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

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Message-ID: <alpine.DEB.2.02.1406101512340.32203@chino.kir.corp.google.com>
Date:	Tue, 10 Jun 2014 15:16:10 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Li Zefan <lizefan@...wei.com>
cc:	Gu Zheng <guz.fnst@...fujitsu.com>,
	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
Subject: Re: [PATCH] mm/mempolicy: fix sleeping function called from invalid
 context

On Tue, 10 Jun 2014, Li Zefan wrote:

> > 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.
>  

Ok, I don't want to fix it in cpusets if cgroups will eventually prevent 
it, so I need an understanding of the long term plan.  Will cgroups 
continue to allow migration during fork(), Tejun?

> > 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?
> 

It wouldn't be too difficult with a cgroup post fork callback into the 
cpuset code to rebind the nodemask if it has changed, but with my above 
concern those might be yanked out eventually :)
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