Re: [PATCH -rt] memcg: use migrate_disable()/migrate_enable( ) in memcg_check_events()

On Wed, Nov 16, 2011 at 09:12:38AM -0500, Steven Rostedt wrote:

On Wed, 2011-11-16 at 17:16 +0800, Yong Zhang wrote:

quoted

Looking at commit 4799401f [memcg: Fix race condition in
memcg_check_events() with this_cpu usage], we just want
to disable migration. So use the right API in -rt. This
will cure below warning.

quoted

 mm/memcontrol.c |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 6aff93c..afa1954 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c

@@ -722,7 +722,7 @@ static void __mem_cgroup_target_update(struct mem_cgroup *memcg, int target)
  */
 static void memcg_check_events(struct mem_cgroup *memcg, struct page *page)
 {
-	preempt_disable();
+	migrate_disable();

No this won't work. Not even for -rt. If we disable migration but not
preemption, then two tasks can take this path. And the checks in
__memcg_event_check() will be corrupted because nothing is protecting
the updates from two tasks going into the same path.

I assumed that we only care about migration, but obviously I'm wrong.

Perhaps a local_lock would work.

Yeah, will try tglx's patch later.

Thanks,
Yong