DROP DATABASE very slow after innodb undo log truncate

(gdb) print buf_pool.mutex

bq. $1 = {m_mutex = {__data = {__lock = 2, __count = 0, __owner = 219, __nusers = 1, __kind = 3, __spins = 8, __elision = 0, __list = {__prev = 0x0, __next = 0x0}}, 

bq.     __size = "\002\000\000\000\000\000\000\000\333\000\000\000\001\000\000\000\003\000\000\000\b", '\000' <repeats 18 times>, __align = 2}, m_psi = 0x7fe0094c2bc0}

bq.

(gdb) print buf_pool.flush_list_mutex

$2 = {m_mutex = {__data = {__lock = 2, __count = 0, __owner = 44, __nusers = 2, __kind = 3, __spins = 10, __elision = 0, __list = {__prev = 0x0, __next = 0x0}}, 

    __size = "\002\000\000\000\000\000\000\000,\000\000\000\002\000\000\000\003\000\000\000\n", '\000' <repeats 18 times>, __align = 2}, m_psi = 0x7fe0094c2c40}

(gdb) thread apply all backtrace