Hello. We are using a backup procedure based on a combination of LVM snapshots and rsync. In particular, the procedure is the following:

... mysql -h -u -p -e "FLUSH TABLES WITH READ LOCK; \! lvcreate -L -s -n "

mount

...

rsync .......

At the end, we change the working directory, unmount  and lvremove . Sometimes, randomly, the umount command works but the lvremove fails and we are left with the lvm snapshot unmounted but we are unable to remove it. We tried to force a removal without success with the following error:

Logical volume contains a filesystem in use

We didn't find any process using that device using lsof and we can't find any handler keeping the device busy. Even a mariadb restart doesn't fix it and the only solution is to reboot the server. The only thing we noticed is a message upon the snapshot creation:

File descriptor 4 (socket:[201844800]) leaked on lvcreate invocation. Parent PID 1564033: mysql

We know that this is the mysql unix socket. Have you ever had such an issue? Thanks a lot. Kind regards. Riccardo