linux-kernel - Re: [RFC][PATCH 5/7] cgroup: make sure parent won't be destroyed before its children

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Date:	Thu, 4 Apr 2013 06:53:53 -0700
From:	Tejun Heo <tj@...nel.org>
To:	Michal Hocko <mhocko@...e.cz>
Cc:	Li Zefan <lizefan@...wei.com>, linux-mm@...ck.org,
	LKML <linux-kernel@...r.kernel.org>,
	Cgroups <cgroups@...r.kernel.org>,
	Glauber Costa <glommer@...allels.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Johannes Weiner <hannes@...xchg.org>
Subject: Re: [RFC][PATCH 5/7] cgroup: make sure parent won't be destroyed
 before its children

Hey,

On Thu, Apr 04, 2013 at 01:37:50PM +0200, Michal Hocko wrote:
> On Wed 03-04-13 17:13:08, Li Zefan wrote:
> > Suppose we rmdir a cgroup and there're still css refs, this cgroup won't
> > be freed. Then we rmdir the parent cgroup, and the parent is freed due
> > to css ref draining to 0. Now it would be a disaster if the child cgroup
> > tries to access its parent.
> 
> Hmm, I am not sure what is the correct layer for this to handle - cgroup
> core or memcg. But we have enforced that in mem_cgroup_css_online where
> we take an additional reference to the memcg.
> 
> Handling it in the memcg code would have an advantage of limiting an
> additional reference only to use_hierarchy cases which is sufficient
> as we never touch the parent otherwise (parent_mem_cgroup).

But what harm does an additional reference do?  And given that there
are cgroup core interfaces which access ->parent, I think it'd be a
good idea that parent always exists while there are children.

Thanks.

-- 
tejun
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