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 :)

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