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

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Date:	Tue, 28 Oct 2014 23:52:50 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Kirill Tkhai <ktkhai@...allels.com>
Cc:	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>,
	Kirill Tkhai <tkhai@...dex.ru>
Subject: Re: [PATCH] sched: Fix race between task_group and sched_task_group

On 10/28, Kirill Tkhai wrote:
>
> Shouldn't we do that in separate patch? How about this?

Up to Peter, but I think a separate patch is fine.

> [PATCH]sched: Remove lockdep check in sched_move_task()
>
> sched_move_task() is the only interface to change sched_task_group:
> cpu_cgrp_subsys methods and autogroup_move_group() use it.

Yes, but...

> Everything is synchronized by task_rq_lock(), so cpu_cgroup_attach()
> is ordered with other users of sched_move_task(). This means we do
> no need RCU here: if we've dereferenced a tg here, the .attach method
> hasn't been called for it yet.
>
> Thus, we should pass "true" to task_css_check() to silence lockdep
> warnings.

In theory, I am not sure.

However, I never really understood this code and today I forgot everything,
please correct me.

> @@ -7403,8 +7403,12 @@ void sched_move_task(struct task_struct *tsk)
>  	if (unlikely(running))
>  		put_prev_task(rq, tsk);
>
> -	tg = container_of(task_css_check(tsk, cpu_cgrp_id,
> -				lockdep_is_held(&tsk->sighand->siglock)),
> +	/*
> +	 * All callers are synchronized by task_rq_lock(); we do not use RCU
> +	 * which is pointless here. Thus, we pass "true" to task_css_check()
> +	 * to prevent lockdep warnings.
> +	 */
> +	tg = container_of(task_css_check(tsk, cpu_cgrp_id, true),
>  			  struct task_group, css);

Why this can't race with cgroup_task_migrate() if it is called by
cgroup_post_fork() ?

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.

task_group = tsk->cgroups[cpu_cgrp_id] can't go away because yes, if we
race with migrate then ->attach() was not called. But it seems that in
theory it is not safe to dereference tsk->cgroups.

Oleg.

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