通过唯一索引S锁与X锁来了解MySQL死锁套路
在初学者从源码理解MySQL死锁问题中介绍了使用调试 MySQL 源码的方式来查看死锁的过程,这篇文章来讲讲一个常见的案例。
这次我们讲一段唯一索引 S 锁与 X 锁的爱恨情仇
我们来看一个简化过的例子
# 构造数据 CREATE TABLE `t1` ( `id` int(11) NOT NULL AUTO_INCREMENT, `name` varchar(10), `level` int(11), PRIMARY KEY (`id`), UNIQUE KEY `uk_name` (`name`) ); INSERT INTO `t1` (`name`, `level`) VALUES ('A',0); # 出现问题的sql语句如下,并发情况下就会出现死锁 INSERT ignore INTO `t1` (`name`, `level`) VALUES ('A',0); update t1 set level = 1 where name = "A";
我们用之前介绍过的源码分析方式,先来看下这两条语句分别加什么锁,然后分析死锁形成的过程。
第一条语句
INSERT ignore INTO t1 (name, level) VALUES ('A',0);
在调试中得到的结果如下
可以看到这条语句对唯一键 uk_name 加共享锁(S锁),而且成功。
第二条语句
update t1 set level = 1 where name = "A";
通过唯一键更新数据库字段。
这种情况在之前的文章已经介绍过,会对唯一索引加 X 锁,然后对主键索引加 X 锁
这样就可以非常轻松的复现死锁的问题了,步骤如下
1.开启两个 session,分别 begin
2.session1 执行INSERT ignore INTO t1 (name, level) VALUES ('A',0);
3.session2 执行INSERT ignore INTO t1 (name, level) VALUES ('A',0);
4.session1 执行update t1 set level = 1 where name = "A"; 进入等待状态
5.session2 执行update t1 set level = 1 where name = "A";,死锁产生,被回滚,同时事务 1 执行成功
详细的锁状态变化如下
t1 | t2 | 备注 |
---|---|---|
INSERT IGNORE INTO | - | t1成功获得uk的S锁 DB_SUCCESS |
- | INSERT IGNORE INTO | t2成功获得uk的S锁 DB_SUCCESS |
UPDATE | - | t1尝试获得uk的X锁,但没有成功,处于等待状态 DB_LOCK_WAIT |
- | UPDATE | t2尝试获得uk的X锁,发现死锁产生 DB_DEADLOCK |
- | Deadlock | t2释放S锁 |
成功 | - | - |
死锁日志如下:
LATEST DETECTED DEADLOCK ------------------------ 181208 23:00:52 *** (1) TRANSACTION: TRANSACTION 53A7, ACTIVE 162 sec starting index read mysql tables in use 1, locked 1 LOCK WAIT 3 lock struct(s), heap size 376, 2 row lock(s) MySQL thread id 12, OS thread handle 0x700010522000, query id 1424 localhost root Updating update t1 set level = 1 where name = "A" *** (1) WAITING FOR THIS LOCK TO BE GRANTED: RECORD LOCKS space id 89 page no 4 n bits 72 index `uk_name` of table `lock_demo2`.`t1` trx id 53A7 lock_mode X locks rec but not gap waiting Record lock, heap no 2 PHYSICAL RECORD: n_fields 2; compact format; info bits 0 0: len 1; hex 41; asc A;; 1: len 4; hex 80000001; asc ;; *** (2) TRANSACTION: TRANSACTION 53A8, ACTIVE 8 sec starting index read mysql tables in use 1, locked 1 3 lock struct(s), heap size 376, 2 row lock(s) MySQL thread id 96, OS thread handle 0x70001062e000, query id 1425 localhost root Updating update t1 set level = 1 where name = "A" *** (2) HOLDS THE LOCK(S): RECORD LOCKS space id 89 page no 4 n bits 72 index `uk_name` of table `lock_demo2`.`t1` trx id 53A8 lock mode S Record lock, heap no 2 PHYSICAL RECORD: n_fields 2; compact format; info bits 0 0: len 1; hex 41; asc A;; 1: len 4; hex 80000001; asc ;; *** (2) WAITING FOR THIS LOCK TO BE GRANTED: RECORD LOCKS space id 89 page no 4 n bits 72 index `uk_name` of table `lock_demo2`.`t1` trx id 53A8 lock_mode X locks rec but not gap waiting Record lock, heap no 2 PHYSICAL RECORD: n_fields 2; compact format; info bits 0 0: len 1; hex 41; asc A;; 1: len 4; hex 80000001; asc ;; *** WE ROLL BACK TRANSACTION (2)
来详细看一下这个死锁日志
*** (1) WAITING FOR THIS LOCK TO BE GRANTED: RECORD LOCKS space id 89 page no 4 n bits 72 index uk_name of table lock_demo2.t1 trx id 53A7 lock_mode X locks rec but not gap waiting
事务 1 想获取 uk_name 唯一索引上的 X 锁 (非 gap 锁的记录锁)
*** (2) HOLDS THE LOCK(S): RECORD LOCKS space id 89 page no 4 n bits 72 index uk_name of table lock_demo2.t1 trx id 53A8 lock mode S
事务 2 持有uk_name 唯一索引上的 S 锁(共享锁)
*** (2) WAITING FOR THIS LOCK TO BE GRANTED: RECORD LOCKS space id 89 page no 4 n bits 72 index uk_name of table lock_demo2.t1 trx id 53A8 lock_mode X locks rec but not gap waiting
事务 2 想获得 uk_name 唯一索引上的 X 锁(非 gap 锁的记录锁)
跟之前理论上推断的结论是一致的