put threads with GLOBAL READ lock (executing ftwrl) into high priority queue

Version: Percona Server 5.6.19

1. With thread pool enabled:
root@sb 05:49:05>show variables like '%thread%pool%';
+-------------------------------+--------------+
| Variable_name | Value |
+-------------------------------+--------------+
| thread_pool_high_prio_mode | transactions |
| thread_pool_high_prio_tickets | 4294967295 |
| thread_pool_idle_timeout | 60 |
| thread_pool_max_threads | 100000 |
| thread_pool_oversubscribe | 3 |
| thread_pool_size | 32 |
| thread_pool_stall_limit | 30 |
+-------------------------------+--------------+
7 rows in set (0.00 sec)

2. start sysbench , with high concurrency updates, for example, 500 threads

3.
root@(none) 05:51:37>flush tables with read lock;
Query OK, 0 rows affected (0.01 sec)

root@(none) 05:51:44>show processlist; ----------- hang in same session.

^C^Z

I think it's better to put threads that own global mdl lock (by executing FLUSH TABLES WITH READ LOCK) into high priority queue.
PS : I am not completely sure if this is the exact reason that lead to hang. Still working on this issue.