Issue with mount options on mount/umount

Hello!

It seems that if 'umount' happens before some cgroup cleanup processes finish, we will not be able to mount this cgroup with changed options.

For example, consider this test case (where I'm playing with noprefix):

#!/bin/sh

mkdir /tmp/bla
mkdir /dev/cgroup

mount -t cgroup -o cpuset,noprefix bla /dev/cgroup mkdir /dev/cgroup/ltp
rmdir /dev/cgroup/ltp

#sleep 1

umount /dev/cgroup
rmdir /dev/cgroup

for i in $(seq 2); do
echo $i
echo $i > /dev/kmsg
mount -t cgroup -o cpuset bla /tmp/bla; ls /tmp/bla | grep
effective_cpus; umount /tmp/bla
done

rmdir /tmp/bla

If it's executed as is, it reports:

[root@ol6-x64 cgroup]# ./test.sh
1
effective_cpus
2
cpuset.effective_cpus

and in dmesg:

[root@ol6-x64 cgroup]# dmesg|tail
1
cgroup: new mount options do not match the existing superblock, will be ignored 2

If we uncomment that 'sleep 1' the test will report cpuset.effective_cpus for both the iterations.

The kernel is v4.5-rc7-159-g7f02bf6. However, this issue is applicable to older versions as well.

Is this behavior a bug or feature? If it's a feature, then what is the motivation for this behavior?

Thank you.