linux-kernel - mount cgroup with "already mounted or cgroup busy"

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Date:   Wed, 30 Oct 2019 09:32:41 +0000
From:   "Li,Rongqing" <lirongqing@...du.com>
To:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:     "tj@...nel.org" <tj@...nel.org>,
        "lizefan@...wei.com" <lizefan@...wei.com>,
        "hannes@...xchg.org" <hannes@...xchg.org>,
        "cgroups@...r.kernel.org" <cgroups@...r.kernel.org>
Subject: mount cgroup with "already mounted or cgroup busy"

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