mount cgroup with "already mounted or cgroup busy"

[Li,Rongqing]

Hi:

I meet a issue, and not sure if it is normal
Using the below script, the last line mount will fail after unmount
And it will fail until reboot system

mount -t cgroup -o cpu,cpuset xx cgroup
mkdir cgroup/x
rmdir cgroup/x
umount cgroup
mount -t cgroup -o cpu xx cgroup
mount: xx already mounted or cgroup busy

but if I add sleep, it will success

mount -t cgroup -o cpu,cpuset xx cgroup
mkdir cgroup/x
rmdir cgroup/x
sleep 10  <<<<==========
umount cgroup
mount -t cgroup -o cpu xx cgroup

- Li RongQing