记录一次Mysql死锁排查过程

NNVvocapvd 4年前
   <p>以前接触到的数据库死锁,都是批量更新时加锁顺序不一致而导致的死锁,但是上周却遇到了一个很难理解的死锁。借着这个机会又重新学习了一下mysql的死锁知识以及常见的死锁场景。在多方调研以及和同事们的讨论下终于发现了这个死锁问题的成因,收获颇多。虽然是后端程序员,我们不需要像DBA一样深入地去分析与锁相关的源码,但是如果我们能够掌握基本的死锁排查方法,对我们的日常开发还是大有裨益的。</p>    <h2>死锁起因</h2>    <p>先介绍一下数据库和表情况,因为涉及到公司内部真是的数据,所以以下都做了模拟,不会影响具体的分析。</p>    <p>我们采用的是5.5版本的mysql数据库,事务隔离级别是默认的RR(Repeatable-Read),采用innodb引擎。假设存在test表:</p>    <pre>  <code class="language-sql">CREATE TABLE `test` (    `id` int(11) unsigned NOT NULL AUTO_INCREMENT,    `a` int(11) unsigned DEFAULT NULL,    PRIMARY KEY (`id`),    UNIQUE KEY `a` (`a`)  ) ENGINE=InnoDB AUTO_INCREMENT=100 DEFAULT CHARSET=utf8;  </code></pre>    <p>表的结构很简单,一个主键id,另一个唯一索引a。表里的数据如下:</p>    <pre>  <code class="language-sql">mysql> select * from test;  +----+------+  | id | a    |  +----+------+  |  1 |    1 |  |  2 |    2 |  |  4 |    4 |  +----+------+  3 rows in set (0.00 sec)  </code></pre>    <p>出现死锁的操作如下:</p>    <table>     <thead>      <tr>       <th>步骤</th>       <th>事务1</th>       <th>事务2</th>      </tr>     </thead>     <tbody>      <tr>       <td>1</td>       <td> </td>       <td>begin</td>      </tr>      <tr>       <td>2</td>       <td> </td>       <td>delete from test where a = 2;</td>      </tr>      <tr>       <td>3</td>       <td>begin</td>       <td> </td>      </tr>      <tr>       <td>4</td>       <td>delete from test where a = 2; (事务1卡住)</td>       <td> </td>      </tr>      <tr>       <td>5</td>       <td>提示出现死锁:ERROR 1213 (40001): Deadlock found when trying to get lock; try restarting transaction</td>       <td>insert into test (id, a) values (10, 2);</td>      </tr>     </tbody>    </table>    <p>然后我们可以通过 SHOW ENGINE INNODB STATUS; 来查看死锁日志:</p>    <pre>  <code class="language-sql">------------------------  LATEST DETECTED DEADLOCK  ------------------------  170219 13:31:31  *** (1) TRANSACTION:  TRANSACTION 2A8BD, ACTIVE 11 sec starting index read  mysql tables in use 1, locked 1  LOCK WAIT 2 lock struct(s), heap size 376, 1 row lock(s)  MySQL thread id 448218, OS thread handle 0x2abe5fb5d700, query id 18923238 renjun.fangcloud.net 121.41.41.92 root updating  delete from test where a = 2  *** (1) WAITING FOR THIS LOCK TO BE GRANTED:  RECORD LOCKS space id 0 page no 923 n bits 80 index `a` of table `oauthdemo`.`test` trx id 2A8BD lock_mode X waiting  Record lock, heap no 3 PHYSICAL RECORD: n_fields 2; compact format; info bits 32   0: len 4; hex 00000002; asc     ;;   1: len 4; hex 00000002; asc     ;;    *** (2) TRANSACTION:  TRANSACTION 2A8BC, ACTIVE 18 sec inserting  mysql tables in use 1, locked 1  4 lock struct(s), heap size 1248, 3 row lock(s), undo log entries 2  MySQL thread id 448217, OS thread handle 0x2abe5fd65700, query id 18923239 renjun.fangcloud.net 121.41.41.92 root update  insert into test (id,a) values (10,2)  *** (2) HOLDS THE LOCK(S):  RECORD LOCKS space id 0 page no 923 n bits 80 index `a` of table `oauthdemo`.