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案例 6：GDB 定位 MySQL5.7 特定版本 hang 死的故障

案例 6：GDB 定位 MySQL5.7 特定版本 hang 死的故障

作者：洪斌 1 问题背景 某环境上有一组Percona MySQL 5.7.23-23 的半同步主从,我们采用Prometheus 监控框架，按其接口规 范自研了独立的exporter 用于监控数据采集,类似于mysqld_exporter,工作方式大致为:

  1. 开启一个http 端口

  2. 采集本机MySQL 状态(简单的show 语句)

  3. 将状态发布到http 端口 exporter 工作模式比较简单,理论上不会对数据库造成严重影响。但在数据库节点上部署启动exporter 后，发现数据库的连接状态异常,对业务产生了一定影响。

  4. 检查连接可用性 1.1 通过TCP 检查连接可用性 – 新连接无法建立!! 1.2 通过Socket 检查连接可用性 – 仍然无法建立连接

  5. 已连接的业务报错查询超时

  6. 检查mysql-error.log – 没有任何内容

  7. 检查系统日志 – 没有任何相关信息

  8. 观察系统资源占用,无异常占用 为何再正常不过的状态采集会导致如此大的问题? 由于日志中已经无法继续判断原因. 所以迅速采集调 用栈信息: pstack $(pgrep -xn mysqld) Tips： 如何采集mysql 调用栈信息： https://www.ibm.com/developerworks/cn/linux/l-cn-deadlock/index.html 2 问题分析 梳理exporter 对数据库发起的SQL 语句:

  9. show global variables

  10. show global status

  11. show master status

  12. show slave status 通过SQL 语句列表和调用栈信息,大致可以观测出MySQL 内部出现了Mutex 死锁.死锁关系如下: 1.SHOW GLOBAL VARIABLES 持有LOCK_global_system_variables, 等待LOCK_log 2.SLAVE SQL THREAD 持有LOCK_log, 等待LOCK_status 3.SHOW GLOBAL STATUS 持有LOCK_status, 等待LOCK_global_system_variables 我们来看下每部分的调用栈信息： 源码部分基于Percona MySQL 5.7.23-23 2.1 SHOW GLOBAL VARIABLES Thread 18 (Thread 0x7fad69737700 (LWP 3588)): #0 0x00007fdd3527642d in __lll_lock_wait () from /lib64/libpthread.so.0 #1 0x00007fdd35271dcb in _L_lock_812 () from /lib64/libpthread.so.0

GDB 定位MySQL5.7 特定版本hang 死的故障分析#92108 #2 0x00007fdd35271c98 in pthread_mutex_lock () from /lib64/libpthread.so.0 #3 0x0000000000d60a5e in PolyLock_lock_log::rdlock() () #4 0x0000000000c3e4d5 in sys_var::value_ptr(THD*, THD*, enum_var_type, st_mysql_lex_string*) () #5 0x0000000000d1ce70 in get_one_variable_ext(THD*, THD*, st_mysql_show_var const*, enum_var_type, enum_mysql_show_type, system_status_var*, charset_info_st const**, char*, unsigned long*) () #6 0x0000000000d1cee1 in get_one_variable(THD*, st_mysql_show_var const*, enum_var_type, enum_mysql_show_type, system_status_var*, charset_info_st const**, char*, unsigned long*) () #7 0x0000000000d2234c in show_status_array(THD*, char const*, st_mysql_show_var*, enum_var_type, system_status_var*, char const*, TABLE_LIST*, bool, Item*) () #8 0x0000000000d27ec6 in fill_variables(THD*, TABLE_LIST*, Item*) () #9 0x0000000000d1550c in do_fill_table(THD*, TABLE_LIST*, QEP_TAB*) () #10 0x0000000000d286ac in get_schema_tables_result(JOIN*, enum_schema_table_state) () #11 0x0000000000d0afed in JOIN::prepare_result() () #12 0x0000000000c9a2cf in JOIN::exec() () #13 0x0000000000d0b93d in handle_query(THD*, LEX*, Query_result*, unsigned long long, unsigned long long) () #14 0x00000000007625d8 in execute_sqlcom_select(THD*, TABLE_LIST*) () #15 0x0000000000ccd191 in mysql_execute_command(THD*, bool) () #16 0x0000000000cd06f5 in mysql_parse(THD*, Parser_state*) () #17 0x0000000000cd12bd in dispatch_command(THD*, COM_DATA const*, enum_server_command) () #18 0x0000000000cd2cff in do_command(THD*) () #19 0x0000000000d9c8f8 in handle_connection () #20 0x0000000000f18904 in pfs_spawn_thread () #21 0x00007fdd3526fe25 in start_thread () from /lib64/libpthread.so.0 #22 0x00007fdd3344434d in clone () from /lib64/libc.so.6 关键代码 /sql\set_var.cc:267/ uchar *sys_var::value_ptr(THD *running_thd, …) { mysql_mutex_assert_owner(&LOCK_global_system_variables); } /sql\sys_vars.cc:3788/ void PolyLock_lock_log::rdlock() { mysql_mutex_lock(mysql_bin_log.get_log_lock()); }

