Shutdown never completes : Waiting for master thread to exit

When MariaDB is shutdown (via systemd), it never finishes.

The mysqld process jumps to 100% CPU, and the log file repeats

2017-07-22 15:06:58 140459974318848 [Note] InnoDB: Waiting for master thread to exit

at regular intervals.

Killing (with kill -KILL) the mysqld process is the only way to proceed. I've left it for hours with no change.
This is a tiny DB install :

# du -ksh /var/lib/mysql

273M    /var/lib/mysql