Re: [PATCH] mm/mempolicy: fix sleeping function called from invalid context

Li Zefan
2014-06-24T02:28:12Z

On 2014/6/21 5:01, Tejun Heo wrote:
> Hello, Li.
>
> Sorry about the long delay.
>
> On Tue, Jun 10, 2014 at 10:58:45AM +0800, Li Zefan wrote:
>> Yes, this is a long-standing issue. Besides the race you described, the child
>> task's mems_allowed can be wrong if the cpuset's nodemask changes before the
>> child has been added to the cgroup's tasklist.
>>
>> I remember Tejun once said he wanted to disallow task migration between
>> cgroups during fork, and that should fix this problem.
>
> I'm having trouble remembering but yeah enforcing stricter behavior
> across fork could be beneficial. Hmmm... the problem with making
> forks exclusive against migrations is that we'll end up adding more
> locking to the fork path which isn't too nice.
>
> Hmmm... other controllers (cgroup_freezer) can reliably synchronize
> the child's state to the cgroup it belongs to. Why can't cpuset? Is
> there something fundamentally missing in the cgroup API?
>

cgroup_freezer uses the fork callback. We can also do this for cpuset as
suggested by David, which adds a little bit overhead to the fork path.

David, care to send out a patch?

>>> It needs to be slightly rewritten to work properly without negatively
>>> impacting the latency of fork(). Do you have the cycles to do it?
>>>
>>
>> Sounds you have other idea?
>
> I don't think the suggested patch breaks anything more than it was
> broken before and we should probably apply it for the time being. Li?
>

Yeah, we should apply Gu Zheng's patch any way.