linux-kernel - Re: 3.5-rc3: BUG: Dentry still in use (1) [unmount of cgroup cgroup]

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Message-ID: <4FEE98EE.1050409@huawei.com>
Date:	Sat, 30 Jun 2012 14:13:02 +0800
From:	Li Zefan <lizefan@...wei.com>
To:	"tj@...nel.org" <tj@...nel.org>
CC:	shyju pv <shyju.pv@...wei.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"cgroups@...r.kernel.org" <cgroups@...r.kernel.org>,
	Sanil kumar <sanil.kumar@...wei.com>,
	Masanari Iida <standby24x7@...il.com>
Subject: Re: 3.5-rc3: BUG: Dentry still in use (1) [unmount of cgroup cgroup]

On 2012/6/30 0:58, tj@...nel.org wrote:

> Hey,
> 
> On Fri, Jun 29, 2012 at 10:20:11AM +0800, Li Zefan wrote:
>>> Can you please elaborate a bit?  I'm not really following?  Where does
>>> the last root->d_count-- come from?
>>
>>
>> From my limited knowledge about vfs internal, seems the parent's refcnt won't go down
>> to 0 before its children. When mkdir, the parent's refcnt will be incremented, and
>> after rmdir, dput(subdir) will drop subdir's refcnt and then drop parent's.
>>
>> So when dropping the subdir's refcnt and leading the superblock to be killed, the root's
>> dentry is still > 0.
> 
> Heh, yeah, I thought you found who was holding out on the refcnt. :)
> 


dput will drop both the subdir and the root's dentry refcnt, but kill_sb will be called
inbetween.

So it's bad to have dentry refcnts dangling after umount. I've made a patch so css will
pin cgroup instead of cgroup dentry.

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