Tianon, I bumped into this issue recently.
The problem appears to be that cgroupfs-umount tries to unmount all
/sys/fs/cgroup subdirectories, even if the mount wasn't originally created by
cgroupfs-mount. elogind mounts /sys/fs/cgroup/{elogind,unified} and is still
using them.
The solution would seem to me for cgroupfs-umount to only try to unmount the
mountpoints it created, ie those derived from /proc/cgroups.
Thanks.
Mark
