之前描述Mysql 5.6.15 Replication中碰到的死锁的情况，这次尝试debug下原因。
debug的过程
用参数–gdb启动mysql，按照步骤重现bug（让slave “show slave status”时卡住）。然后用gdb attach到slave mysql实例上。
(gdb) thread apply all bt
输出所有线程的backtrace，找到show slave status卡住的线程和位置
Thread 2 (Thread 0x7f583c166700 (LWP 2440)):
#0 0x00007f583f484054 in __lll_lock_wait () from /lib64/libpthread.so.0
#1 0x00007f583f47f3be in _L_lock_995 () from /lib64/libpthread.so.0
#2 0x00007f583f47f326 in pthread_mutex_lock () from /lib64/libpthread.so.0
#3 0x0000000000aa3cde in safe_mutex_lock (mp=0x3516ae8, try_lock=0 '\000', file=0xfb8e58 "/home/vagrant/mysql-5.6.12/sql/rpl_slave.cc", line=2611) at /home/vagrant/mysql-5.6.12/mysys/thr_mutex.c:152
#4 0x0000000000a4b993 in inline_mysql_mutex_lock (that=0x3516ae8, src_file=0xfb8e58 "/home/vagrant/mysql-5.6.12/sql/rpl_slave.cc", src_line=2611) at /home/vagrant/mysql-5.6.12/include/mysql/psi/mysql_thread.h:686
#5 0x0000000000a53cb3 in show_slave_status (thd=0x352e3d0, mi=0x34b4f20) at /home/vagrant/mysql-5.6.12/sql/rpl_slave.cc:2611
#6 0x00000000007d45f4 in mysql_execute_command (thd=0x352e3d0) at /home/vagrant/mysql-5.6.12/sql/sql_parse.cc:2766
#7 0x00000000007ddc46 in mysql_parse (thd=0x352e3d0, rawbuf=0x7f57ec005010 "show slave status", length=17, parser_state=0x7f583c165660) at /home/vagrant/mysql-5.6.12/sql/sql_parse.cc:6187
#8 0x00000000007d1019 in dispatch_command (command=COM_QUERY, thd=0x352e3d0, packet=0x3534e51 "", packet_length=17) at /home/vagrant/mysql-5.6.12/sql/sql_parse.cc:1334
#9 0x00000000007d017b in do_command (thd=0x352e3d0) at /home/vagrant/mysql-5.6.12/sql/sql_parse.cc:1036
#10 0x0000000000797a08 in do_handle_one_connection (thd_arg=0x352e3d0) at /home/vagrant/mysql-5.6.12/sql/sql_connect.cc:977
#11 0x00000000007974e4 in handle_one_connection (arg=0x352e3d0) at /home/vagrant/mysql-5.6.12/sql/sql_connect.cc:893
#12 0x0000000000aea87a in pfs_spawn_thread (arg=0x351b510) at /home/vagrant/mysql-5.6.12/storage/perfschema/pfs.cc:1855
#13 0x00007f583f47d851 in start_thread () from /lib64/libpthread.so.0
#14 0x00007f583e3e890d in clone () from /lib64/libc.so.6
可以看到show slave status卡在
#5 0x0000000000a53cb3 in show_slave_status (thd=0x352e3d0, mi=0x34b4f20) at /home/vagrant/mysql-5.6.12/sql/rpl_slave.cc:2611
查找源码可以看到show slave status卡在获取锁mi->rli->data_lock上
(科普下缩写: mi=master info, rli=relay log info
在gdb中运行命令
(gdb) thread 2
(gdb) f 5
(gdb) print mi->rli->data_lock
切换到thread 2堆栈第5层的上下文，打印出mi->rli->data_lock变量，输出如下
$1 = {m_mutex = {global = {__data = {__lock = 0, __count = 0, __owner = 0, __nusers = 0, __kind = 2, __spins = 0,
__list = {__prev = 0x0, __next = 0x0}},
__size = '\000' <repeats 16 times>, "\002", '\000' <repeats 22 times>, __align = 0}, mutex = {__data = {
__lock = 2, __count = 0, __owner = 2435, __nusers = 1, __kind = 3, __spins = 0, __list = {__prev = 0x0,
__next = 0x0}},
__size = "\002\000\000\000\000\000\000\000\203\t\000\000\001\000\000\000\003", '\000' <repeats 22 times>,
__align = 2}, file = 0xfa4520 "/home/vagrant/mysql-5.6.12/sql/log_event.cc", line = 7259, count = 1,
thread = 140016942216960}, m_psi = 0x0}
