linux-kernel - Re: [RFC][PATCH 3/5] memcg: Disable preemption in memcg_check

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Date:	Tue, 20 Sep 2011 10:33:17 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Christoph Lameter <cl@...ux.com>,
	Greg Thelen <gthelen@...gle.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Daisuke Nishimura <nishimura@....nes.nec.co.jp>
Subject: Re: [RFC][PATCH 3/5] memcg: Disable preemption in
 memcg_check_events()

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@...hat.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@...xchg.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);
>  }
>  
>  /*


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