Messages in this thread | | | Date | Tue, 02 Dec 2008 21:07:46 +0800 | From | Li Zefan <> | Subject | Re: BUG: CGROUPS [2.6.28-rc6] Task migration does not clean reference to group |
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sudhir kumar wrote: > 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.
Ah, it's not a bug. :)
> 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
Seems whenever we create a background job, we'll have a pin to the directory where the job is created. So though rmdir() removed the directory, the dentry's refcnt is still >0, so cgroup_diput() in which number_of_cgroups is decreased is not called. This explains why 'num_cgroups' showed in /proc/cgroups is not 1, and why umount failed.
You can try: # mount -t ext3 /dev/sda6 /mnt # cd /mnt # sleep 10000 & # cd .. # umount /mnt umount: /mnt: device is busy
And umount is OK for the below example: # mount -t ext3 /dev/sda6 /mnt # sleep 10000 & # umount /mnt
> 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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