160921 13:17:23 [ERROR] mysqld got signal 11 ;
This could be because you hit a bug. It is also possible that this binary
or one of the libraries it was linked against is corrupt, improperly built,
or misconfigured. This error can also be caused by malfunctioning hardware.

To report this bug, see https://mariadb.com/kb/en/reporting-bugs

We will try our best to scrape up some info that will hopefully help
diagnose the problem, but since we have already crashed, 
something is definitely wrong and this may fail.

Server version: 10.1.17-MariaDB-1~jessie
key_buffer_size=67108864
read_buffer_size=2097152
max_used_connections=34
max_threads=202
thread_count=10
It is possible that mysqld could use up to 
key_buffer_size + (read_buffer_size + sort_buffer_size)*max_threads = 1310752 K  bytes of memory
Hope that's ok; if not, decrease some variables in the equation.

Thread pointer: 0x0x7f7773ef2008
Attempting backtrace. You can use the following information to find out
where mysqld died. If you see no messages after this, something went
terribly wrong...
stack_bottom = 0x7f78c3d2c1f8 thread_stack 0x48400
/usr/sbin/mysqld(my_print_stacktrace+0x2e)[0x7f78c93f6a2e]
/usr/sbin/mysqld(handle_fatal_signal+0x2d5)[0x7f78c8f32905]
/lib/x86_64-linux-gnu/libpthread.so.0(+0xf8d0)[0x7f78c855f8d0]
/usr/sbin/mysqld(_ZN8MDL_lock11Ticket_list13remove_ticketEP10MDL_ticket+0x11)[0x7f78c8e84a81]
/usr/sbin/mysqld(_ZN8MDL_lock13remove_ticketEP7LF_PINSMS_NS_11Ticket_listEP10MDL_ticket+0x8c)[0x7f78c8e8569c]
/usr/sbin/mysqld(_ZN11MDL_context12release_lockE17enum_mdl_durationP10MDL_ticket+0x24)[0x7f78c8e86864]
/usr/sbin/mysqld(_ZN11MDL_context27release_locks_stored_beforeE17enum_mdl_durationP10MDL_ticket+0x3c)[0x7f78c8e868ec]
/usr/sbin/mysqld(_ZN11MDL_context27release_transactional_locksEv+0x15)[0x7f78c8e86d05]
/usr/sbin/mysqld(_Z21mysql_execute_commandP3THD+0xa75)[0x7f78c8da7505]
/usr/sbin/mysqld(_Z11mysql_parseP3THDPcjP12Parser_state+0x26e)[0x7f78c8dafd8e]
/usr/sbin/mysqld(+0x420589)[0x7f78c8db0589]
/usr/sbin/mysqld(_Z16dispatch_command19enum_server_commandP3THDPcj+0x16ec)[0x7f78c8db21fc]
/usr/sbin/mysqld(_Z10do_commandP3THD+0x16e)[0x7f78c8db2e3e]
/usr/sbin/mysqld(_Z24do_handle_one_connectionP3THD+0x182)[0x7f78c8e7d312]
/usr/sbin/mysqld(handle_one_connection+0x40)[0x7f78c8e7d4d0]
/lib/x86_64-linux-gnu/libpthread.so.0(+0x80a4)[0x7f78c85580a4]
/lib/x86_64-linux-gnu/libc.so.6(clone+0x6d)[0x7f78c670262d]

Trying to get some variables.
Some pointers may be invalid and cause the dump to abort.
Query (0x7f775e999020): REMOVED FOR OBVIOUS RESONS!
Connection ID (thread ID): 1008924
Status: NOT_KILLED

Optimizer switch: index_merge=on,index_merge_union=on,index_merge_sort_union=on,index_merge_intersection=on,index_merge_sort_intersection=off,engine_condition_pushdown=off,index_condition_pushdown=on,derived_merge=on,derived_with_keys=on,firstmatch=on,loosescan=on,materialization=on,in_to_exists=on,semijoin=on,partial_match_rowid_merge=on,partial_match_table_scan=on,subquery_cache=on,mrr=off,mrr_cost_based=off,mrr_sort_keys=off,outer_join_with_cache=on,semijoin_with_cache=on,join_cache_incremental=on,join_cache_hashed=on,join_cache_bka=on,optimize_join_buffer_size=off,table_elimination=on,extended_keys=on,exists_to_in=on,orderby_uses_equalities=off

The manual page at http://dev.mysql.com/doc/mysql/en/crashing.html contains
information that should help you find out what is causing the crash.