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

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Date:	Tue, 12 Feb 2013 17:24:42 +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 Tue 12-02-13 17:13:32, Michal Hocko wrote:
> On Tue 12-02-13 16:43:30, Michal Hocko wrote:
> [...]
> The example was not complete:
> 
> > Wait a moment. But what prevents from the following race?
> > 
> > rcu_read_lock()
> 
> cgroup_next_descendant_pre
> css_tryget(css);
> memcg = mem_cgroup_from_css(css)		atomic_add(CSS_DEACT_BIAS, &css->refcnt)
> 
> > 						mem_cgroup_css_offline(memcg)
> 
> We should be safe if we did synchronize_rcu() before root->dead_count++,
> no?
> Because then we would have a guarantee that if css_tryget(memcg)
> suceeded then we wouldn't race with dead_count++ it triggered.
> 
> > 						root->dead_count++
> > iter->last_dead_count = root->dead_count
> > iter->last_visited = memcg
> > 						// final
> > 						css_put(memcg);
> > // last_visited is still valid
> > rcu_read_unlock()
> > [...]
> > // next iteration
> > rcu_read_lock()
> > iter->last_dead_count == root->dead_count
> > // KABOOM

Ohh I have missed that we took a reference on the current memcg which
will be stored into last_visited. And then later, during the next
iteration it will be still alive until we are done because previous
patch moved css_put to the very end.
So this race is not possible. I still need to think about parallel
iteration and a race with removal.

-- 
Michal Hocko
SUSE Labs
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