我正在使用MySQL 5.5。我注意到在并发方案中发生了特殊的死锁，并且我认为不应该发生此死锁。

使用两个同时运行的mysql客户端会话，像这样重现：

mysql会话1 ：

create table parent (id int(11) primary key);
insert into parent values (1);
create table child (id int(11) primary key, parent_id int(11), foreign key (parent_id) references parent(id));

begin;
insert into child (id, parent_id) values (10, 1);
-- this will create shared lock on parent(1)

mysql会话2 ：

begin;
-- try and get exclusive lock on parent row
select id from parent where id = 1 for update;
-- this will block because of shared lock in session 1

mysql会话1 ：

-- try and get exclusive lock on parent row
select id from parent where id = 1 for update;
-- observe that mysql session 2 transaction has been rolled back

mysql会话2 ：

ERROR 1213 (40001): Deadlock found when trying to get lock; try restarting transaction

报告的信息show engine innodb status是这样的：

------------------------
LATEST DETECTED DEADLOCK
------------------------
161207 10:48:56
*** (1) TRANSACTION:
TRANSACTION 107E67, ACTIVE 43 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 13074, OS thread handle 0x7f68eccfe700, query id 5530424 localhost root statistics
select id from parent where id = 1 for update
*** (1) WAITING FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 0 page no 3714 n bits 72 index `PRIMARY` of table `foo`.`parent` trx id 107E67 lock_mode X locks rec but not gap waiting
Record lock, heap no 2 PHYSICAL RECORD: n_fields 3; compact format; info bits 0
 0: len 4; hex 80000001; asc     ;;
 1: len 6; hex 000000107e65; asc     ~e;;
 2: len 7; hex 86000001320110; asc     2  ;;

*** (2) TRANSACTION:
TRANSACTION 107E66, ACTIVE 52 sec starting index read
mysql tables in use 1, locked 1
5 lock struct(s), heap size 1248, 2 row lock(s), undo log entries 1
MySQL thread id 12411, OS thread handle 0x7f68ecfac700, query id 5530425 localhost root statistics
select id from parent where id = 1 for update
*** (2) HOLDS THE LOCK(S):
RECORD LOCKS space id 0 page no 3714 n bits 72 index `PRIMARY` of table `foo`.`parent` trx id 107E66 lock mode S locks rec but not gap
Record lock, heap no 2 PHYSICAL RECORD: n_fields 3; compact format; info bits 0
 0: len 4; hex 80000001; asc     ;;
 1: len 6; hex 000000107e65; asc     ~e;;
 2: len 7; hex 86000001320110; asc     2  ;;

*** (2) WAITING FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 0 page no 3714 n bits 72 index `PRIMARY` of table `foo`.`parent` trx id 107E66 lock_mode X locks rec but not gap waiting
Record lock, heap no 2 PHYSICAL RECORD: n_fields 3; compact format; info bits 0
 0: len 4; hex 80000001; asc     ;;
 1: len 6; hex 000000107e65; asc     ~e;;
 2: len 7; hex 86000001320110; asc     2  ;;

*** WE ROLL BACK TRANSACTION (1)

您可以看到事务（1）不显示任何已获取的S或X锁；它只是试图获取独占锁而被阻止。据我了解，既然没有周期，那么在这种情况下就不会出现僵局。

这是一个已知的MySQL错误吗？有其他人遇到吗？使用了什么解决方法？

这些是我们可以采取的可能的步骤：

• 减少对外键的使用（在生产场景中，我们仅软删除引用表中的行，但是很麻烦）
• 预先获取排他锁，而不是隐式共享锁（这将降低我们的并发吞吐量）
• 更改逻辑，这样我们就不再需要在同一事务中对父级进行排他锁，而这会增加子行（风险较大和难度较大）
• 将我们的MySQL版本更改为不显示此行为的版本

还有其他我们不考虑的选择吗？