GreatSQL 死锁案例分析
1.背景概述 客户业务发生死锁的报错，根据业务程序日志及业务流程，发现造成死锁的原因是：事务1 delete + insert ，事务2 delete + insert 2个事务交替执行导致的死锁；由于GAP锁阻塞了插入意向锁，并且当delete的数据存在时死锁不会发生，当delete的数据不存在时，会发生死锁。 2.问题复现 本次测试基于 GreatSQL-8.0.32-24，隔离级别为 RR 2.1 创建测试表 greatsql> create database test; greatsql> create table test(c1 int unique key, c2 int, c3 int, c4 int); greatsql> insert into test values (1,1,1,1),(3,3,3,3),(5,5,5,5),(9,9,9,9); greatsql> select * from test; +------+------+------+------+ | c1 | c2 | c3 | c4 | +------+------+------+------+ | 1 | 1 | 1 | 1 | | 3 | 3 | 3 | 3 | | 5 | 5 | 5 | 5 | | 9 | 9 | 9 | 9 | +------+------+------+------+ 4 rows in set (0.01 sec) 2.2 事务执行顺序 按以下事务执行顺序，如果要删除的数据存在，则不会发生死锁；如果要删除的数据不存，并且要删除的数据在同一个GAP锁的区间内则会发生死锁； 时间 事务1 事务2 T1 BEGIN; BEGIN; T2 delete from test where c1=?; T3 delete from test where c1=?; T4 insert into test value(?,?,?,?); T5 insert into test value(?,?,?,?); 2.3 当delete的数据存在时 事务1：delete greatsql> begin; Query OK, 0 rows affected (0.00 sec) greatsql> delete from test where c1=3; Query OK, 1 row affected (0.00 sec) 查看锁信息： greatsql> select THREAD_ID,OBJECT_SCHEMA,OBJECT_NAME,INDEX_NAME,LOCK_TYPE,LOCK_MODE,LOCK_STATUS,LOCK_DATA from performance_schema.data_locks; +-----------+---------------+-------------+-----------------+-----------+---------------+-------------+-------------------+ | THREAD_ID | OBJECT_SCHEMA | OBJECT_NAME | INDEX_NAME | LOCK_TYPE | LOCK_MODE | LOCK_STATUS | LOCK_DATA | +-----------+---------------+-------------+-----------------+-----------+---------------+-------------+-------------------+ | 59 | test | test | NULL | TABLE | IX | GRANTED | NULL | | 59 | test | test | c1 | RECORD | X,REC_NOT_GAP | GRANTED | 3, 0x000000000201 | | 59 | test | test | GEN_CLUST_INDEX | RECORD | X,REC_NOT_GAP | GRANTED | 0x000000000201 | +-----------+---------------+-------------+-----------------+-----------+---------------+-------------+-------------------+ 3 rows in set (0.00 sec) 此时事务1给 3, 0x000000000201 这条数据加了 记录锁 X,REC_NOT_GAP 事务2：delete greatsql> begin; Query OK, 0 rows affected (0.00 sec) greatsql> delete from test where c1=5; Query OK, 1 row affected (0.00 sec) 查看锁信息： greatsql> select THREAD_ID,OBJECT_SCHEMA,OBJECT_NAME,INDEX_NAME,LOCK_TYPE,LOCK_MODE,LOCK_STATUS,LOCK_DATA from performance_schema.data_locks; +-----------+---------------+-------------+-----------------+-----------+---------------+-------------+-------------------+ | THREAD_ID | OBJECT_SCHEMA | OBJECT_NAME | INDEX_NAME | LOCK_TYPE | LOCK_MODE | LOCK_STATUS | LOCK_DATA | +-----------+---------------+-------------+-----------------+-----------+---------------+-------------+-------------------+ | 57 | test | test | NULL | TABLE | IX | GRANTED | NULL | | 57 | test | test | c1 | RECORD | X,REC_NOT_GAP | GRANTED | 5, 0x000000000202 | | 57 | test | test | GEN_CLUST_INDEX | RECORD | X,REC_NOT_GAP | GRANTED | 0x000000000202 | | 59 | test | test | NULL | TABLE | IX | GRANTED | NULL | | 59 | test | test | c1 | RECORD | X,REC_NOT_GAP | GRANTED | 