本篇内容主要讲解“MySQL的RR模式下死锁”，感兴趣的朋友不妨来看看。本文介绍的方法操作简单快捷，实用性强。下面就让小编来带大家学习“MySQL的RR模式下死锁”吧!
如下构造方式，问为什么RC模式下不会死锁，RR模式下死锁。
drop table tt; CREATE TABLE `tt` ( `id` int(11) NOT NULL, `c1` int(11) DEFAULT NULL, PRIMARY KEY (`id`), UNIQUE KEY `c1` (`c1`) ) ENGINE=InnoDB DEFAULT CHARSET=utf8; insert into tt values(1,1);
| session 1 | session 2 |
|---|---|
| begin; | |
| select * from tt where c1=1 for update; | |
| update tt set id=2 where c1=1; | |
| begin;select * from tt where c1=1 for update;堵塞 | |
| select * from tt where c1=1 for update; | |
| 死锁回滚 |
select * from tt where c1=1 for update;
执行后的加锁行为
RR
---TRANSACTION 231106, ACTIVE 9 sec3 lock struct(s), heap size 1160, 2 row lock(s) MySQL thread id 11, OS thread handle 140737153623808, query id 303 localhost root TABLE LOCK table `test`.`tt` trx id 231106 lock mode IX RECORD LOCKS space id 127 page no 4 n bits 72 index c1 of table `test`.`tt` trx id 231106 lock_mode X(LOCK_X) locks rec but not gap(LOCK_REC_NOT_GAP) Record lock, heap no 2 PHYSICAL RECORD: n_fields 2; compact format; info bits 0 0: len 4; hex 80000001; asc ;; 1: len 4; hex 80000001; asc ;; RECORD LOCKS space id 127 page no 3 n bits 72 index PRIMARY of table `test`.`tt` trx id 231106 lock_mode X(LOCK_X) locks rec but not gap(LOCK_REC_NOT_GAP) Record lock, heap no 2 PHYSICAL RECORD: n_fields 4; compact format; info bits 0 0: len 4; hex 80000001; asc ;; 1: len 6; hex 0000000386c0; asc ;; 2: len 7; hex aa0000003f0110; asc ? ;; 3: len 4; hex 80000001; asc ;;
RC
---TRANSACTION 231105, ACTIVE 7 sec3 lock struct(s), heap size 1160, 2 row lock(s) MySQL thread id 11, OS thread handle 140737153623808, query id 295 localhost root TABLE LOCK table `test`.`tt` trx id 231105 lock mode IX RECORD LOCKS space id 127 page no 4 n bits 72 index c1 of table `test`.`tt` trx id 231105 lock_mode X(LOCK_X) locks rec but not gap(LOCK_REC_NOT_GAP) Record lock, heap no 2 PHYSICAL RECORD: n_fields 2; compact format; info bits 0 0: len 4; hex 80000001; asc ;; 1: len 4; hex 80000001; asc ;; RECORD LOCKS space id 127 page no 3 n bits 72 index PRIMARY of table `test`.`tt` trx id 231105 lock_mode X(LOCK_X) locks rec but not gap(LOCK_REC_NOT_GAP) Record lock, heap no 2 PHYSICAL RECORD: n_fields 4; compact format; info bits 0 0: len 4; hex 80000001; asc ;; 1: len 6; hex 0000000386c0; asc ;; 2: len 7; hex aa0000003f0110; asc ? ;; 3: len 4; hex 80000001; asc ;;
我们可以看到因为 c1是主键因此加锁方式不管怎么样都是LOCK_X|LOCK_REC_NOT_GAP，主键上也是同样的。就是锁住了二级唯一索引和主键的相关记录。
update tt set id=2 where c1=1;
执行后的加锁行为
这一句比较重要，在二级唯一索引c1上会做一个删除和插入操作，也就是会将原来的1,1记录标记为del flag，同时插入2,1这条记录，这会引起一个锁的继承操作(lock_rec_inherit_to_gap_if_gap_lock调用会出现GAP LOCK)，但是之前还会涉及到唯一性检查因此还涉及到LOCK_S锁和next key lock的存在(对于二级索引做唯一性检查始终是next key lock)。这里的del flag也是形成死锁的重要原因。(对于二级索引的update操作总是先删除然后插入记录，主键则会进行判断是否可以容下新的记录)
RR
