From 89dc991f0f5272c307c746fdd57d0bff382b1ba2 Mon Sep 17 00:00:00 2001 From: Johannes Weiner Date: Wed, 12 Jun 2013 14:05:09 -0700 Subject: 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 Reported-by: Tejun Heo Reviewed-by: Tejun Heo Acked-by: Michal Hocko Cc: KAMEZAWA Hiroyuki Cc: Glauber Costa Cc: [3.10+] Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/memcontrol.c | 12 +++++------- 1 file changed, 5 insertions(+), 7 deletions(-) (limited to 'mm/memcontrol.c') diff --git a/mm/memcontrol.c b/mm/memcontrol.c index 931e38c6f09..194721839cf 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; - } } } -- cgit v1.2.3-18-g5258