Our MariaDB server is crashing on long semaphor wait. It is happening usually at night when database backups are running (but sometimes also in the middle of the day). I'm 100% sure it's related to one of our users because when i moved him to different server, the problem moves with him. There were enough of memory and we had the same problem with MySQL 5.5. Everytime it happend, the mentioned user is running simultanious SELECT and TRUNCATE on the same table.