---TRANSACTION 231106, ACTIVE 1626 sec5 lock struct(s), heap size 1160, 5 row lock(s), undo log entries 2MySQL thread id 11, OS thread handle 140737153623808, query id 305 localhost root TABLE LOCK table `test`.`tt` trx id 231106 lock mode IX RECORD LOCKS space id 127 page no 4 n bits 72 index c1 of table `test`.`tt` trx id 231106 lock_mode X(LOCK_X) locks rec but not gap(LOCK_REC_NOT_GAP) Record lock, heap no 2 PHYSICAL RECORD: n_fields 2; compact format; info bits 32 0: len 4; hex 80000001; asc ;; 1: len 4; hex 80000001; asc ;; RECORD LOCKS space id 127 page no 3 n bits 72 index PRIMARY of table `test`.`tt` trx id 231106 lock_mode X(LOCK_X) locks rec but not gap(LOCK_REC_NOT_GAP) Record lock, heap no 2 PHYSICAL RECORD: n_fields 4; compact format; info bits 32 0: len 4; hex 80000001; asc ;; 1: len 6; hex 0000000386c2; asc ;; 2: len 7; hex 2c000000410dca; asc , A ;; 3: len 4; hex 80000001; asc ;; RECORD LOCKS space id 127 page no 4 n bits 72 index c1 of table `test`.`tt` trx id 231106 lock mode S(LOCK_S) locks gap and rec(LOCK_ORDINARY[next_key_lock]) Record lock, heap no 1 PHYSICAL RECORD: n_fields 1; compact format; info bits 0 0: len 8; hex 73757072656d756d; asc supremum;; Record lock, heap no 2 PHYSICAL RECORD: n_fields 2; compact format; info bits 32 0: len 4; hex 80000001; asc ;; 1: len 4; hex 80000001; asc ;; RECORD LOCKS space id 127 page no 4 n bits 72 index c1 of table `test`.`tt` trx id 231106 lock mode S(LOCK_S) locks gap before rec(LOCK_GAP) Record lock, heap no 3 PHYSICAL RECORD: n_fields 2; compact format; info bits 0 0: len 4; hex 80000001; asc ;; 1: len 4; hex 80000002; asc ;;
RC
5 lock struct(s), heap size 1160, 5 row lock(s), undo log entries 2MySQL thread id 11, OS thread handle 140737153623808, query id 316 localhost root TABLE LOCK table `test`.`tt` trx id 231123 lock mode IX RECORD LOCKS space id 128 page no 4 n bits 72 index c1 of table `test`.`tt` trx id 231123 lock_mode X(LOCK_X) locks rec but not gap(LOCK_REC_NOT_GAP) Record lock, heap no 2 PHYSICAL RECORD: n_fields 2; compact format; info bits 32 0: len 4; hex 80000001; asc ;; 1: len 4; hex 80000001; asc ;; RECORD LOCKS space id 128 page no 3 n bits 72 index PRIMARY of table `test`.`tt` trx id 231123 lock_mode X(LOCK_X) locks rec but not gap(LOCK_REC_NOT_GAP) Record lock, heap no 2 PHYSICAL RECORD: n_fields 4; compact format; info bits 32 0: len 4; hex 80000001; asc ;; 1: len 6; hex 0000000386d3; asc ;; 2: len 7; hex 370000003206e2; asc 7 2 ;; 3: len 4; hex 80000001; asc ;; RECORD LOCKS space id 128 page no 4 n bits 72 index c1 of table `test`.`tt` trx id 231123 lock mode S(LOCK_S) locks gap and rec(LOCK_ORDINARY[next_key_lock]) Record lock, heap no 1 PHYSICAL RECORD: n_fields 1; compact format; info bits 0 0: len 8; hex 73757072656d756d; asc supremum;; Record lock, heap no 2 PHYSICAL RECORD: n_fields 2; compact format; info bits 32 0: len 4; hex 80000001; asc ;; 1: len 4; hex 80000001; asc ;; RECORD LOCKS space id 128 page no 4 n bits 72 index c1 of table `test`.`tt` trx id 231123 lock mode S(LOCK_S) locks gap before rec(LOCK_GAP) Record lock, heap no 3 PHYSICAL RECORD: n_fields 2; compact format; info bits 0 0: len 4; hex 80000001; asc ;; 1: len 4; hex 80000002; asc ;;
