1.背景概述
客户业务产生死锁的报错，依据业务程序日志及业务流程，发现形成死锁的原因是：业务1 delete insert ，业务2 delete insert 2个业务交替履行导致的死锁；因为GAP锁堵塞了刺进意向锁，而且当delete的数据存在时死锁不会产生，当delete的数据不存在时，会产生死锁。
2.问题复现
本次测验根据 GreatSQL-8.0.32-24，阻隔级别为 RR
2.1 创建测验表
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 业务履行顺序
按以下业务履行顺序，假如要删去的数据存在，则不会产生死锁；假如要删去的数据不存，而且要删去的数据在同一个GAP锁的区间内则会产生死锁；
| 时间 | 业务1 | 业务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
业务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)
检查锁信息：
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)
此刻业务1给 3, 0x000000000201 这条数据加了 记载锁 X,REC_NOT_GAP
业务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)
检查锁信息：
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)
此刻业务2给 5, 0x000000000202 这条数据加了 记载锁 X,REC_NOT_GAP
业务1：insert
greatsql> insert into test value(3,3,3,3);
Query OK, 1 row affected (0.00 sec)
检查锁信息：
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)
此刻业务1给 被delete删去的数据 3, 0x000000000201 ，刺进的数据 3, 0x000000000202，以及相邻的下一条数据 5, 0x000000000202 加了 空隙锁 S,GAP
业务2：insert
greatsql> insert into test value(5,5,5,5);
Query OK, 1 row affected (0.01 sec)
检查锁信息：
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)
此刻业务2给 被delete删去的数据 5, 0x000000000202 ，刺进的数据 5, 0x000000000207，以及相邻的下一条数据 9, 0x000000000203 加了 空隙锁 S,GAP
因为GAP锁之间是彼此兼容的，所以没有产生锁等候及死锁，此刻业务1，业务2都履行完结，能够正常提交。
2.4 当delete的数据不存在时
业务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)
检查锁信息：
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)
此刻业务1给 9, 0x000000000203 这条数据加了 空隙锁 X,GAP
业务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)
检查锁信息：
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)
此刻业务2给 9, 0x000000000203 这条数据加了 空隙锁 X,GAP 空隙锁能够彼此兼容，因而没有报错
业务1：insert
greatsql> insert into test value(6,6,6,6);
---hang住，处于锁等候
检查锁信息：
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)
此刻业务1，要请求给 9, 0x000000000203 这条数据加 空隙锁，刺进意向锁 X,GAP,INSERT_INTENTION；因为业务2现已加了 空隙锁 X,GAP 空隙锁与刺进意向锁并不兼容，因而业务1的insert处于锁等候状态
业务2：insert
greatsql> insert into test value(7,7,7,7);
ERROR 1213 (40001): Deadlock found when trying to get lock; try restarting transaction
检查锁信息：
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)
业务2回滚，只要业务1的加锁信息。
因为此刻业务2，要请求给 9, 0x000000000203 这条数据加 空隙锁，刺进意向锁因为业务1现已加了 空隙锁 X,GAP 空隙锁与刺进意向锁并不兼容，因而业务2的insert处于锁等候状态，2个业务彼此等候锁导致死锁，此刻业务2回滚。
3.总结
此次死锁的产生主要是GAP 锁 和 刺进意向锁的冲突，主张让业务修改一下逻辑，先判别数据是否存在 select 一下，存在的话 delete 后在 insert ； 不存在的话直接 insert 不必delete了。
Enjoy GreatSQL :)
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