2.2 SLAVE SQL THREAD Thread 25 (Thread 0x7fad7c0f5700 (LWP 47842)): #0 0x00007fdd3527642d in __lll_lock_wait () from /lib64/libpthread.so.0 #1 0x00007fdd35271e01 in _L_lock_1022 () from /lib64/libpthread.so.0 #2 0x00007fdd35271da2 in pthread_mutex_lock () from /lib64/libpthread.so.0 #3 0x0000000000e92d37 in MYSQL_BIN_LOG::publish_coordinates_for_global_status() const () #4 0x0000000000e99791 in MYSQL_BIN_LOG::ordered_commit(THD*, bool, bool) () #5 0x0000000000e9ccbf in MYSQL_BIN_LOG::commit(THD*, bool) () #6 0x00000000008084d1 in ha_commit_trans(THD*, bool, bool) () #7 0x0000000000d7c9c9 in trans_commit(THD*) () #8 0x0000000000e65efb in Xid_log_event::do_commit(THD*) () #9 0x0000000000e69b45 in Xid_apply_log_event::do_apply_event(Relay_log_info const*) () #10 0x0000000000e6f5dd in Log_event::apply_event(Relay_log_info*) () #11 0x0000000000eb87f6 in apply_event_and_update_pos(Log_event**, THD*, Relay_log_info*) () #12 0x0000000000ec4e90 in handle_slave_sql () #13 0x0000000000f18904 in pfs_spawn_thread () #14 0x00007fdd3526fe25 in start_thread () from /lib64/libpthread.so.0 #15 0x00007fdd3344434d in clone () from /lib64/libc.so.6 关键代码 /sql\binlog.cc:10424// void MYSQL_BIN_LOG::publish_coordinates_for_global_status(void) const { mysql_mutex_assert_owner(&LOCK_log); mysql_mutex_lock(&LOCK_status); } 2.3 SHOW GLOBAL STATUS Thread 14 (Thread 0x7fad69633700 (LWP 3613)): #0 0x00007fdd3527642d in __lll_lock_wait () from /lib64/libpthread.so.0 #1 0x00007fdd35271e01 in _L_lock_1022 () from /lib64/libpthread.so.0 #2 0x00007fdd35271da2 in pthread_mutex_lock () from /lib64/libpthread.so.0 #3 0x0000000000d222eb in show_status_array(THD, char const*, st_mysql_show_var*, enum_var_type, system_status_var*, char const*, TABLE_LIST*, bool, Item*) () #4 0x0000000000d29860 in fill_status(THD*, TABLE_LIST*, Item*) () #5 0x0000000000d1550c in do_fill_table(THD*, TABLE_LIST*, QEP_TAB*) ()

GDB 定位MySQL5.7 特定版本hang 死的故障分析#92108 #6 0x0000000000d286ac in get_schema_tables_result(JOIN*, enum_schema_table_state) () #7 0x0000000000d0afed in JOIN::prepare_result() () #8 0x0000000000c9a2cf in JOIN::exec() () #9 0x0000000000d0b93d in handle_query(THD*, LEX*, Query_result*, unsigned long long, unsigned long long) () #10 0x00000000007625d8 in execute_sqlcom_select(THD*, TABLE_LIST*) () #11 0x0000000000cca8a6 in mysql_execute_command(THD*, bool) () #12 0x0000000000cd06f5 in mysql_parse(THD*, Parser_state*) () #13 0x0000000000cd12bd in dispatch_command(THD*, COM_DATA const*, enum_server_command) () #14 0x0000000000cd2cff in do_command(THD*) () #15 0x0000000000d9c8f8 in handle_connection () #16 0x0000000000f18904 in pfs_spawn_thread () #17 0x00007fdd3526fe25 in start_thread () from /lib64/libpthread.so.0 #18 0x00007fdd3344434d in clone () from /lib64/libc.so.6 关键代码 /*sql\sql_show.cc:8020// int fill_status(THD *thd, TABLE_LIST *tables, Item *cond) { … mysql_mutex_lock(&LOCK_status); } /sql\sql_show.cc:3061/ static bool show_status_array(THD *thd, const char *wild,SHOW_VAR *variables, …) { … mysql_mutex_lock(&LOCK_global_system_variables); 2.4 问题1: 新连接为什么无法建立? 连接请求时的调用栈 Thread 6 (Thread 0x7fad6942b700 (LWP 4343)): #0 0x00007fdd3527642d in __lll_lock_wait () from /lib64/libpthread.so.0 #1 0x00007fdd35271e01 in _L_lock_1022 () from /lib64/libpthread.so.0 #2 0x00007fdd35271da2 in pthread_mutex_lock () from /lib64/libpthread.so.0 #3 0x0000000000c88ff7 in THD::init() () #4 0x0000000000c8be9f in THD::THD(bool) () #5 0x0000000000e54a1a in Channel_info::create_thd() () #6 0x0000000000d9fede in Channel_info_tcpip_socket::create_thd() () #7 0x0000000000d9c7c2 in handle_connection ()

