Re: [PATCH] memcg: Fix race condition in memcg_check_events() with this_cpu usage

Andrew Morton
2011-09-26T23:46:36Z

On Mon, 26 Sep 2011 09:43:22 +0900
KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:

> On Fri, 23 Sep 2011 20:54:42 -0400
> Steven Rostedt <rostedt@goodmis.org> 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.
> >
> > [ Added this_cpu to __this_cpu conversion by Johannes ]
> >
> > Cc: Greg Thelen <gthelen@google.com>
> > Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> > Cc: Balbir Singh <balbir@linux.vnet.ibm.com>
> > Cc: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
> > Cc: Andrew Morton <akpm@linux-foundation.org>
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > Cc: Peter Zijlstra <peterz@infradead.org>
> > Cc: Christoph Lameter <cl@linux.com>
> > Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
> > Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
> > Link: http://lkml.kernel.org/r/20110919212641.015320989@goodmis.org
>
> Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
>
> Andrew, could you pick this up ?

The patch needed rework due to other changes we have pending in there.

From: Steven Rostedt <srostedt@redhat.com>
Subject: memcg: Fix race condition in memcg_check_events() with this_cpu usage

Various code in memcontrol.c () calls this_cpu_read() on the calculations
to be done from two different percpu variables, or does an open-coded
read-modify-write on a single percpu variable.

Disable preemption throughout these operations so that the writes go to
the correct palces.

[ Added this_cpu to __this_cpu conversion by Johannes ]

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Cc: Greg Thelen <gthelen@google.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Balbir Singh <balbir@linux.vnet.ibm.com>
Cc: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Christoph Lameter <cl@linux.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

mm/memcontrol.c | 10 ++++++----
1 file changed, 6 insertions(+), 4 deletions(-)

diff -puN mm/memcontrol.c~memcg-fix-race-condition-in-memcg_check_events-with-this_cpu-usage mm/memcontrol.c
--- a/mm/memcontrol.c~memcg-fix-race-condition-in-memcg_check_events-with-this_cpu-usage
+++ a/mm/memcontrol.c
@@ -687,8 +687,8 @@ static bool __memcg_event_check(struct m
{
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);
}
@@ -697,7 +697,7 @@ static void __mem_cgroup_target_update(s
{
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:
@@ -713,7 +713,7 @@ static void __mem_cgroup_target_update(s
return;
}

- this_cpu_write(memcg->stat->targets[target], next);
+ __this_cpu_write(memcg->stat->targets[target], next);
}

/*
@@ -722,6 +722,7 @@ static void __mem_cgroup_target_update(s
*/
static void memcg_check_events(struct mem_cgroup *memcg, struct page *page)
{
+ preempt_disable();
/* threshold event is triggered in finer grain than soft limit */
if (unlikely(__memcg_event_check(memcg, MEM_CGROUP_TARGET_THRESH))) {
mem_cgroup_threshold(memcg);
@@ -741,6 +742,7 @@ static void memcg_check_events(struct me
}
#endif
}
+ preempt_enable();
}

static struct mem_cgroup *mem_cgroup_from_cont(struct cgroup *cont)
_