linux-kernel - Re: [PATCH cgroup/for-3.7-fixes 1/2] Revert "cgroup: Remove task_lock() from cgroup_post

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Message-ID: <20121020223709.GA5626@htj.dyndns.org>
Date:	Sat, 20 Oct 2012 15:37:09 -0700
From:	Tejun Heo <tj@...nel.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Li Zefan <lizefan@...wei.com>,
	containers@...ts.linux-foundation.org, cgroups@...r.kernel.org,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH cgroup/for-3.7-fixes 1/2] Revert "cgroup: Remove
 task_lock() from cgroup_post_fork()"

Hello, Frederic.

On Sat, Oct 20, 2012 at 02:21:43PM -0400, Frederic Weisbecker wrote:
> CPU 0
>                     CPU 1
> 
> cgroup_task_migrate {
>         task_lock(p)
>         rcu_assign_pointer(tsk->cgroups, newcg);
>         task_unlock(tsk);
> 
>         write_lock(&css_set_lock);
>         if (!list_empty(&tsk->cg_list))
>             list_move(&tsk->cg_list, &newcg->tasks);
>         write_unlock(&css_set_lock);
> 
>                           write_lock(&css_set_lock);
> 	put_css_set(oldcg);
>          list_add(&child->cg_list, &child->cgroups->tasks); (1)

Man, that's confusing. :)

> On (1), child->cgroups should have the value of newcg and not oldcg
> due to the memory ordering implied by the locking of css_set_lock. Now
> I can't guarantee that because I'm no memory ordering expert. And even
> if it's safe, it's so very non obvious that I now agree with you:
> let's revert  the patch and restart with a better base by gathering
> all the cgroup fork code in the current cgroup_post_fork place.

Aye aye, let's move everything to cgroup_post_fork() and then we don't
have to worry about grabbing task_lock multiple times.

Thanks.

-- 
tejun
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