Re: [RFC][PATCH 3/5] memcg: Disable preemption in memcg_check_events()

Steven Rostedt
2011-09-20T14:33:21Z

On Tue, 2011-09-20 at 16:24 +0200, Johannes Weiner wrote:
> On Tue, Sep 20, 2011 at 04:20:31PM +0200, Johannes Weiner wrote:
> > On Mon, Sep 19, 2011 at 05:20:43PM -0400, Steven Rostedt wrote:
> > > From: Steven Rostedt <srostedt@redhat.com>
> > >
> > > The code in memcg_check_events() calls this_cpu_read() on
> > > different variables without disabling preemption, and can cause
> > > the calculations to be done from two different CPU variables.
> > >
> > > Disable preemption throughout the check to keep apples and oranges
> > > from becoming a mixed drink.
> >
> > Makes sense, thanks!
> >
> > Since the atomic versions are no longer required with preemption
> > disabled explicitely, could you also make the this_cpu ops in
> > __memcg_event_check and __mem_cgroup_target_update non-atomic in the
> > same go?

Although this patch set is RFC, I probably should go ahead and push this
one forward, as it looks to be a real bug.

>
> Sorry, shouldn't be on you, can you fold this in?

Sure, thanks!

-- Steve

>
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
> ---
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index b76011a..9d4ba65 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -683,8 +683,8 @@ static bool __memcg_event_check(struct mem_cgroup *memcg, int target)
> {
> unsigned long val, next;
>
> - val = this_cpu_read(memcg->stat->events[MEM_CGROUP_EVENTS_COUNT]);
> - next = this_cpu_read(memcg->stat->targets[target]);
> + val = __this_cpu_read(memcg->stat->events[MEM_CGROUP_EVENTS_COUNT]);
> + next = __this_cpu_read(memcg->stat->targets[target]);
> /* from time_after() in jiffies.h */
> return ((long)next - (long)val < 0);
> }
> @@ -693,7 +693,7 @@ static void __mem_cgroup_target_update(struct mem_cgroup *memcg, int target)
> {
> unsigned long val, next;
>
> - val = this_cpu_read(memcg->stat->events[MEM_CGROUP_EVENTS_COUNT]);
> + val = __this_cpu_read(memcg->stat->events[MEM_CGROUP_EVENTS_COUNT]);
>
> switch (target) {
> case MEM_CGROUP_TARGET_THRESH:
> @@ -709,7 +709,7 @@ static void __mem_cgroup_target_update(struct mem_cgroup *memcg, int target)
> return;
> }
>
> - this_cpu_write(memcg->stat->targets[target], next);
> + __this_cpu_write(memcg->stat->targets[target], next);
> }
>
> /*