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

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Date:	Mon, 9 Jun 2014 17:58:17 +0800
From:	Gu Zheng <guz.fnst@...fujitsu.com>
To:	David Rientjes <rientjes@...gle.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Tejun Heo <tj@...nel.org>, <linux-mm@...ck.org>,
	Cgroups <cgroups@...r.kernel.org>, <stable@...r.kernel.org>,
	Li Zefan <lizefan@...wei.com>
Subject: Re: [PATCH] mm/mempolicy: fix sleeping function called from invalid
 context

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
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