From mboxrd@z Thu Jan  1 00:00:00 1970
From: Tejun Heo <tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
Subject: Re: [RFC][PATCH 5/7] cgroup: make sure parent won't be destroyed
 before its children
Date: Thu, 4 Apr 2013 06:53:53 -0700
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To: Michal Hocko <mhocko-AlSwsSmVLrQ@public.gmane.org>
Cc: Li Zefan <lizefan-hv44wF8Li93QT0dZR+AlfA@public.gmane.org>, linux-mm-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org, LKML <linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>, Cgroups <cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>, Glauber Costa <glommer-bzQdu9zFT3WakBO8gow8eQ@public.gmane.org>, KAMEZAWA Hiroyuki <kamezawa.hiroyu-+CUm20s59erQFUHtdCDX3A@public.gmane.org>, Johannes Weiner <hannes-druUgvl0LCNAfugRpC6u6w@public.gmane.org>

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