BUG: CGROUPS [2.6.28-rc6] Task migration does not clean reference to group

[sudhir kumar]

Hi,
I see an interesting bug in the cgroups. I found that cat /proc/cgroups
does not decrease the num_cgroups field when a group is deleted. It looks
to me that somewhere we miss to delete the reference, which leads to a failure
to unmount the cgroups filesystem.
The steps to produce the bug are:

mkdir /cpu
mount -t cgroup -ocpu cgroup /cpu
cd cpu
mkdir group1;
cd group1;
/./while &              # an infinite loop
echo pid > tasks        # pid is of infinite task
cd ..                   # we are now in root group
echo pid > /tasks       # move the task to root group
rmdir group1;           # Succeeds even when I was inside this group
in another shell
cd ..
umount /cpu;    # this fails for me as below
[root@malik-laptop /]# umount cpu/
umount: cpu/: device is busy

And I see that cat /proc/cgroups shows me still num_cgroups as 2;

I checked it on two kernels.
[root@malik-laptop cpu]# uname -a
Linux malik-laptop.in.ibm.com 2.6.26 #1 SMP Tue Aug 12 16:39:18 IST
2008 i686 i686 i386 GNU/Linux

[root@e325b sudhir]# uname -a
Linux e325b.in.ibm.com 2.6.28-rc6 #1 SMP Mon Dec 1 15:53:09 IST 2008
i686 athlon i386 GNU/Linux

A look on /proc/cgroups
root@e325b sudhir]# cat /proc/cgroups
#subsys_name    hierarchy       num_cgroups     enabled
#cpuset  0       1       1
#debug   0       1       1
#ns      0       1       1
#cpu     8       2       1
#cpuacct 0       1       1
#freezer 0       1       1
#

Please let me know if further information is required or I am doing
something wrong ?

Thanks
Sudhir
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