From mboxrd@z Thu Jan  1 00:00:00 1970
From: Li Zefan <lizefan-hv44wF8Li93QT0dZR+AlfA@public.gmane.org>
Subject: Re: [PATCH] cgroup: missing rcu read lock around task_css_set
Date: Wed, 5 Mar 2014 08:57:06 +0800
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To: Tejun Heo <tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
Cc: Sasha Levin <sasha.levin-QHcLZuEGTsvQT0dZR+AlfA@public.gmane.org>, containers-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org

On 2014/3/5 3:47, Tejun Heo wrote:
> On Tue, Mar 04, 2014 at 12:20:45PM -0500, Sasha Levin wrote:
>>> 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.
>>
>> Hi Tejun,
>>
>> It bisects down to your patch: "cgroup: drop task_lock() protection
>> around task->cgroups". I'll look into it later unless it's obvious
>> to you.
> 
> Hmmm... maybe I'm confused and PF_EXITING is not set there and
> task_lock was what held off the lockdep warning.  Confused....
> 

Because this cgroup_exit() is called in a failure path in copy_process().