Re: [patch 1/2] mm: memcontrol: fix lockless reclaim hierarchyiterator

From: Tejun Heo
Date: Wed Jun 05 2013 - 19:06:37 EST

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@xxxxxxxxxx>
> Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: stable@xxxxxxxxxx [3.10+]

Reviewed-by: Tejun Heo <tj@xxxxxxxxxx>

Oops, right, the references were reversed too.


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