I tried to stop a qemu-img copy or clone and it broke everything. It was fine to "stop" it from the GUI but a process still persisted so I killed the relevant qemu-img and the kernel went crazy. It also may not have helped that I tried to lvremove a different volume (an unused disk). But either way it breaks LVM (you cannot even run lvdisplay) so a reboot is necessary.
Jan 17 06:45:21 testserver kernel: [ 5680.439337] systemd-udevd D 0&nbs........
In short the solution is just to use vgremove for the actual /dev/mapper device:
vgremove /dev/mapper/backups-backuplv
box mnt # mdadm --manage /dev/md8 --stop
mdadm: Cannot get exclusive access to /dev/md8:Perhaps a running process, mounted filesystem or active volume group?
box mnt # lv
lvchange lvconvert lvcreate l........