linux-kernel - Re: [RFC][PATCH 8/9 v2] cgroup: avoid creating new cgroup under a cgroup being destroyed

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Message-ID: <20120427204035.GN26595@google.com>
Date:	Fri, 27 Apr 2012 13:40:35 -0700
From:	Tejun Heo <tj@...nel.org>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	Linux Kernel <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"cgroups@...r.kernel.org" <cgroups@...r.kernel.org>,
	Michal Hocko <mhocko@...e.cz>,
	Johannes Weiner <hannes@...xchg.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Glauber Costa <glommer@...allels.com>,
	Han Ying <yinghan@...gle.com>,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	kamezawa.hiroyuki@...il.com
Subject: Re: [RFC][PATCH 8/9 v2] cgroup: avoid creating new cgroup under a
 cgroup being destroyed

On Fri, Apr 27, 2012 at 03:04:14PM +0900, KAMEZAWA Hiroyuki wrote:
> When ->pre_destroy() is called, it should be guaranteed that
> new child cgroup is not created under a cgroup, where pre_destroy()
> is running. If not, ->pre_destroy() must check children and
> return -EBUSY, which causes warning.
> 
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>

Hmm... I'm getting confused more.  Why do we need these cgroup changes
at all?  cgroup still has cgrp->count check and
cgroup_clear_css_refs() after pre_destroy() calls.  The order of
changes should be,

* Make memcg pre_destroy() not fail; however, pre_destroy() should
  still be ready to be retried.  That's the defined interface.

* cgroup core updated to drop pre_destroy() retrying and guarantee
  that pre_destroy() invocation will happen only once.

* memcg and other cgroups can update their pre_destroy() if the "won't
  be retried" part can simplify their implementations.

So, there's no reason to be updating cgroup pre_destroy() semantics at
this point and these updates actually break cgroup API as it currently
stands.  The only change necessary is memcg's pre_destroy() not
returning zero.

Thanks.

-- 
tejun
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