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

[Frederic Weisbecker <fweisbec-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

2012/10/20 Frederic Weisbecker <fweisbec-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>:
> 2012/10/19 Tejun Heo <tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>:
>> Hello, Frederic.
>>
>> On Fri, Oct 19, 2012 at 03:44:20PM -0400, Frederic Weisbecker wrote:
>>> > For -stable, I think it's better to revert.  If you want to remove
>>> > task_lock, let's do it for 3.8.
>>>
>>> I don't think that a wrong comment justifies a patch to stable.
>>
>> I'm not really sure whether it's safe or not.  It seems all usages are
>> protected by write locking css_set_lock but maybe I'm missing
>> something and as the commit is born out of confusion, I'm very
>> inclined to revert it by default.  Are you sure this one is safe?
>
> Thinking about it further, one scenario is worrying me but it
> eventually looks safe but by accident.
>
> 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)

gmail mangled everything :(