linux-kernel - Re: [patch for-3.2-rc3] cpusets: stall when updating mems_allowed for mempolicy or disjoint nodemask

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Message-ID: <alpine.DEB.2.00.1111171328120.15918@chino.kir.corp.google.com>
Date:	Thu, 17 Nov 2011 13:33:14 -0800 (PST)
From:	David Rientjes <rientjes@...gle.com>
To:	Miao Xie <miaox@...fujitsu.com>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Paul Menage <paul@...lmenage.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [patch for-3.2-rc3] cpusets: stall when updating mems_allowed
 for mempolicy or disjoint nodemask

On Thu, 17 Nov 2011, Miao Xie wrote:

> Oh~, David
> 
> I find these is another problem, please take account of the following case:
> 
>   2-3 -> 1-2 -> 0-1
> 
> the user change mems_allowed twice continuously, the task may see the empty
> mems_allowed.
> 
> So, it is still dangerous.
> 

With this patch, we're protected by task_lock(tsk) to determine whether we 
want to take the exception, i.e. whether need_loop is false, and the 
setting of tsk->mems_allowed.  So this would see the nodemask change at 
the individual steps from 2-3 -> 1-2 -> 0-1, not some inconsistent state 
in between or directly from 2-3 -> 0-1.  The only time we don't hold 
task_lock(tsk) to change tsk->mems_allowed is when tsk == current and in 
that case we're not concerned about intermediate reads to its own nodemask 
while storing to a mask where MAX_NUMNODES > BITS_PER_LONG.

Thus, there's no problem here with regard to such behavior if we exclude 
mempolicies, which this patch does.
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