`test` trx id 2A8BC lock_mode X locks rec but not gap  Record lock, heap no 3 PHYSICAL RECORD: n_fields 2; compact format; info bits 32   0: len 4; hex 00000002; asc     ;;   1: len 4; hex 00000002; asc     ;;    *** (2) WAITING FOR THIS LOCK TO BE GRANTED:  RECORD LOCKS space id 0 page no 923 n bits 80 index `a` of table `oauthdemo`.`test` trx id 2A8BC lock mode S waiting  Record lock, heap no 3 PHYSICAL RECORD: n_fields 2; compact format; info bits 32   0: len 4; hex 00000002; asc     ;;   1: len 4; hex 00000002; asc     ;;    *** WE ROLL BACK TRANSACTION (1)  </code></pre>    <h2>分析</h2>    <h3>阅读死锁日志</h3>    <p>遇到死锁,第一步就是阅读死锁日志。死锁日志通常分为两部分,上半部分说明了事务1在等待什么锁:</p>    <pre>  <code class="language-sql">170219 13:31:31  *** (1) TRANSACTION:  TRANSACTION 2A8BD, ACTIVE 11 sec starting index read  mysql tables in use 1, locked 1  LOCK WAIT 2 lock struct(s), heap size 376, 1 row lock(s)  MySQL thread id 448218, OS thread handle 0x2abe5fb5d700, query id 18923238 renjun.fangcloud.net 121.41.41.92 root updating  delete from test where a = 2  *** (1) WAITING FOR THIS LOCK TO BE GRANTED:  RECORD LOCKS space id 0 page no 923 n bits 80 index `a` of table `oauthdemo`.`test` trx id 2A8BD lock_mode X waiting  Record lock, heap no 3 PHYSICAL RECORD: n_fields 2; compact format; info bits 32   0: len 4; hex 00000002; asc     ;;   1: len 4; hex 00000002; asc     ;;  </code></pre>    <p>从日志里我们可以看到事务1当前正在执行 delete from test where a = 2 ,该条语句正在申请索引a的X锁,所以提示 lock_mode X waiting 。</p>    <p>然后日志的下半部分说明了事务2当前持有的锁以及等待的锁:</p>    <pre>  <code class="language-sql">*** (2) TRANSACTION:  TRANSACTION 2A8BC, ACTIVE 18 sec inserting  mysql tables in use 1, locked 1  4 lock struct(s), heap size 1248, 3 row lock(s), undo log entries 2  MySQL thread id 448217, OS thread handle 0x2abe5fd65700, query id 18923239 renjun.fangcloud.net 121.41.41.92 root update  insert into test (id,a) values (10,2)  *** (2) HOLDS THE LOCK(S):  RECORD LOCKS space id 0 page no 923 n bits 80 index `a` of table `oauthdemo`.`test` trx id 2A8BC lock_mode X locks rec but not gap  Record lock, heap no 3 PHYSICAL RECORD: n_fields 2; compact format; info bits 32   0: len 4; hex 00000002; asc     ;;   1: len 4; hex 00000002; asc     ;;    *** (2) WAITING FOR THIS LOCK TO BE GRANTED:  RECORD LOCKS space id 0 page no 923 n bits 80 index `a` of table `oauthdemo`.`test` trx id 2A8BC lock mode S waiting  Record lock, heap no 3 PHYSICAL RECORD: n_fields 2; compact format; info bits 32   0: len 4; hex 00000002; asc     ;;   1: len 4; hex 00000002; asc     ;;  </code></pre>    <p>从日志的 HOLDS THE LOCKS(S) 块中我们可以看到事务2持有索引a的X锁,并且是记录锁(Record Lock)。该锁是通过事务2在步骤2执行的delete语句申请的。由于是RR隔离模式下的基于唯一索引的等值查询(Where a = 2),所以会申请一个记录锁,而非next-key锁。