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SubjectRe: [patch 1/2] mm: memcontrol: fix lockless reclaim hierarchy iterator
On Wed 05-06-13 18:53:45, 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@kernel.org>
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
> Cc: stable@kernel.org [3.10+]

Acked-by: Michal Hocko <mhocko@suse.cz>

> ---
> mm/memcontrol.c | 12 +++++-------
> 1 file changed, 5 insertions(+), 7 deletions(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 010d6c1..e2cbb44 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -1199,7 +1199,6 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
>
> mz = mem_cgroup_zoneinfo(root, nid, zid);
> iter = &mz->reclaim_iter[reclaim->priority];
> - last_visited = iter->last_visited;
> if (prev && reclaim->generation != iter->generation) {
> iter->last_visited = NULL;
> goto out_unlock;
> @@ -1218,13 +1217,12 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
> * is alive.
> */
> dead_count = atomic_read(&root->dead_count);
> - smp_rmb();
> - last_visited = iter->last_visited;
> - if (last_visited) {
> - if ((dead_count != iter->last_dead_count) ||
> - !css_tryget(&last_visited->css)) {
> + if (dead_count == iter->last_dead_count) {
> + smp_rmb();
> + last_visited = iter->last_visited;
> + if (last_visited &&
> + !css_tryget(&last_visited->css))
> last_visited = NULL;
> - }
> }
> }
>
> --
> 1.8.3
>

--
Michal Hocko
SUSE Labs


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