umount doesn't seem to really unmount

[Stefan Ring]

>> $ sudo service mysqld start
> Does not work for me. It must be something else.

It seems to happen with any service that has PrivateTmp set:

$ grep PrivateTmp /usr/lib/systemd/system/*.service
/usr/lib/systemd/system/colord-sane.service:PrivateTmp=yes
/usr/lib/systemd/system/colord.service:PrivateTmp=yes
/usr/lib/systemd/system/cups.service:PrivateTmp=true
/usr/lib/systemd/system/httpd-event.service:PrivateTmp=true
/usr/lib/systemd/system/httpd.service:PrivateTmp=true
/usr/lib/systemd/system/httpd-worker.service:PrivateTmp=true
/usr/lib/systemd/system/mysqld.service:PrivateTmp=true
/usr/lib/systemd/system/openvpn at .service:PrivateTmp=true

I guess that the XFS filesystem must be mounted at the time the
service gets started for this to happen. In that case, the service
seems to hold a hidden reference to XFS which get released when the
service is stopped.