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

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Date:   Sat, 12 Nov 2016 13:12:29 +0100
From:   Mike Galbraith <efault@....de>
To:     Peter Zijlstra <peterz@...radead.org>,
        Oleg Nesterov <oleg@...hat.com>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        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, 2016-11-09 at 18:50 +0100, Peter Zijlstra wrote:
> 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.

Yeah, I've forgotten nearly everything about it too.  I do recall that
the autogroup always existed for both manual on/off and migrations
cgroup <--> root.

I think autogroup should go away.  Systemthing infestation is nearly
everywhere these days, and it wants to do ever more stupid shite that
will render it useless anyway.

	-Mike