memcg: correctly order reading PCG_USED and pc->mem_cgroup

The placement of the read-side barrier is confused: the writer first
sets pc->mem_cgroup, then PCG_USED.  The read-side barrier has to be
between testing PCG_USED and reading pc->mem_cgroup.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Acked-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Cc: Balbir Singh <balbir@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 9 insertions, 18 deletions

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 5b562b375cbd..db76ef726293 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -836,13 +836,12 @@ void mem_cgroup_rotate_lru_list(struct page *page, enum lru_list lru)
                 return;
 
         pc = lookup_page_cgroup(page);
-        /*
-         * Used bit is set without atomic ops but after smp_wmb().
-         * For making pc->mem_cgroup visible, insert smp_rmb() here.
-         */
-        smp_rmb();
         /* unused or root page is not rotated. */
-        if (!PageCgroupUsed(pc) || mem_cgroup_is_root(pc->mem_cgroup))
+        if (!PageCgroupUsed(pc))
+                return;
+        /* Ensure pc->mem_cgroup is visible after reading PCG_USED. */
+        smp_rmb();
+        if (mem_cgroup_is_root(pc->mem_cgroup))
                 return;
         mz = page_cgroup_zoneinfo(pc);
         list_move(&pc->lru, &mz->lists[lru]);
@@ -857,14 +856,10 @@ void mem_cgroup_add_lru_list(struct page *page, enum lru_list lru)
                 return;
         pc = lookup_page_cgroup(page);
         VM_BUG_ON(PageCgroupAcctLRU(pc));
-        /*
-         * Used bit is set without atomic ops but after smp_wmb().
-         * For making pc->mem_cgroup visible, insert smp_rmb() here.
-         */
-        smp_rmb();
         if (!PageCgroupUsed(pc))
                 return;
-
+        /* Ensure pc->mem_cgroup is visible after reading PCG_USED. */
+        smp_rmb();
         mz = page_cgroup_zoneinfo(pc);
         /* huge page split is done under lru_lock. so, we have no races. */
         MEM_CGROUP_ZSTAT(mz, lru) += 1 << compound_order(page);
@@ -1031,14 +1026,10 @@ mem_cgroup_get_reclaim_stat_from_page(struct page *page)
                 return NULL;
 
         pc = lookup_page_cgroup(page);
-        /*
-         * Used bit is set without atomic ops but after smp_wmb().
-         * For making pc->mem_cgroup visible, insert smp_rmb() here.
-         */
-        smp_rmb();
         if (!PageCgroupUsed(pc))
                 return NULL;
-
+        /* Ensure pc->mem_cgroup is visible after reading PCG_USED. */
+        smp_rmb();
         mz = page_cgroup_zoneinfo(pc);
         if (!mz)
                 return NULL;