linux-kernel - Re: [PATCH 2/2] cgroup: make css->refcnt clearing on cgroup removal optional

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Date:	Fri, 18 May 2012 20:28:56 +0200
From:	Sasha Levin <levinsasha928@...il.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Li Zefan <lizefan@...wei.com>,
	containers@...ts.linux-foundation.org, cgroups@...r.kernel.org,
	linux-kernel@...r.kernel.org, fweisbec@...il.com, rni@...gle.com,
	ctalbott@...gle.com, Vivek Goyal <vgoyal@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...e.cz>,
	Balbir Singh <bsingharora@...il.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Subject: Re: [PATCH 2/2] cgroup: make css->refcnt clearing on cgroup removal optional

On Fri, May 18, 2012 at 7:55 PM, Tejun Heo <tj@...nel.org> wrote:
> On Thu, May 17, 2012 at 12:33:05AM +0200, Sasha Levin wrote:
>> Hi Tejun,
>>
>> On Sun, Apr 1, 2012 at 8:54 PM, Tejun Heo <tj@...nel.org> wrote:
>> > Hello, Li.
>> >
>> > On Sun, Apr 01, 2012 at 10:00:26AM +0800, Li Zefan wrote:
>> >> > Signed-off-by: Tejun Heo <tj@...nel.org>
>> >>
>> >> Both patches look good.
>> >>
>> >> Acked-by: Li Zefan <lizefan@...wei.com>
>> >
>> > Alright, adding the patches to cgroup/for-3.5.
>>
>> This patch allows for race when removing a cgroup since one of the
>> css's may still have a dentry ref when the cgroup is removed, no? Is
>> there a plan to deal with that before this patch gets pushed into 3.5?
>
> Can you please elaborate?

>From what I gathered, that patch means that you'll have refs to
dentries while you're trying to complete the unmount of the cgroup,
which would make something really angry.

I tested it by running two scripts to race with eachother:

First:

while [ true ];
do
mount -t cgroup dummy cgroup/
mkdir cgroup/0
rmdir cgroup/0
umount cgroup/
done

Second:

while [ true ];
do
mount -t cgroup dummy cgroup/
umount cgroup/
done

Now, if you give it a go, you'll see a BUG() and a system hang pretty fast.
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