linux-kernel - Re: [PATCH v3 4/7] memcg: remove memcg from the reclaim iterators

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Message-ID: <20130213125617.GD23562@dhcp22.suse.cz>
Date:	Wed, 13 Feb 2013 13:56:17 +0100
From:	Michal Hocko <mhocko@...e.cz>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Ying Han <yinghan@...gle.com>, Tejun Heo <htejun@...il.com>,
	Glauber Costa <glommer@...allels.com>,
	Li Zefan <lizefan@...wei.com>
Subject: Re: [PATCH v3 4/7] memcg: remove memcg from the reclaim iterators

On Wed 13-02-13 11:34:59, Michal Hocko wrote:
> On Tue 12-02-13 12:37:41, Johannes Weiner wrote:
> > On Tue, Feb 12, 2013 at 06:12:16PM +0100, Michal Hocko wrote:
> [...]
> > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > > index 727ec39..31bb9b0 100644
> > > --- a/mm/memcontrol.c
> > > +++ b/mm/memcontrol.c
> > > @@ -144,8 +144,13 @@ struct mem_cgroup_stat_cpu {
> > >  };
> > >  
> > >  struct mem_cgroup_reclaim_iter {
> > > -	/* last scanned hierarchy member with elevated css ref count */
> > > +	/*
> > > +	 * last scanned hierarchy member. Valid only if last_dead_count
> > > +	 * matches memcg->dead_count of the hierarchy root group.
> > > +	 */
> > >  	struct mem_cgroup *last_visited;
> > > +	unsigned int last_dead_count;
> > 
> > Since we read and write this without a lock, I would feel more
> > comfortable if this were a full word, i.e. unsigned long.  That
> > guarantees we don't see any partial states.
> 
> OK. Changed. Although I though that int is read/modified atomically as
> well if it is aligned to its size.

Ohh, I guess what was your concern. If last_dead_count was int then it
would fit into the same full word slot with generation and so the
parallel read-modify-update cycle could be an issue.

-- 
Michal Hocko
SUSE Labs
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