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When high priority replication slave applier encounters lock conflict in innodb, it will force the conflicting lock holder transaction (victim) to rollback. This is a must in multi-master sychronous replication model to avoid cluster lock-up. This high priority victim abort (aka "brute force" (BF) abort), is started from innodb lock manager while holding the victim's transaction's (trx) mutex. Depending on the execution state of the victim transaction, it may happen that the BF abort will call for THD::awake() to wake up the victim transaction for the rollback. Now, if BF abort requires THD::awake() to be called, then the applier thread executed locking protocol of: victim trx mutex -> victim THD::LOCK_thd_data If, at the same time another DBMS super user issues KILL command to abort the same victim, it will execute locking protocol of: victim THD::LOCK_thd_data -> victim trx mutex. These two locking protocol acquire mutexes in opposite order, hence unresolvable mutex locking deadlock may occur. The fix in this commit adds THD::wsrep_aborter flag to synchronize who can kill the victim This flag is set both when BF is called for from innodb and by KILL command. Either path of victim killing will bail out if victim's wsrep_killed is already set to avoid mutex conflicts with the other aborter execution. THD::wsrep_aborter records the aborter THD's ID. This is needed to preserve the right to kill the victim from different locations for the same aborter thread. It is also good error logging, to see who is reponsible for the abort. A new test case was added in galera.galera_bf_kill_debug.test for scenario where wsrep applier thread and manual KILL command try to kill same idle victim |
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