After upgrade MariaDB to
# mysql --version
mysql  Ver 15.1 Distrib 10.0.13-MariaDB, for Linux (x86_64) using readline 5.1
# systemctl start mysql.service fails with error
rcmysql[9612]: Starting service MySQL Something went wrong
This is apparently bug #914370, but here it is not just a cosmetic problem.
Error log says it can't create socket file. But /var/run/mysql/ contains socket
as well as pid files. Apparently, those files were not deleted during DB
shutdown. After deleting content of /var/run/mysql/, MariaDB starts fine.
Following restarts also work.