linux-kernel - Re: [RFC 2/5] memcg: rework mem_cgroup

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Message-ID: <20121119151130.GB16803@dhcp22.suse.cz>
Date:	Mon, 19 Nov 2012 16:11:30 +0100
From:	Michal Hocko <mhocko@...e.cz>
To:	linux-mm@...ck.org
Cc:	linux-kernel@...r.kernel.org,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Ying Han <yinghan@...gle.com>, Tejun Heo <htejun@...il.com>,
	Glauber Costa <glommer@...allels.com>
Subject: Re: [RFC 2/5] memcg: rework mem_cgroup_iter to use cgroup iterators

On Tue 13-11-12 16:30:36, Michal Hocko wrote:
[...]
> +		/*
> +		 * Root is not visited by cgroup iterators so it needs a special
> +		 * treatment.
> +		 */
> +		if (!last_visited) {
> +			css = &root->css;
> +		} else {
> +			struct cgroup *next_cgroup;
> +
> +			next_cgroup = cgroup_next_descendant_pre(
> +					last_visited->css.cgroup,
> +					root->css.cgroup);
> +			if (next_cgroup)
> +				css = cgroup_subsys_state(next_cgroup,
> +						mem_cgroup_subsys_id);

This is not correct because cgroup_next_descendant_pre expects pos to be
NULL for the first iteration but the way we do iterate (visit the root
first) means that the second iteration will have last_visited != NULL
and if root doesn't have any children the iteration would go unleashed
to to the endless loop. We need something like:
	struct cgroup *prev_cgroup = (last_visited == root) ? NULL 
					: last_visited->css.cgroup;
	next_cgroup = cgroup_next_descendant_pre(prev_cgroup,
				root->css.gtoup);

-- 
Michal Hocko
SUSE Labs
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