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

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Date:	Tue, 28 Oct 2014 00:04:27 +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 10/27, Kirill Tkhai wrote:
>
> +static void cpu_cgroup_fork(struct task_struct *task)
> +{
> +	sched_move_task(task);
> +}
> +
>  static int cpu_cgroup_can_attach(struct cgroup_subsys_state *css,
>  				 struct cgroup_taskset *tset)
>  {
> @@ -8205,6 +8210,7 @@ struct cgroup_subsys cpu_cgrp_subsys = {
>  	.css_free	= cpu_cgroup_css_free,
>  	.css_online	= cpu_cgroup_css_online,
>  	.css_offline	= cpu_cgroup_css_offline,
> +	.fork		= cpu_cgroup_fork,

Agreed, but it seems that sched_move_task() -> task_css_check() can
complain if CONFIG_PROVE_RCU...

cpu_cgroup_exit() too calls sched_move_task() without any lock, but
there is the PF_EXITING check and init_css_set can't go away.

perhaps sched_move_task() should just take rcu_read_lock() and use
task_css() ? This lockdep_is_held(siglock) looks ugly, and iiuc we
need it to shut up the warning if autogroup_move_group() is the caller.

Oleg.

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