mtflush threads stall on shutdown

--innodb_use_mtflush=1 --innodb_mtflush_threads=32

--source include/have_innodb.inc

--source include/mysqladmin_shutdown.inc

...

2016-11-08 12:35:07 139764986319616 [Warning] InnoDB: 32 threads created by InnoDB had not exited at shutdown!

...

                if (srv_use_mtflush) {

                        /* g. Exit the multi threaded flush threads */

                        buf_mtflu_io_thread_exit();

                }