linux-kernel - Re: [PATCH 3/5] mempolicy: fix a race in shared_policy

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Date:	Fri, 07 Sep 2012 18:59:40 -0400
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	mgorman@...e.de
CC:	akpm@...ux-foundation.org, kosaki.motohiro@...fujitsu.com,
	davej@...hat.com, cl@...ux.com, ben@...adent.org.uk,
	ak@...ux.intel.com, hughd@...gle.com, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH 3/5] mempolicy: fix a race in shared_policy_replace()

First off, thank you very much for reworking this for me. I haven't got a chance
to get a test machine for this.

> shared_policy_replace() use of sp_alloc() is unsafe. 1) sp_node cannot
> be dereferenced if sp->lock is not held and 2) another thread can modify
> sp_node between spin_unlock for allocating a new sp node and next spin_lock.
> The bug was introduced before 2.6.12-rc2.
> 
> Kosaki's original patch for this problem was to allocate an sp node and policy
> within shared_policy_replace and initialise it when the lock is reacquired. I
> was not keen on this approach because it partially duplicates sp_alloc(). As
> the paths were sp->lock is taken are not that performance critical this
> patch converts sp->lock to sp->mutex so it can sleep when calling sp_alloc().

Looks make sense.

Acked-by: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>


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