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

Gu Zheng
2014-06-09T09:58:17Z

Hi David,

On 06/09/2014 05:13 PM, David Rientjes wrote:

> On Mon, 9 Jun 2014, Gu Zheng wrote:
>
>>> I think your patch addresses the problem that you're reporting but misses
>>> the larger problem with cpuset.mems rebinding on fork(). When the
>>> forker's task_struct is duplicated (which includes ->mems_allowed) and it
>>> races with an update to cpuset_being_rebound in update_tasks_nodemask()
>>> then the task's mems_allowed doesn't get updated.
>>
>> Yes, you are right, this patch just wants to address the bug reported above.
>> The race condition you mentioned above inherently exists there, but it is yet
>> another issue, the rcu lock here makes no sense to it, and I think we need
>> additional sync-mechanisms if want to fix it.
>
> Yes, the rcu lock is not providing protection for any critical section
> here that requires (1) the forker's cpuset to be stored in
> cpuset_being_rebound or (2) the forked thread's cpuset to be rebound by
> the cpuset nodemask update, and no race involving the two.
>
>> But thinking more, though the current implementation has flaw, but I worry
>> about the negative effect if we really want to fix it. Or maybe the fear
>> is unnecessary.:)
>>
>
> It needs to be slightly rewritten to work properly without negatively
> impacting the latency of fork(). Do you have the cycles to do it?
>

To be honest, I'm busy with other schedule. And if you(or other
guys) have proper proposal, please go ahead.

To Tejun, Li and Andrew:
Any comment? Or could you apply this *bug fix* first?

Regards,
Gu