Comment 5 for bug 1505118

In Percona Server 5.6, we introduced:
https://www.percona.com/doc/percona-server/5.6/scalability/multiple_user_level_locks.html#multiple-user-level-locks
living now in MDL context, the same way as it is in MySQL 5.7 now:
http://dev.mysql.com/doc/refman/5.7/en/miscellaneous-functions.html#function_get-lock

The function *mysql_ull_cleanup* we can see in stack traces is not even present in MySQL Community 5.6:
$ grep -r 'mysql_ull_cleanup' mysql-5.6.30/

$ grep -r 'mysql_ull_cleanup' mysql-5.7.12/
mysql-5.7.12/sql/item_func.h:void mysql_ull_cleanup(THD *thd);
mysql-5.7.12/sql/item_func.cc:void mysql_ull_cleanup(THD *thd)
mysql-5.7.12/sql/item_func.cc: DBUG_ENTER("mysql_ull_cleanup");
mysql-5.7.12/sql/sql_class.cc: mysql_ull_cleanup(this);

$ grep -r 'mysql_ull_cleanup' Percona-XtraDB-Cluster-5.6.27-76.0/
Percona-XtraDB-Cluster-5.6.27-76.0/sql/item_func.h:void mysql_ull_cleanup(THD *thd);
Percona-XtraDB-Cluster-5.6.27-76.0/sql/wsrep_thd.cc: mysql_ull_cleanup(thd);
Percona-XtraDB-Cluster-5.6.27-76.0/sql/item_func.cc:void mysql_ull_cleanup(THD *thd)
Percona-XtraDB-Cluster-5.6.27-76.0/sql/item_func.cc: DBUG_ENTER("mysql_ull_cleanup");
Percona-XtraDB-Cluster-5.6.27-76.0/sql/sql_class.cc: mysql_ull_cleanup(this);

So in addition to the fact that user locks are not supported in Galera:
http://galeracluster.com/documentation-webpages/limitations.html#table-locking
https://blueprints.launchpad.net/codership-mysql/+spec/get-lock-support

now they are even more complex in PXC 5.6.19+, which I think may lead to those crashes.