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>

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 5b562b3..db76ef7 100644 (file)
--- 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;