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

[Li Zefan]

On 2014/6/9 17:13, 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.
>

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.
 
>> 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?
> 

Sounds you have other idea?

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