linux-kernel - Re: [PATCH] cgroup: fix dentry still in use bug when dropping css refs after umount

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Date:	Tue, 10 Jul 2012 10:11:05 +0800
From:	Li Zefan <lizefan@...wei.com>
To:	Tejun Heo <tj@...nel.org>
CC:	shyju pv <shyju.pv@...wei.com>,
	Sanil kumar <sanil.kumar@...wei.com>,
	Masanari Iida <standby24x7@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Cgroups <cgroups@...r.kernel.org>, <viro@...iv.linux.org.uk>,
	<levinsasha928@...il.com>
Subject: Re: [PATCH] cgroup: fix dentry still in use bug when dropping css
 refs after umount

On 2012/7/8 14:35, Tejun Heo wrote:

> Hello, Li.
> 
> On Wed, Jul 04, 2012 at 02:19:15PM +0800, Li Zefan wrote:
>>> I think we're horridly broken for
>>> root cgroup tho - and it has been broken for very long time.  I think
>>> it's mostly hidden because most (all?) controllers short-circuit root
>>> cgroup.  Eh, well....
>>>
>>
>> Could you elaborate a bit on what's broken for root cgroup?
> 
> If someone holds css ref of a root cgroup, AFAICS nothing is
> preventing the cgroup hierarchy from being unmounted and root cgroup
> destroyed.
> 


Right, but that should be safe. The css objects of the root cgroup are
allocated at boot, and won't be destroyed at umount.

Furthermore when a cgroup hierarchy is going to be unmounted, those css's
will be made to point to a cgroup named dummytop in rebind_subsystems(),
and there's a syncronize_rcu() in the end of the function, so accessing
css->cgroup is always safe.

In this case, dummytop->dentry is NULL, and that's safe too, because
cgroup_path() is aware of this case.

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