3, 0x000000000201 | | 59 | test | test | GEN_CLUST_INDEX | RECORD | X,REC_NOT_GAP | GRANTED | 0x000000000201 | +-----------+---------------+-------------+-----------------+-----------+---------------+-------------+-------------------+ 6 rows in set (0.00 sec) 此时事务2给 5, 0x000000000202 这条数据加了 记录锁 X,REC_NOT_GAP 事务1：insert greatsql> insert into test value(3,3,3,3); Query OK, 1 row affected (0.00 sec) 查看锁信息： greatsql> select THREAD_ID,OBJECT_SCHEMA,OBJECT_NAME,INDEX_NAME,LOCK_TYPE,LOCK_MODE,LOCK_STATUS,LOCK_DATA from performance_schema.data_locks; +-----------+---------------+-------------+-----------------+-----------+---------------+-------------+-------------------+ | THREAD_ID | OBJECT_SCHEMA | OBJECT_NAME | INDEX_NAME | LOCK_TYPE | LOCK_MODE | LOCK_STATUS | LOCK_DATA | +-----------+---------------+-------------+-----------------+-----------+---------------+-------------+-------------------+ | 57 | test | test | NULL | TABLE | IX | GRANTED | NULL | | 57 | test | test | c1 | RECORD | X,REC_NOT_GAP | GRANTED | 5, 0x000000000202 | | 57 | test | test | GEN_CLUST_INDEX | RECORD | X,REC_NOT_GAP | GRANTED | 0x000000000202 | | 59 | test | test | NULL | TABLE | IX | GRANTED | NULL | | 59 | test | test | c1 | RECORD | X,REC_NOT_GAP | GRANTED | 3, 0x000000000201 | | 59 | test | test | GEN_CLUST_INDEX | RECORD | X,REC_NOT_GAP | GRANTED | 0x000000000201 | | 59 | test | test | c1 | RECORD | S,GAP | GRANTED | 3, 0x000000000201 | | 59 | test | test | c1 | RECORD | S,GAP | GRANTED | 5, 0x000000000202 | | 59 | test | test | c1 | RECORD | S,GAP | GRANTED | 3, 0x000000000206 | +-----------+---------------+-------------+-----------------+-----------+---------------+-------------+-------------------+ 9 rows in set (0.00 sec) 此时事务1给 被delete删除的数据 3, 0x000000000201 ，插入的数据 3, 0x000000000202，以及相邻的下一条数据 5, 0x000000000202 加了 间隙锁 S,GAP 事务2：insert greatsql> insert into test value(5,5,5,5); Query OK, 1 row affected (0.01 sec) 查看锁信息： greatsql> select THREAD_ID,OBJECT_SCHEMA,OBJECT_NAME,INDEX_NAME,LOCK_TYPE,LOCK_MODE,LOCK_STATUS,LOCK_DATA from performance_schema.data_locks; +-----------+---------------+-------------+-----------------+-----------+---------------+-------------+-------------------+ | THREAD_ID | OBJECT_SCHEMA | OBJECT_NAME | INDEX_NAME | LOCK_TYPE | LOCK_MODE | LOCK_STATUS | LOCK_DATA | +-----------+---------------+-------------+-----------------+-----------+---------------+-------------+-------------------+ | 57 | test | test | NULL | TABLE | IX | GRANTED | NULL | | 57 | test | test | c1 | RECORD | X,REC_NOT_GAP | GRANTED | 5, 0x000000000202 | | 57 | test | test | GEN_CLUST_INDEX | RECORD | X,REC_NOT_GAP | GRANTED | 0x000000000202 | | 57 | test | test | c1 | RECORD | S,GAP | GRANTED | 5, 0x000000000202 | | 57 | test | test | c1 | RECORD | S,GAP | GRANTED | 9, 0x000000000203 | | 57 | test | test | c1 | RECORD | S,GAP | GRANTED | 5, 0x000000000207 | | 59 | test | test | NULL | TABLE | IX | GRANTED | NULL | | 59 | test | test | c1 | RECORD | X,REC_NOT_GAP | GRANTED | 3, 0x000000000201 | | 59 | test | test | GEN_CLUST_INDEX | RECORD | X,REC_NOT_GAP | GRANTED | 0x000000000201 | | 59 | test | test | c1 | RECORD | S,GAP | GRANTED | 3, 0x000000000201 | | 59 | test | test | c1 | RECORD | S,GAP | GRANTED | 5, 0x000000000202 | | 59 | test | test | c1 | RECORD | S,GAP | GRANTED | 3, 0x000000000206 | +-----------+---------------+-------------+-----------------+-----------+---------------+-------------+-------------------+ 12 rows in set (0.00 sec) 此时事务2给 被delete删除的数据 5, 0x000000000202 ，插入的数据 5, 0x000000000207，以及相邻的下一条数据 9, 0x000000000203 加了 间隙锁 S,GAP 由于GAP锁之间是相互兼容的，所以没有发生锁等待及死锁，此时事务1，事务2都执行完成，可以正常提交。 2.4 当delete的数据不存在时 事务1：delete greatsql> begin; Query OK, 0 rows affected (0.01 sec) greatsql> delete from test where c1=6; Query OK, 0 rows affected (0.00 sec) 查看锁信息： greatsql> select THREAD_ID,OBJECT_SCHEMA,OBJECT_NAME,INDEX_NAME,LOCK_TYPE,LOCK_MODE,LOCK_STATUS,LOCK_DATA from performance_schema.data_locks; +-----------+---------------+-------------+------------+-----------+-----------+-------------+-------------------+ | THREAD_ID | OBJECT_SCHEMA | OBJECT_NAME | INDEX_NAME | LOCK_TYPE | LOCK_MODE | LOCK_STATUS | LOCK_DATA | +-----------+---------------+-------------+------------+-----------+-----------+-------------+-------------------+ | 61 | test | test | NULL | TABLE | IX | GRANTED | NULL | | 61 | test | test | c1 | RECORD | X,GAP | GRANTED | 9, 0x000000000203 | +-----------+---------------+-------------+------------+-----------+-----------+-------------+-------------------+ 2 rows in set (0.00 sec) 此时事务1给 9, 0x000000000203 这条数据加了 间隙锁 X,GAP 事务2：delete greatsql> begin; Query OK, 0 rows affected (0.00 sec) greatsql> delete from test where c1=7; Query OK, 0 rows affected (0.00 sec) 查看锁信息： greatsql> select THREAD_ID,OBJECT_SCHEMA,OBJECT_NAME,INDEX_NAME,LOCK_TYPE,LOCK_MODE,LOCK_STATUS,LOCK_DATA from performance_schema.data_locks; +-----------+---------------+-------------+------------+-----------+-----------+-------------+-------------------+ | THREAD_ID | OBJECT_SCHEMA | OBJECT_NAME | INDEX_NAME | LOCK_TYPE | LOCK_MODE | LOCK_STATUS | LOCK_DATA | +-----------+---------------+-------------+------------+-----------+-----------+-------------+-------------------+ | 62 | test | test | NULL | TABLE | IX | GRANTED | NULL | | 62 | test | test | c1 | RECORD | X,GAP | GRANTED | 9, 0x000000000203 | | 61 | test | test | NULL | TABLE | IX | GRANTED | NULL | | 61 | test | test | c1 | RECORD | X,GAP | GRANTED | 9, 0x000000000203 | +-----------+---------------+-------------+------------+-----------+-----------+-------------+-------------------+ 4 rows in set (0.00 sec) 此时事务2给 9, 0x000000000203 这条数据加了 间隙锁 X,GAP 间隙锁可以相互兼容，因此没有报错 事务1：insert greatsql> insert into test value(6,6,6,6); ---hang住，处于锁等待 查看锁信息： greatsql> select THREAD_ID,OBJECT_SCHEMA,OBJECT_NAME,INDEX_NAME,LOCK_TYPE,LOCK_MODE,LOCK_STATUS,LOCK_DATA from