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

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Date:	Thu, 28 Jun 2012 14:07:51 +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]

tj@...nel.org wrote:

> On Sun, Jun 24, 2012 at 07:08:07PM +0000, shyju pv wrote:
>> Hi,
>> Observed a crash on 3.5-rc3 with cgroup tests from LTP(Stable April,2012 release)on Dell Inspiron 1526[Intel(R) Core2Duo T7250, MEM 4GB] and also on another x86 quad core target(4GB RAM).
>> LTP test case: cgroup_regression_test.sh(test 4,7 and 9 crashes randomly when the test cases are executed in order)


I wrote thoese test scripts. ;)

>> Shyju
>>
>> [  532.805905] BUG: Dentry ffff8801164db490{i=491b,n=/} still in use (1) [unmount of cgroup cgrou
> 
> Hmm... fa980ca87d "cgroup: superblock can't be released with active
> dentries" is supposed to have fixed that.  Looking into it.
> 


I think I know what happened here:

umount
  deativate_super(sb)
                       dput(subdir)
                         subdir->d_count--
                         d_release(subdir)
                           deactivate_super(sb)
                             shrink_dcache_for_umount(sb)
                               BUG(root->d_count)!!
                         root->d_count--

I use this script to reproduce the bug:

mount -t cgroup -o cpu xxx /mnt
mkdir /mnt/sub
sleep 100 < /mnt/sub &
kill $!
wait $!
rmdir /mnt/sub
umount /mnt

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