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

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Date:	Wed, 4 Jul 2012 14:19:15 +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 2012/7/4 6:52, Tejun Heo wrote:

> Hello,
> 
> Shyju, can you please test the following patch?
> 
> Sasha, can you please re-test your test case with this patch?  I don't
> think fa980ca87d "cgroup: superblock can't be released with active
> dentries" did anything useful.  cgroup always calls d_iput() and
> d_release() in succession from the same context.  It may just have
> stretched the timing a bit to hide the race.
> 
> Li, this patch reverts fa980ca87d and wraps dput() in css_dput_fn()
> with s_active refs.  Positive dentry ref means that we have active ref
> on sb, so wrapping the final dput() with extra s_active ref should
> avoid premature super destruction. 


> 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?

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