performance_schema.data_locks; +-----------+---------------+-------------+------------+-----------+------------------------+-------------+-------------------+ | THREAD_ID | OBJECT_SCHEMA | OBJECT_NAME | INDEX_NAME | LOCK_TYPE | LOCK_MODE | LOCK_STATUS | LOCK_DATA | +-----------+---------------+-------------+------------+-----------+------------------------+-------------+-------------------+ | 62 | test | test | NULL | TABLE | IX | GRANTED | NULL | | 62 | test | test | c1 | RECORD | X,GAP | GRANTED | 9, 0x000000000203 | | 61 | test | test | NULL | TABLE | IX | GRANTED | NULL | | 61 | test | test | c1 | RECORD | X,GAP | GRANTED | 9, 0x000000000203 | | 61 | test | test | c1 | RECORD | X,GAP,INSERT_INTENTION | WAITING | 9, 0x000000000203 | +-----------+---------------+-------------+------------+-----------+------------------------+-------------+-------------------+ 5 rows in set (0.00 sec) 此时事务1，要申请给 9, 0x000000000203 这条数据加 间隙锁，插入意向锁 X,GAP,INSERT_INTENTION；由于事务2已经加了 间隙锁 X,GAP 间隙锁与插入意向锁并不兼容，因此事务1的insert处于锁等待状态 事务2：insert greatsql> insert into test value(7,7,7,7); ERROR 1213 (40001): Deadlock found when trying to get lock; try restarting transaction 查看锁信息： greatsql> select THREAD_ID,OBJECT_SCHEMA,OBJECT_NAME,INDEX_NAME,LOCK_TYPE,LOCK_MODE,LOCK_STATUS,LOCK_DATA from performance_schema.data_locks; +-----------+---------------+-------------+-----------------+-----------+------------------------+-------------+------------------------+ | THREAD_ID | OBJECT_SCHEMA | OBJECT_NAME | INDEX_NAME | LOCK_TYPE | LOCK_MODE | LOCK_STATUS | LOCK_DATA | +-----------+---------------+-------------+-----------------+-----------+------------------------+-------------+------------------------+ | 61 | test | test | NULL | TABLE | IX | GRANTED | NULL | | 61 | test | test | c1 | RECORD | X,GAP | GRANTED | 9, 0x000000000203 | | 61 | test | test | c1 | RECORD | X,GAP | GRANTED | 6, 0x000000000213 | | 61 | test | test | GEN_CLUST_INDEX | RECORD | X | GRANTED | supremum pseudo-record | | 61 | test | test | GEN_CLUST_INDEX | RECORD | X,GAP | GRANTED | 0x000000000213 | | 61 | test | test | c1 | RECORD | X,GAP,INSERT_INTENTION | GRANTED | 9, 0x000000000203 | +-----------+---------------+-------------+-----------------+-----------+------------------------+-------------+------------------------+ 6 rows in set (0.01 sec) 事务2回滚，只有事务1的加锁信息。 由于此时事务2，要申请给 9, 0x000000000203 这条数据加 间隙锁，插入意向锁由于事务1已经加了 间隙锁 X,GAP 间隙锁与插入意向锁并不兼容，因此事务2的insert处于锁等待状态，2个事务相互等待锁导致死锁，此时事务2回滚。 3.总结 此次死锁的发生主要是GAP 锁 和 插入意向锁的冲突，建议让业务修改一下逻辑，先判断数据是否存在 select 一下，存在的话 delete 后在 insert ； 不存在的话直接 insert 不用delete了。 Enjoy GreatSQL :) 关于 GreatSQL GreatSQL是适用于金融级应用的国内自主开源数据库，具备高性能、高可靠、高易用性、高安全等多个核心特性，可以作为MySQL或Percona Server的可选替换，用于线上生产环境，且完全免费并兼容MySQL或Percona Server。 相关链接： GreatSQL社区 Gitee GitHub Bilibili GreatSQL社区： 社区有奖建议反馈： https://greatsql.cn/thread-54-1-1.html 社区博客有奖征稿详情： https://greatsql.cn/thread-100-1-1.html （对文章有疑问或者有独到见解都可以去社区官网提出或分享哦~） 技术交流群： 微信&QQ群： QQ群：533341697 微信群：添加GreatSQL社区助手（微信号：wanlidbc ）好友，待社区助手拉您进群。
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