#8 0x0000000000f18904 in pfs_spawn_thread () #9 0x00007fdd3526fe25 in start_thread () from /lib64/libpthread.so.0 #10 0x00007fdd3344434d in clone () from /lib64/libc.so.6 连接的初始化需要LOCK_global_system_variables, 此mutex 被占用,导致新连接无法初始化 /sql\sql_class.cc:1629/ void THD::init(void) { mysql_mutex_lock(&LOCK_global_system_variables); … mysql_mutex_unlock(&LOCK_global_system_variables); /* 新连接请求发起时一直被挂起,没有任何响应,现象如下: [root@localhost /]# mysql -uroot -p -S /mysql/data/mysqld.sock Enter password: ^C^C^C^C^C 2.5 问题2:查询操作为什么超时? 查询操作的调用栈 Thread 16 (Thread 0x7fad696b5700 (LWP 3597)): #0 0x00007fdd3527642d in __lll_lock_wait () from /lib64/libpthread.so.0 #1 0x00007fdd35271e01 in _L_lock_1022 () from /lib64/libpthread.so.0 #2 0x00007fdd35271da2 in pthread_mutex_lock () from /lib64/libpthread.so.0 #3 0x0000000000876508 in Item_func_get_system_var::fix_length_and_dec() () #4 0x000000000087953d in Item_func::fix_fields(THD*, Item**) () #5 0x0000000000c72485 in setup_fields(THD*, Bounds_checked_array<Item*>, List&, unsigned long, List, bool, bool) () #6 0x0000000000d072d5 in st_select_lex::prepare(THD) () #7 0x0000000000d0ba79 in handle_query(THD*, LEX*, Query_result*, unsigned long long, unsigned long long) () #8 0x00000000007625d8 in execute_sqlcom_select(THD*, TABLE_LIST*) () #9 0x0000000000ccd191 in mysql_execute_command(THD*, bool) () #10 0x0000000000cd06f5 in mysql_parse(THD*, Parser_state*) () #11 0x0000000000cd12bd in dispatch_command(THD*, COM_DATA const*, enum_server_command) () #12 0x0000000000cd2cff in do_command(THD*) () #13 0x0000000000d9c8f8 in handle_connection () #14 0x0000000000f18904 in pfs_spawn_thread ()

GDB 定位MySQL5.7 特定版本hang 死的故障分析#92108 #15 0x00007fdd3526fe25 in start_thread () from /lib64/libpthread.so.0 #16 0x00007fdd3344434d in clone () from /lib64/libc.so.6 SQL 查询时也需要LOCK_global_system_variables, 此mutex 被占用,导致查询被阻塞 /sql\item_func.cc:7275/ void Item_func_get_system_var::fix_length_and_dec() { … mysql_mutex_lock(&LOCK_global_system_variables); } 3 问题结论 经过一轮分析,我们基本找到问题原因. 但心中还存在疑问：

  1. LOCK_global_system_variables 是热点的mutex,大概率会出现争用,但不至于死锁。
  2. show global variables 操作为何需要LOCK_log?
  3. 这样常用的查询真的会稳定导致数据库死锁么? 进一步搜索,发现社区已有类似的缺陷反馈: https://bugs.mysql.com/bug.php?id=92108 该缺陷可理解为: MySQL 5.7.22 引入了如下两个参数:

binlog_transaction_dependency_tracking https://dev.mysql.com/doc/refman/5.7/en/replication-options-binary- log.html#sysvar_binlog_transaction_dependency_tracking 2. binlog_transaction_dependency_history_size https://dev.mysql.com/doc/refman/5.7/en/replication-options-binary- log.html#sysvar_binlog_transaction_dependency_history_size 在5.7.23 版本中,读取这两个参数需要LOCK_log.从而会导致各种各样的死锁(bug 正文中提到了另外一 种场景)。 在5.7.25 版本以后,这两个参数的读取使用了另外一种影响更低的lock,从而避免了本文以及bug 中提到 的死锁。 4 问题处理 对于环境中出现死锁的这一组Percona MySQL 5.7.23-23 半同步主从,我们使用5.7.25-28 版本进行了升 级后此问题消失,至此可以基本确定导致问题的根本原因。

案例 5：记一次 MySQL semaphore crash 的分析案例 7：delete 大表 slave 回放巨慢的问题分析