot for the life of me still think.


> although this is a bit off-topic. Another question is that I fail to
> understand why sched_autogroup_create_attach() does autogroup_create()
> and changes signal->autogroup even if !sysctl_sched_autogroup_enabled.

I really cannot remember back that far, but it could be to allow
flipping it back on. Then again, I don't think the fork path puts new
tasks in, even if the autogroup exists.