cgroup umount bug

cgroup umount bug

[含黛]

Hi,

    I found a mount/umount problem while using cgroup: I mount a cpu
cgroup (or other subsystems) and create some subgroups under it, but
later I try to umount it without rmdir those subgroups. I find that I
can umount it without any errors but actually the super block still
exist. This is because the sb->s_active count is greater than 1 which
was added by cgroup_create. But later if I want to mount both cpu and
memory subsystems, it will be unsuccessful indicating that the
device(cpu subsystem) is busy. This situation will not happen if I rmdir
the subgroups manually before umount it. So my question is:
    Should the umount return success when there are still some subgroups
exist ? If this is okay, then it may lead to some *strange* errors using
cgroup later. If it can not be umount, then it should inform that the
mount point is still busy.


Following is an example of this problem:

mount -t cgroup -o cpu cgroup /mnt
mkdir /mnt/test

…..// do some tests

umount /mnt      /* return successfully */

/* do other subsystems test */
mount -cgroup -o cpu,cpuset cgroup /mnt
mount: cgroup already mounted or /mnt busy  /*??? which is strange */


Looking forward to your reply.
Thanks.


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Re: cgroup umount bug

[Serge E. Hallyn]

Quoting 含黛 (handai.szj@taobao.com):
> Hi,
> 
>     I found a mount/umount problem while using cgroup: I mount a cpu
> cgroup (or other subsystems) and create some subgroups under it, but
> later I try to umount it without rmdir those subgroups. I find that I
> can umount it without any errors but actually the super block still
> exist. This is because the sb->s_active count is greater than 1 which
> was added by cgroup_create. But later if I want to mount both cpu and
> memory subsystems, it will be unsuccessful indicating that the
> device(cpu subsystem) is busy. This situation will not happen if I rmdir
> the subgroups manually before umount it. So my question is:
>     Should the umount return success when there are still some subgroups

Yes.  The fs was unmounted from that directory.

> exist ? If this is okay, then it may lead to some *strange* errors using
> cgroup later. If it can not be umount, then it should inform that the
> mount point is still busy.

> Following is an example of this problem:
> 
> mount -t cgroup -o cpu cgroup /mnt
> mkdir /mnt/test
> 
> …..// do some tests
> 
> umount /mnt      /* return successfully */
> 
> /* do other subsystems test */
> mount -cgroup -o cpu,cpuset cgroup /mnt
> mount: cgroup already mounted or /mnt busy  /*??? which is strange */

I'm not sure from what you write whether you're aware of this, but
to work around this, you can then

 mount -t cgroup -o cpu /mnt
 for i in `cat /mnt/test/tasks`; do
  echo $i > /mnt/tasks
 done
 rmdir /mnt/test
 umount /mnt

and now you can 

 mount -cgroup -o cpu,cpuset cgroup /mnt

Being unable to umount /mnt while child cgroups exist or having all
child cgroups cleared and removed automatically would both be less
useful behaviors.

thanks,
-serge