看到锁的owner是线程(LWP 2435)，为Thread 3
Thread 3的backtrace如下
Thread 3 (Thread 0x7f583c1a7700 (LWP 2435)):
#0 0x00007f583f4817bb in pthread_cond_timedwait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
#1 0x0000000000aa429d in safe_cond_timedwait (cond=0x7f57f4000ba8, mp=0x7f57f4000b38, abstime=0x7f583c1a60f0, file=0xedc960 "/home/vagrant/mysql-5.6.12/include/mysql/psi/mysql_thread.h", line=1199) at /home/vagrant/mysql-5.6.12/mysys/thr_mutex.c:278
#2 0x00000000007121f4 in inline_mysql_cond_timedwait (that=0x7f57f4000ba8, mutex=0x7f57f4000b38, abstime=0x7f583c1a60f0, src_file=0xedcb98 "/home/vagrant/mysql-5.6.12/sql/mdl.cc", src_line=1306) at /home/vagrant/mysql-5.6.12/include/mysql/psi/mysql_thread.h:1199
#3 0x0000000000713111 in MDL_wait::timed_wait (this=0x7f57f4000b38, owner=0x7f57f4000a50, abs_timeout=0x7f583c1a60f0, set_status_on_timeout=true, wait_state_name=0x14d0488) at /home/vagrant/mysql-5.6.12/sql/mdl.cc:1306
#4 0x0000000000714811 in MDL_context::acquire_lock (this=0x7f57f4000b38, mdl_request=0x7f583c1a6180, lock_wait_timeout=31536000) at /home/vagrant/mysql-5.6.12/sql/mdl.cc:2241
#5 0x000000000063656a in ha_commit_trans (thd=0x7f57f4000a50, all=true) at /home/vagrant/mysql-5.6.12/sql/handler.cc:1396 (COMMIT LOCK)
#6 0x00000000008a010b in trans_commit (thd=0x7f57f4000a50) at /home/vagrant/mysql-5.6.12/sql/transaction.cc:228
#7 0x0000000000a081bb in Xid_log_event::do_commit (this=0x7f57f4004730, thd=0x7f57f4000a50) at /home/vagrant/mysql-5.6.12/sql/log_event.cc:7174
#8 0x0000000000a0886e in Xid_log_event::do_apply_event (this=0x7f57f4004730, rli=0x3516650) at /home/vagrant/mysql-5.6.12/sql/log_event.cc:7310 (rli->data_lock)
#9 0x00000000009fd956 in Log_event::apply_event (this=0x7f57f4004730, rli=0x3516650) at /home/vagrant/mysql-5.6.12/sql/log_event.cc:3049
#10 0x0000000000a55e31 in apply_event_and_update_pos (ptr_ev=0x7f583c1a68a0, thd=0x7f57f4000a50, rli=0x3516650) at /home/vagrant/mysql-5.6.12/sql/rpl_slave.cc:3374
#11 0x0000000000a56e45 in exec_relay_log_event (thd=0x7f57f4000a50, rli=0x3516650) at /home/vagrant/mysql-5.6.12/sql/rpl_slave.cc:3742
#12 0x0000000000a5c334 in handle_slave_sql (arg=0x34b4f20) at /home/vagrant/mysql-5.6.12/sql/rpl_slave.cc:5552
#13 0x0000000000aea87a in pfs_spawn_thread (arg=0x350a800) at /home/vagrant/mysql-5.6.12/storage/perfschema/pfs.cc:1855
#14 0x00007f583f47d851 in start_thread () from /lib64/libpthread.so.0
#15 0x00007f583e3e890d in clone () from /lib64/libc.so.6
可以看到Thread 3卡在commit lock上，同时查源码看到Thread 3同时占有了rli->data_lock (log_event.cc:7259)
锁的状态
按照bug的描述，
- flush tables with read lock; 会持有commit lock
- IO thread (Thread 3)会持有rli->data_lock，并等待commit lock
- show slave status; 会等待rli->data_lock
结果导致show slave status卡住不可用
臆测一下解决方法
鉴于功底不深，只能臆测一下
- IO thread持有锁rli->data_lock的原因是要更新relay log的状态，然后进行commit(Xid_log_event::do_apply_event (log_event.cc:7248))。在commit的时候不会更新rli的数据。
- show slave status不会更新rli的数据，需要锁rli->data_lock的原因是要一致性数据。
因此可能的解决方案是IO thread持有读写锁，进行commit时转为持有读锁。show slave status只使用读锁。
只是臆测下解决方法，待bug #70307修掉时再学习。