到这里RR和RC加锁并没有什么不同，因为都是唯一值，同时锁继承也都是二级索引上的都是LOCK_S|LOCK_ORDINARY[next_key_lock]，但是下面就会出现不同了。
select * from tt where c1=1 for update;
实际上这个时候存在2条c1=1的记录只有1,1标记为删除了，1,2没有提交，都是需要访问的。
然后堵塞行为为：
RR
LOCK WAIT 2 lock struct(s), heap size 1160, 1 row lock(s) MySQL thread id 10, OS thread handle 140737153824512, query id 350 localhost root statistics select * from tt where c1=1 for update ------- TRX HAS BEEN WAITING 11 SEC FOR THIS LOCK TO BE GRANTED: RECORD LOCKS space id 129 page no 4 n bits 72 index c1 of table `test`.`tt` trx id 231146 lock_mode X(LOCK_X) locks gap and rec(LOCK_ORDINARY[next_key_lock]) waiting(LOCK_WAIT) Record lock, heap no 2 PHYSICAL RECORD: n_fields 2; compact format; info bits 32 0: len 4; hex 80000001; asc ;; 1: len 4; hex 80000001; asc ;;
RC
LOCK WAIT 2 lock struct(s), heap size 1160, 1 row lock(s) MySQL thread id 10, OS thread handle 140737153824512, query id 325 localhost root statistics select * from tt where c1=1 for update ------- TRX HAS BEEN WAITING 9 SEC FOR THIS LOCK TO BE GRANTED: RECORD LOCKS space id 128 page no 4 n bits 72 index c1 of table `test`.`tt` trx id 231128 lock_mode X(LOCK_X) locks rec but not gap(LOCK_REC_NOT_GAP) waiting(LOCK_WAIT) Record lock, heap no 2 PHYSICAL RECORD: n_fields 2; compact format; info bits 32 0: len 4; hex 80000001; asc ;; 1: len 4; hex 80000001; asc ;;
我们这里可以看到对于RR模式下唯一键c1的1,1已经删除了。我做了debug发现这里会在函数中row_search_mvcc加锁前做判断如下：
if (!set_also_gap_locks || srv_locks_unsafe_for_binlog || trx->isolation_level <= TRX_ISO_READ_COMMITTED || (unique_search && !rec_get_deleted_flag(rec, comp)) || dict_index_is_spatial(index)) {
goto no_gap_lock;
} else {
lock_type = LOCK_ORDINARY;
}我们抛开其他来分析这两句
trx->isolation_level <= TRX_ISO_READ_COMMITTED
如果是RC模式则直接上LOCK_REC_NOT_GAP及只锁住记录本身
(unique_search && !rec_get_deleted_flag(rec, comp))
如果不是RC，如果是二级唯一索引并且没有被标记为del flag则标记为LOCK_REC_NOT_GAP。但是如果标记为del flag则标记为LOCK_ORDINARY就是next key lock。
select * from tt where c1=1 for update;
如上这个语句会访问1,1标记为删除了，1,2没有提交 的两个记录。这个时候就出现了不同。
RC
只需要唯一索引 1,1上 LOCK_REC_NOT_GAP|LOCK_X 及记录索引，这个锁在本事物的第一句语句上已经获得了，因此直接通过了，不需要做检测。
RR
需要在唯一索引 1,1上 LOCK_ORDINARY|LOCK_X 也就是就是next key lock。这个锁在本事物中并没有获取过，因此需要重新的检测所的兼容性，最终加入了等待列表。
我们来看一下函数lock_rec_lock_slow，我做debug的时候明显看到了不同的逻辑：
if (lock_rec_has_expl(mode, block, heap_no, trx)) {
/* The trx already has a strong enough lock on rec: do 1,1 key lock RR NEX KEY LOCK stronager
nothing */
lock_rec_print(mode,block,heap_no,index,thr_get_trx(thr));
err = DB_SUCCESS;
} else { const lock_t* wait_for = lock_rec_other_has_conflicting(
