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

[Frederic Weisbecker]

2012/10/21 Tejun Heo <tj@xxxxxxxxxx>:
> 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. :)

Sorry and I'm currently stuck in some airport and too lazy to reorder
the above lines :)

>
>> 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.

Agreed. and Acked-by: Frederic Weisbecker <fweisbec@xxxxxxxxx>
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