cgroup umount bug

[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
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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