linux-kernel - Re: Q: cgroup: Questions about possible issues in cgroup locking

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Message-ID: <20111221130848.GA19679@redhat.com>
Date:	Wed, 21 Dec 2011 14:08:48 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Li Zefan <lizf@...fujitsu.com>, Tejun Heo <tj@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Mandeep Singh Baines <msb@...omium.org>,
	Containers <containers@...ts.linux-foundation.org>,
	Cgroups <cgroups@...r.kernel.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Paul Menage <paul@...lmenage.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: Q: cgroup: Questions about possible issues in cgroup locking

On 12/21, Frederic Weisbecker wrote:
> Hi,
>
> Starring at some parts of cgroups, I have a few questions:
>
> - Is cgroup_enable_task_cg_list()'s while_each_thread() safe
> against concurrent exec()? The leader may change in de_thread()
> and invalidate the test done in while_each_thread().

Yes. Oh, we need to do something with while_each_thread.

> - do_each_thread() also needs RCU and cgroup_enable_task_cg_list()
> seems to remind it. But it seems there is at least one caller that
> doesn't call rcu_read_lock(): update_cpu_mask() -> update_tasks_cpumask() -> cgroup_scan_tasks()

I don't see any caller which takes rcu_read_lock...

> - By the time we call cgroup_post_fork(), it is ready to be woken up
> and usable by the scheduler.

No, the new child can't run until do_fork()->wake_up_new_task().

> - Is the check for use_task_css_set_links in cgroup_post_fork() safe? given
> it is checked outside css_set_lock?
>
> Imagine this:
>
> CPU 0                                                        CPU 1
> ----                                                         -----
>
> cgroup_enable_task_cg() {
> 	uset_tasks_css_set_links = 1
> 	for_each_thread() {
> 		add tasks in the list
> 	}
> }
>                                                            do_fork() {
>                                                                cgroup_post_fork() {
>                                                                      use_tasks_css_set_links appears
>                                                                      to be equal to 0 due to write/read
>                                                                       not flushed. New task won't
>                                                                       appear to the list.

Yes, I was thinking about this too.

Or (I think) they can race "contrariwise". CPU_1 creates the new child,
then CPU_0 sets uset_tasks_css_set_links = 1. But afaics there is no any
guarantee that CPU_0 sees the result of list_add_tail_rcu().

Oleg.

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