tp_timeout_handler acquires the LOCK_thd_kill before calling set_killed which tries to acquire the same lock.

Thread 164 (Thread 0x77d05f7ff160 (LWP 526603)):

#0  0x00007fffab927d28 in __lll_lock_wait (futex=0x200, private=<optimized out>) at lowlevellock.c:46

#1  0x00007fffab91c784 in __GI___pthread_mutex_lock (mutex=0x77936c0026e0) at ../nptl/pthread_mutex_lock.c:135

#2  0x000000011b1fdb78 in inline_mysql_mutex_lock (src_line=3829, src_file=0x11b9be898 "/home/danielgb/mariadb-server/sql/sql_class.h", that=0x77936c0026e0) at /home/danielgb/mariadb-server/include/mysql/psi/mysql_thread.h:712

#3  THD::set_killed (killed_err_msg_arg=0x0, killed_errno_arg=0, killed_arg=KILL_WAIT_TIMEOUT, this=0x77936c000ce8) at /home/danielgb/mariadb-server/sql/sql_class.h:3829

#4  tp_timeout_handler (c=0x10c43dc4a20) at /home/danielgb/mariadb-server/sql/threadpool_common.cc:473

#5  0x000000011b2b6fc4 in timeout_check (timer=<optimized out>) at /home/danielgb/mariadb-server/sql/threadpool_generic.cc:609

#6  timer_thread (param=0x11c0effd0 <pool_timer>) at /home/danielgb/mariadb-server/sql/threadpool_generic.cc:674

#7  0x00007fffab918770 in start_thread (arg=0x7fffab918670 <start_thread>) at pthread_create.c:465

#8  0x00007fffab035e38 in clone () at ../sysdeps/unix/sysv/linux/powerpc/powerpc64/clone.S:82

(gdb) up

#3  THD::set_killed (killed_err_msg_arg=0x0, killed_errno_arg=0, killed_arg=KILL_WAIT_TIMEOUT, this=0x77936c000ce8) at /home/danielgb/mariadb-server/sql/sql_class.h:3829

3829        mysql_mutex_lock(&LOCK_thd_kill);

(gdb) p LOCK_thd_kill

$2 = {m_mutex = {__data = {__lock = 2, __count = 0, __owner = 526603, __nusers = 1, __kind = 3, __spins = 0, __elision = 0, __list = {__prev = 0x0, __next = 0x0}}, 

    __size = "\002\000\000\000\000\000\000\000\v\t\b\000\001\000\000\000\003", '\000' <repeats 22 times>, __align = 2}, m_psi = 0x0}