</p>    <p>从日志的 WAITING FOR THIS LOCK TO BE GRANTED 块中我们可以看到事务2正在申请S锁,也就是共享锁。该锁是 insert into test (id,a) values (10,2) 语句申请的。 insert语句在普通情况下是会申请排他锁,也就是X锁,但是这里出现了S锁。这是因为a字段是一个唯一索引,所以insert语句会在插入前进行一次duplicate key的检查,为了使这次检查成功,需要申请S锁防止其他事务对a字段进行修改。</p>    <p>那么为什么该S锁会失败呢?这是 <strong>对同一个字段的锁的申请是需要排队的</strong> 。S锁前面还有一个未申请成功的X锁,所以S锁必须等待,所以形成了循环等待,死锁出现了。</p>    <p>通过阅读死锁日志,我们可以清楚地知道两个事务形成了怎样的循环等待,再加以分析,就可以逆向推断出循环等待的成因,也就是死锁形成的原因。</p>    <h3>死锁形成流程图</h3>    <p>为了让大家更好地理解死锁形成的原因,我们再通过表格的形式阐述死锁形成的流程:</p>    <table>     <thead>      <tr>       <th>步骤</th>       <th>事务1</th>       <th>事务2</th>      </tr>     </thead>     <tbody>      <tr>       <td>1</td>       <td> </td>       <td>begin</td>      </tr>      <tr>       <td>2</td>       <td> </td>       <td>delete from test where a = 2; 执行成功,事务2占有a=2下的X锁,类型为记录锁。</td>      </tr>      <tr>       <td>3</td>       <td>begin</td>       <td> </td>      </tr>      <tr>       <td>4</td>       <td>delete from test where a = 2; 事务1希望申请a=2下的X锁,但是由于事务2已经申请了一把X锁,两把X锁互斥,所以X锁申请进入锁请求队列。</td>       <td> </td>      </tr>      <tr>       <td>5</td>       <td>出现死锁,事务1权重较小,所以被选择回滚(成为牺牲品)。</td>       <td>insert into test (id, a) values (10, 2); 由于a字段建立了唯一索引,所以需要申请S锁以便检查duplicate key,由于插入的a的值还是2,所以排在X锁后面。但是前面的X锁的申请只有在事务2commit或者rollback之后才能成功,此时形成了循环等待,死锁产生。</td>      </tr>     </tbody>    </table>    <h2>拓展</h2>    <p>在排查死锁的过程中,有个同事还发现了上述场景会产生另一种死锁,该场景无法通过手工复现,只有高并发场景下才有可能复现。</p>    <p>该死锁对应的日志这里就不贴出了,与上一个死锁的核心差别是事务2等待的锁从S锁换成了X锁,也就是 lock_mode X locks gap before rec insert intention waiting 。我们还是通过表格来详细说明该死锁产生的流程:</p>    <table>     <thead>      <tr>       <th>步骤</th>       <th>事务1</th>       <th>事务2</th>      </tr>     </thead>     <tbody>      <tr>       <td>1</td>       <td> </td>       <td>begin</td>      </tr>      <tr>       <td>2</td>       <td> </td>       <td>delete from test where a = 2; 执行成功,事务2占有a=2下的X锁,类型为记录锁。</td>      </tr>      <tr>       <td>3</td>       <td>begin</td>       <td> </td>      </tr>      <tr>       <td>4</td>       <td> </td>       <td>【insert第1阶段】insert into test (id, a) values (10, 2); 事务2申请S锁进行duplicate key进行检查。检查成功。</td>      </tr>      <tr>       <td>5</td>       <td>delete from test where a = 2; 事务1希望申请a=2下的X锁,但是由于事务2已经申请了一把X锁,两把X锁互斥,所以X锁申请进入锁请求队列。</td>       <td> </td>      </tr>      <tr>       <td>6</td>       <td>出现死锁,事务1权重较小,所以被选择回滚(成为牺牲品)。</td>       <td>【insert第2阶段】insert into test (id, a) values (10, 2); 事务2开始插入数据,S锁升级为X锁,类型为insert intention。同理,X锁进入队列排队,形成循环等待,死锁产生。</td>      </tr>     </tbody>    </table>    <h2>总结</h2>    <p>排查死锁时,首先需要根据死锁日志分析循环等待的场景,然后根据当前各个事务执行的SQL分析出加锁类型以及顺序,逆向推断出如何形成循环等待,这样就能找到死锁产生的原因了。</p>    <p>PS:上述分析都是基于经验的推断,希望其他小伙伴们能够指出当中的错误以及不足指出,谢谢!</p>    <p> </p>    <p>来自:http://www.kissyu.org/2017/02/19/记录一次Mysql死锁排查过程/</p>    <p> </p>