mode, block, heap_no, trx,index); if (wait_for != NULL) { /* If another transaction has a non-gap conflicting
request in the queue, as this transaction does not
have a lock strong enough already granted on the
record, we may have to wait. */
RecLock rec_lock(thr, index, block, heap_no, mode);
err = rec_lock.add_to_waitq(wait_for);
}最终RR下形成了环路
session1 首先获得唯一索引上的 1,1记录的 LOCK_REC_NOT_GAP|LOCK_X
然后session 1做update 获得唯一索引上的 1,1记录的 LOCK_ORDINARY(next key lock)|LOCK_S
然后session 2需要获取唯一索引上的 1,1记录的 LOCK_ORDINARY(next key lock)|LOCK_X 发生等待
然后session 1需要获取唯一索引上的 1,1记录的 LOCK_ORDINARY(next key lock)|LOCK_X 加入等待队列进行等待
死锁发生因此发生，而RC模式下最后两部需要获取的都是 LOCK_REC_NOT_GAP|LOCK_X，虽然session 2处于等待但是session因为已经获得相同级别的锁不会在进行检测加锁等待，而直接通过了。
下面是死锁的记录：
*** (1) WAITING FOR THIS LOCK TO BE GRANTED: RECORD LOCKS space id 117 page no 4 n bits 72 index c1 of table `t1`.`tt` trx id 230530 lock_mode X(LOCK_X) locks gap and rec(LOCK_ORDINARY[next_key_lock]) waiting(LOCK_WAIT) Record lock, heap no 2 PHYSICAL RECORD: n_fields 2; compact format; info bits 32 0: len 4; hex 80000001; asc ;; 1: len 4; hex 80000001; asc ;; *** (2) TRANSACTION: TRANSACTION 230525, ACTIVE 68 sec starting index read mysql tables in use 1, locked 16 lock struct(s), heap size 1160, 6 row lock(s), undo log entries 2MySQL thread id 6, OS thread handle 140737153423104, query id 156 localhost root statistics select * from tt where c1=1 for update *** (2) HOLDS THE LOCK(S): RECORD LOCKS space id 117 page no 4 n bits 72 index c1 of table `t1`.`tt` trx id 230525 lock_mode X(LOCK_X) locks rec but not gap(LOCK_REC_NOT_GAP) Record lock, heap no 2 PHYSICAL RECORD: n_fields 2; compact format; info bits 32 0: len 4; hex 80000001; asc ;; 1: len 4; hex 80000001; asc ;; *** (2) WAITING FOR THIS LOCK TO BE GRANTED: RECORD LOCKS space id 117 page no 4 n bits 72 index c1 of table `t1`.`tt` trx id 230525 lock_mode X(LOCK_X) locks gap and rec(LOCK_ORDINARY[next_key_lock]) waiting(LOCK_WAIT) Record lock, heap no 2 PHYSICAL RECORD: n_fields 2; compact format; info bits 32 0: len 4; hex 80000001; asc ;; 1: len 4; hex 80000001; asc ;; *** WE ROLL BACK TRANSACTION (1)
最后留下几个栈帧以备分析使用
锁继承
#0 lock_rec_set_nth_bit (lock=0x30b1230, i=3) at /root/mysqlall/percona-server-locks-detail-5.7.22/storage/innobase/include/lock0priv.ic:91#1 0x0000000001a5d44a in RecLock::lock_alloc (trx=0x7fffd7803b10, index=0x7ffe7459ff80, mode=546, rec_id=..., size=9) at /root/mysqlall/percona-server-locks-detail-5.7.22/storage/innobase/lock/lock0lock.cc:1484#2 0x0000000001a5d826 in RecLock::create (this=0x7fffec0c0eb0, trx=0x7fffd7803b10, owns_trx_mutex=false, add_to_hash=true, prdt=0x0) at /root/mysqlall/percona-server-locks-detail-5.7.22/storage/innobase/lock/lock0lock.cc:1537#3 0x0000000001a5e60c in lock_rec_add_to_queue (type_mode=546, block=0x7fff9adb8150, heap_no=3, index=0x7ffe7459ff80, trx=0x7fffd7803b10, caller_owns_trx_mutex=false) at /root/mysqlall/percona-server-locks-detail-5.7.22/storage/innobase/lock/lock0lock.cc:1853#4 0x0000000001a611ec in lock_rec_inherit_to_gap_if_gap_lock (block=0x7fff9adb8150, heir_heap_no=3, heap_no=1) at /root/mysqlall/percona-server-locks-detail-5.7.22/storage/innobase/lock/lock0lock.cc:2829#5 0x0000000001a62ea3 in lock_update_insert (block=0x7fff9adb8150, rec=0x7fff9b9c408c "\200") at /root/mysqlall/percona-server-locks-detail-5.7.22/storage/innobase/lock/lock0lock.cc:3659#6 0x0000000001c53c25 in btr_cur_optimistic_insert (flags=0, cursor=0x7fffec0c23f0, offsets=0x7fffec0c24c8, heap=0x7fffec0c13e0, entry=0x7ffe7403bb70, rec=0x7fffec0c24c0, big_rec=0x7fffec0c24b8, n_ext=0, thr=0x7ffe7403ba00, mtr=0x7fffec0c1bc0) at /root/mysqlall/percona-server-locks-detail-5.7.22/storage/innobase/btr/btr0cur.cc:3346#7 0x0000000001b195fe in row_ins_sec_index_entry_low (flags=0, mode=2, index=0x7ffe7459ff80, offsets_heap=0x7ffe7403bf98, heap=0x7ffe7403c448, entry=0x7ffe7403bb70, trx_id=0, thr=0x7ffe7403ba00, dup_chk_only=false) at /root/mysqlall/percona-server-locks-detail-5.7.22/storage/innobase/row/row0ins.cc:3166#8 0x0000000001b1a15e in row_ins_sec_index_entry (index=0x7ffe7459ff80, entry=0x7ffe7403bb70, thr=0x7ffe7403ba00, dup_chk_only=false) at /root/mysqlall/percona-server-locks-detail-5.7.22/storage/innobase/row/row0ins.cc:3421#9 0x0000000001b9e053 in row_upd_sec_index_entry (node=0x7ffe7403b6f8, thr=0x7ffe7403ba00) at /root/mysqlall/percona-server-locks-detail-5.7.22/storage/innobase/row/row0upd.cc:2337#10 0x0000000001b9e1c3 in row_upd_sec_step (node=0x7ffe7403b6f8, thr=0x7ffe7403ba00) at /root/mysqlall/percona-server-locks-detail-5.7.22/storage/innobase/row/row0upd.cc:2364
RR加锁del flag记录
#0 lock_rec_set_nth_bit (lock=0x30b28b8, i=2) at /root/mysqlall/percona-server-locks-detail-5.7.22/storage/innobase/include/lock0priv.ic:91#1 0x0000000001a5d44a in RecLock::lock_alloc (trx=0x7fffd7804080, index=0x7ffe74064ea0, mode=259, rec_id=..., size=9) at /root/mysqlall/percona-server-locks-detail-5.7.22/storage/innobase/lock/lock0lock.cc:1484#2 0x0000000001a5d826 in RecLock::create (this=0x7fffec0c1e00, trx=0x7fffd7804080, owns_trx_mutex=true, add_to_hash=true, prdt=0x0) at /root/mysqlall/percona-server-locks-detail-5.7.22/storage/innobase/lock/lock0lock.cc:1537#3 0x0000000001a5e1c4 in RecLock::add_to_waitq (this=0x7fffec0c1e00, wait_for=0x30b0e58, prdt=0x0) at /root/mysqlall/percona-server-locks-detail-5.7.22/storage/innobase/lock/lock0lock.cc:1731#4 0x0000000001a5ee37 in lock_rec_lock_slow (impl=0, mode=3, block=0x7fff4d027b20, heap_no=2, index=0x7ffe74064ea0, thr=0x7ffe7459ff60) at /root/mysqlall/percona-server-locks-detail-5.7.22/storage/innobase/lock/lock0lock.cc:2004#5 0x0000000001a5f1ce in lock_rec_lock (impl=false, mode=3, block=0x7fff4d027b20, heap_no=2, index=0x7ffe74064ea0, thr=0x7ffe7459ff60) at /root/mysqlall/percona-server-locks-detail-5.7.22/storage/innobase/lock/lock0lock.cc:2082#6 0x0000000001a69a02 in lock_sec_rec_read_check_and_lock (flags=0, block=0x7fff4d027b20, rec=0x7fff4da8c07e "\200", index=0x7ffe74064ea0, offsets=0x7fffec0c2690, mode=LOCK_X, gap_mode=0, thr=0x7ffe7459ff60) at /root/mysqlall/percona-server-locks-detail-5.7.22/storage/innobase/lock/lock0lock.cc:6457#7 0x0000000001b717f7 in sel_set_rec_lock (pcur=0x7ffe7459f6d0, rec=0x7fff4da8c07e "\200", index=0x7ffe74064ea0, offsets=0x7fffec0c2690, mode=3, type=0, thr=0x7ffe7459ff60, mtr=0x7fffec0c2180) at /root/mysqlall/percona-server-locks-detail-5.7.22/storage/innobase/row/row0sel.cc:1270#8 0x0000000001b7ab6a in row_search_mvcc (buf=0x7ffe7405b9c0 "\375\002", mode=PAGE_CUR_GE, prebuilt=0x7ffe7459f4b0, match_mode=1, direction=0) at /root/mysqlall/percona-server-locks-detail-5.7.22/storage/innobase/row/row0sel.cc:5591
RC加锁del flag记录
#0 lock_rec_lock_slow (impl=0, mode=1027, block=0x7fff3310cdf0, heap_no=2, index=0x7ffe74076d90, thr=0x7ffe7459fc20) at /root/mysqlall/percona-server-locks-detail-5.7.22/storage/innobase/lock/lock0lock.cc:1962#1 0x0000000001a5f1ce in lock_rec_lock (impl=false, mode=1027, block=0x7fff3310cdf0, heap_no=2, index=0x7ffe74076d90, thr=0x7ffe7459fc20) at /root/mysqlall/percona-server-locks-detail-5.7.22/storage/innobase/lock/lock0lock.cc:2082#2 0x0000000001a69a02 in lock_sec_rec_read_check_and_lock (flags=0, block=0x7fff3310cdf0, rec=0x7fff33bdc07e "\200", index=0x7ffe74076d90, offsets=0x7fffec0c2690, mode=LOCK_X, gap_mode=1024, thr=0x7ffe7459fc20) at /root/mysqlall/percona-server-locks-detail-5.7.22/storage/innobase/lock/lock0lock.cc:6457#3 0x0000000001b717f7 in sel_set_rec_lock (pcur=0x7ffe7459f6d0, rec=0x7fff33bdc07e "\200", index=0x7ffe74076d90, offsets=0x7fffec0c2690, mode=3, type=1024, thr=0x7ffe7459fc20, mtr=0x7fffec0c2180) at /root/mysqlall/percona-server-locks-detail-5.7.22/storage/innobase/row/row0sel.cc:1270#4 0x0000000001b7ab6a in row_search_mvcc (buf=0x7ffe7403ae80 "\375\002", mode=PAGE_CUR_GE, prebuilt=0x7ffe7459f4b0, match_mode=1, direction=0) at /root/mysqlall/percona-server-locks-detail-5.7.22/storage/innobase/row/row0sel.cc:5591#5 0x00000000019d5493 in ha_innobase::index_read (this=0x7ffe7403cda0, buf=0x7ffe7403ae80 "\375\002", key_ptr=0x7ffe74095600 "", key_len=5, find_flag=HA_READ_KEY_EXACT) at /root/mysqlall/percona-server-locks-detail-5.7.22/storage/innobase/handler/ha_innodb.cc:9536#6 0x0000000000f934aa in handler::index_read_map (this=0x7ffe7403cda0, buf=0x7ffe7403ae80 "\375\002", key=0x7ffe74095600 "", keypart_map=1, find_flag=HA_READ_KEY_EXACT) at /root/mysqlall/percona-server-locks-detail-5.7.22/sql/handler.h:2942
到此，相信大家对“MySQL的RR模式下死锁”有了更深的了解，不妨来实际操作一番吧！这里是亿速云网站，更多相关内容可以进入相关频道进行查询，关注我们，继续学习！
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