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mm: memcontrol: fix lockless reclaim hierarchy iterator 

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.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Reported-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Tejun Heo <tj@kernel.org>
Acked-by: Michal Hocko <mhocko@suse.cz>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Glauber Costa <glommer@parallels.com>
Cc: <stable@kernel.org>		[3.10+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Johannes Weiner 2013-06-12 14:05:09 -07:00 committed by Linus Torvalds
parent 7b57976da4
commit 89dc991f0f
1 changed files with 5 additions and 7 deletions
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12
mm/memcontrol.c
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@ -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;
}
}
}