Mysql: 一個死鎖場景的解決
阿新 • • 發佈:2020-12-27
update對資源搶佔的死鎖
業務場景
檔案系統裡面修改檔案的時候,會對上級檔案的修改時間進行修改。由於會同時修改多個檔案,所以,將他們的父目錄收集起來放在set裡面。最後執行update。注意,所有的操作都在一個事務裡面。
問題原因
最後對修改時間的update操作是無序的(set),而我們在使用update語句去更新的時候會鎖定一行資料,如果事務1鎖定了路徑A的檔案,然後去寫路徑B的檔案,而事務2鎖定了B,等待寫入路徑A就形成了死鎖。
復現
CREATE TABLE IF NOT EXISTS `file` ( `id` int(11) NOT NULL AUTO_INCREMENT, `file_parent` varchar(128) NOT NULL, `file_name` varchar(6) DEFAULT NULL, `create_time` datetime(6) NOT NULL, `modify_time` datetime(6) NOT NULL, PRIMARY KEY (`id`), UNIQUE KEY `index_path` (`file_parent`, `file_name`) ) ENGINE = InnoDB AUTO_INCREMENT = 5 DEFAULT CHARSET = utf8mb4; INSERT INTO cache.file (file_parent, file_name, create_time, modify_time) VALUES('/', 'root', now(), now()); INSERT INTO cache.file (file_parent, file_name, create_time, modify_time) VALUES('/root', 'sub', now(), now());
事務1:
mysql> begin; Query OK, 0 rows affected (0.00 sec) mysql> update file set modify_time = now() where file_parent = '/'; Query OK, 1 row affected (0.00 sec) Rows matched: 1 Changed: 1 Warnings: 0 mysql> update file set modify_tiime = now() where file_parent = '/root'; ERROR 1054 (42S22): Unknown column 'modify_tiime' in 'field list' mysql> update file set modify_time = now() where file_parent = '/root'; Query OK, 1 row affected (25.33 sec) Rows matched: 1 Changed: 1 Warnings: 0
事務2:
mysql> begin; Query OK, 0 rows affected (0.00 sec) mysql> update file set modify_time = now() where file_parent = '/root'; Query OK, 1 row affected (0.00 sec) Rows matched: 1 Changed: 1 Warnings: 0 mysql> update file set modify_time = now() where file_parent ='/'; ERROR 1213 (40001): Deadlock found when trying to get lock; try restarting transaction mysql>
復現的時候需要把死鎖的記錄開啟:
mysql> show variables like "%innodb_print_all_deadlocks%";
+----------------------------+-------+
| Variable_name | Value |
+----------------------------+-------+
| innodb_print_all_deadlocks | OFF |
+----------------------------+-------+
1 row in set (0.01 sec)
mysql> set global innodb_print_all_deadlocks=1
->
-> ;
Query OK, 0 rows affected (0.00 sec)
mysql> show variables like "%innodb_print_all_deadlocks%";
+----------------------------+-------+
| Variable_name | Value |
+----------------------------+-------+
| innodb_print_all_deadlocks | ON |
+----------------------------+-------+
1 row in set (0.00 sec)
資料庫日誌:
------------------------
LATEST DETECTED DEADLOCK
------------------------
2020-12-27 12:25:21 0x7f9ff84fa700
*** (1) TRANSACTION:
TRANSACTION 64536, ACTIVE 1164 sec starting index read
mysql tables in use 1, locked 1
LOCK WAIT 5 lock struct(s), heap size 1136, 4 row lock(s), undo log entries 1
MySQL thread id 9, OS thread handle 140325444028160, query id 79 localhost root updating
update file set modify_time = now() where file_parent = '/root'
*** (1) HOLDS THE LOCK(S):
RECORD LOCKS space id 47 page no 5 n bits 312 index index_path of table `cache`.`file` trx id 64536 lock_mode X
Record lock, heap no 3 PHYSICAL RECORD: n_fields 3; compact format; info bits 0
0: len 1; hex 2f; asc /;;
1: len 4; hex 726f6f74; asc root;;
2: len 4; hex 80000008; asc ;;
*** (1) WAITING FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 47 page no 5 n bits 312 index index_path of table `cache`.`file` trx id 64536 lock_mode X waiting
Record lock, heap no 4 PHYSICAL RECORD: n_fields 3; compact format; info bits 0
0: len 5; hex 2f726f6f74; asc /root;;
1: len 3; hex 737562; asc sub;;
2: len 4; hex 80000007; asc ;;
*** (2) TRANSACTION:
TRANSACTION 64537, ACTIVE 14 sec starting index read
mysql tables in use 1, locked 1
LOCK WAIT 4 lock struct(s), heap size 1136, 4 row lock(s), undo log entries 1
MySQL thread id 13, OS thread handle 140325442389760, query id 80 localhost root updating
update file set modify_time = now() where file_parent ='/'
*** (2) HOLDS THE LOCK(S):
RECORD LOCKS space id 47 page no 5 n bits 312 index index_path of table `cache`.`file` trx id 64537 lock_mode X
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 4 PHYSICAL RECORD: n_fields 3; compact format; info bits 0
0: len 5; hex 2f726f6f74; asc /root;;
1: len 3; hex 737562; asc sub;;
2: len 4; hex 80000007; asc ;;
*** (2) WAITING FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 47 page no 5 n bits 312 index index_path of table `cache`.`file` trx id 64537 lock_mode X waiting
Record lock, heap no 3 PHYSICAL RECORD: n_fields 3; compact format; info bits 0
0: len 1; hex 2f; asc /;;
1: len 4; hex 726f6f74; asc root;;
2: len 4; hex 80000008; asc ;;
*** WE ROLL BACK TRANSACTION (2)
第一個事務持有鎖第二個事務持有的鎖衝突了。互相等待造成死鎖。這是一個典型的資源無序操作造成的死鎖。
如何解決
將無序的操作改為有序即可。