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Date:	Wed, 29 Oct 2014 20:21:42 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Kirill Tkhai <tkhai@...dex.ru>
Cc:	Kirill Tkhai <ktkhai@...allels.com>, linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Burke Libbey <burke.libbey@...pify.com>,
	Vladimir Davydov <vdavydov@...allels.com>
Subject: Re: [PATCH] sched: Fix race between task_group and sched_task_group

On 10/29, Kirill Tkhai wrote:
>
> On 29.10.2014 01:52, Oleg Nesterov wrote:
>
> > And cgroup_task_migrate() can free ->cgroups via call_rcu(). Of course,
> > in practice raw_spin_lock_irq() should also act as rcu_read_lock(), but
> > we should not rely on implementation details.
>
> Do you mean cgroup_task_migrate()->put_css_set_locked()? It's not
> possible there, because old_cset->refcount is lager than 1. We increment
> it in cgroup_migrate_add_src() and real freeing happens in
> cgroup_migrate_finish(). These functions are around task_migrate(), they
> are pair brackets.

Ah, I see.

> If we've got old_cset in
> cgroup_post_fork()->sched_move_task(), the right sched_task_group will
> be installed by attach->sched_move_task().

Yes, yes, this part is clear.

OK, I think the patch is fine then.

Thanks!

Oleg.

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