linux-kernel - Re: [PATCH] cgroup: missing rcu read lock around task_css

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Date:	Mon, 03 Mar 2014 18:00:25 -0500
From:	Sasha Levin <sasha.levin@...cle.com>
To:	Tejun Heo <tj@...nel.org>
CC:	lizefan@...wei.com, containers@...ts.linux-foundation.org,
	cgroups@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] cgroup: missing rcu read lock around task_css_set

On 03/03/2014 05:45 PM, Tejun Heo wrote:
> On Mon, Mar 03, 2014 at 05:43:11PM -0500, Sasha Levin wrote:
>> On 03/03/2014 05:33 PM, Tejun Heo wrote:
>>> On Sat, Mar 01, 2014 at 10:00:11PM -0500, Sasha Levin wrote:
>>>> rcu read lock should be held when calling and working with task_css_set.
>>>>
>>>> This patch also fixes a related lockdep warning.
>>>
>>> Hmmm... PF_EXITING should be visible at that point and cset can't
>>> change anymore.  We prolly need to update lockdep annotation rather
>>> than adding spurious rcu locking around it.  Against which branch is
>>> it?  Can you please post the lockdep warning?
>>
>> I see it on -next.
>
> I think the right thing to do is using task_css_set_check() with
> PF_EXITING check and add comment explaining that cset can no longer
> change.  Can you please write up a patch?

Hrm... there is a PF_EXITING check there already:

	#define task_css_set_check(task, __c)			\
		rcu_dereference_check((task)->cgroups,		\
		lockdep_is_held(&cgroup_mutex) ||		\
		lockdep_is_held(&css_set_rwsem) ||		\
		((task)->flags & PF_EXITING) || (__c))

I see it's not happening on Linus's master so I'll run a bisection to figure out what broke it.


Thanks,
Sasha
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