It is enough to move the shell process (under which umount runs) out of the logind cgroup user.slice/user-1000.slice/session-1.scope into the / cgroup in the cpu,cpuacct controller:
echo $$ | sudo tee /sys/fs/cgroup/cpu,cpuacct/tasks
All other cgroup controllers work fine.
It is enough to move the shell process (under which umount runs) out of the logind cgroup user.slice/ user-1000. slice/session- 1.scope into the / cgroup in the cpu,cpuacct controller:
echo $$ | sudo tee /sys/fs/ cgroup/ cpu,cpuacct/ tasks
All other cgroup controllers work fine.