linux-kernel - Re: [patch 1/2] mm: memcontrol: fix lockless reclaim hierarchy iterator

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Message-ID: <20130605230625.GN10693@mtj.dyndns.org>
Date:	Wed, 5 Jun 2013 16:06:25 -0700
From:	Tejun Heo <tj@...nel.org>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Michal Hocko <mhocko@...e.cz>, linux-mm@...ck.org,
	cgroups@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [patch 1/2] mm: memcontrol: fix lockless reclaim hierarchy
 iterator

On Wed, Jun 05, 2013 at 06:53:45PM -0400, Johannes Weiner wrote:
> The lockless reclaim hierarchy iterator currently has a misplaced
> barrier that can lead to use-after-free crashes.
> 
> The reclaim hierarchy iterator consist of a sequence count and a
> position pointer that are read and written locklessly, with memory
> barriers enforcing ordering.
> 
> The write side sets the position pointer first, then updates the
> sequence count to "publish" the new position.  Likewise, the read side
> must read the sequence count first, then the position.  If the
> sequence count is up to date, it's guaranteed that the position is up
> to date as well:
> 
>   writer:                         reader:
>   iter->position = position       if iter->sequence == expected:
>   smp_wmb()                           smp_rmb()
>   iter->sequence = sequence           position = iter->position
> 
> However, the read side barrier is currently misplaced, which can lead
> to dereferencing stale position pointers that no longer point to valid
> memory.  Fix this.
> 
> Reported-by: Tejun Heo <tj@...nel.org>
> Signed-off-by: Johannes Weiner <hannes@...xchg.org>
> Cc: stable@...nel.org [3.10+]

Reviewed-by: Tejun Heo <tj@...nel.org>

Oops, right, the references were reversed too.

Thanks.

-- 
tejun
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