mysql共享锁问题，何时Mysql会隐性的加上Share锁

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mysql共享锁问题，何时Mysql会隐性的加上Share锁0

我的应用中没有显性的加lock in share mode ，看到的死锁日志却显示是有S锁，不知道何缘故，求解答
LATEST DETECTED DEADLOCK
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121016 12:27:30
*** (1) TRANSACTION:
TRANSACTION 1F73AC, ACTIVE 0 sec inserting
mysql tables in use 1, locked 1
LOCK WAIT 6 lock struct(s), heap size 1024, 5 row lock(s), undo log entries 3
MySQL thread id 6658, OS thread handle 0x4304, query id 194871 localhost 127.0.0.1 root update
insert into dbinstance(dbInstanceIdentifier,userProductId,dbInstanceClass,instanceCreateTime,instanceStatus,engine,engineVersion,dbName,port,autoBackupStartTime,autoBackupEndTime,backupRetentionPeriod,masterUsername,latestRestorableTime,allocStorage,deadTime,masterUserPassword,maintenanceStartTime,maintenanceEndTime,diskType,replicaUserPassword,dbParameters,serviceType,isUpgrade,ddbProductId,snapshotSrc,aggregationItemName,aggregationItemValue,version,sending) values('db1350361619378:myproduct45','myproduct45',2,1350361650050,3,'MySQL','5.5.20','mydb','3306',0,7200,0,'myadmin',0,20,0,null,1440,1439,'share','ozMGkRsoTFeTor3upLqw8uo+ESZDjPq2','\ndbp\n','RDS',0,null,'snap1350361619378:myproduct45',null,null,0,1)
*** (1) WAITING FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 0 page no 536 n bits 368 index `dbInstanceIdentifier_Unique` of table `rds`.`dbinstance` trx id 1F73AC lock_mode X locks gap before rec insert intention waiting
Record lock, heap no 274 PHYSICAL RECORD: n_fields 2; compact format; info bits 32
0: len 27; hex 6462313335303336313631393438363a6d7970726f647563743339; asc db1350361619486:myproduct39;;
1: len 8; hex 000000000000c98c; asc         ;;

*** (2) TRANSACTION:
TRANSACTION 1F73AA, ACTIVE 0 sec starting index read, thread declared inside InnoDB 500
mysql tables in use 1, locked 1
7 lock struct(s), heap size 1024, 6 row lock(s), undo log entries 3
MySQL thread id 6656, OS thread handle 0x45e8, query id 194872 localhost 127.0.0.1 root Updating
update dbinstance set dbParameters ='\ndbp\n' where dbInstanceIdentifier ='db1350361619486:myproduct39'
*** (2) HOLDS THE LOCK(S):
RECORD LOCKS space id 0 page no 536 n bits 368 index `dbInstanceIdentifier_Unique` of table `rds`.`dbinstance` trx id 1F73AA lock mode S
Record lock, heap no 274 PHYSICAL RECORD: n_fields 2; compact format; info bits 32
0: len 27; hex 6462313335303336313631393438363a6d7970726f647563743339; asc db1350361619486:myproduct39;;
1: len 8; hex 000000000000c98c; asc         ;;

Record lock, heap no 276 PHYSICAL RECORD: n_fields 2; compact format; info bits 32
0: len 26; hex 6462313335303336313631393637313a6d7970726f6475637438; asc db1350361619671:myproduct8;;
1: len 8; hex 000000000000c98e; asc         ;;

*** (2) WAITING FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 0 page no 536 n bits 368 index `dbInstanceIdentifier_Unique` of table `rds`.`dbinstance` trx id 1F73AA lock_mode X waiting
Record lock, heap no 274 PHYSICAL RECORD: n_fields 2; compact format; info bits 32
0: len 27; hex 6462313335303336313631393438363a6d7970726f647563743339; asc db1350361619486:myproduct39;;
1: len 8; hex 000000000000c98c; asc         ;;

*** WE ROLL BACK TRANSACTION (1)
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TRANSACTIONS
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Trx id counter 1F894B