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

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Message-ID: <alpine.DEB.2.02.1406090209460.24247@chino.kir.corp.google.com>
Date:	Mon, 9 Jun 2014 02:13:57 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Gu Zheng <guz.fnst@...fujitsu.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

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?
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