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From "Knut Anders Hatlen (JIRA)" <j...@apache.org>
Subject [jira] Commented: (DERBY-3980) Conflicting select then update with REPEATABLE_READ gives lock timeout instead of deadlock
Date Mon, 22 Dec 2008 23:09:44 GMT

    [ https://issues.apache.org/jira/browse/DERBY-3980?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12658710#action_12658710
] 

Knut Anders Hatlen commented on DERBY-3980:
-------------------------------------------

I think I see now why we have a problem with a lower deadlock timeout. Since the updater is
supposed to be granted the lock after two seconds, we need to set the deadlock timeout to
one second in order to trigger the deadlock detection. However, just before the updater has
waited for one second, the first select thread releases its lock and wakes up all waiters.
This early wakeup makes the updater check if it can obtain the lock, which it can't, but it
won't do a deadlock check. It then goes back to sleep for another second, but before it wakes
up to perform deadlock detection, it is granted the lock. (It looks like we allow five early
wakeups before we reduce the time to wait for a deadlock. So for each early wakeup, the deadlock
timeout is effectively increased. This is probably a good way to do it, since early wakeups
indicate that it's probably not a deadlock yet.)

Easy workaround: Set the deadlock timeout to 1 second, and make the select threads hold the
locks for 4 seconds instead of 3 seconds. Then the update thread will wait for two seconds
before it gets the early wakeup (because the first select thread releases the lock), and by
that time it will already have performed deadlock detection.

> Conflicting select then update with REPEATABLE_READ gives lock timeout instead of deadlock
> ------------------------------------------------------------------------------------------
>
>                 Key: DERBY-3980
>                 URL: https://issues.apache.org/jira/browse/DERBY-3980
>             Project: Derby
>          Issue Type: Bug
>          Components: Store
>    Affects Versions: 10.1.3.1, 10.2.2.0, 10.3.3.0, 10.4.2.0, 10.5.0.0
>            Reporter: Kathey Marsden
>         Attachments: derby-3980_javadoc_and_test_diff.txt, derby.log, derby.log.10_1,
javacore.20081209.092827.9800.txt, LiveLockTest_diff.txt, LiveLockTest_with_Deadlock_look_diff.txt,
LockTimeoutWithInserts.java, TryTimeout.java, TryTimeout2.java, TryTimeout2.out.10_1.deadlock,
TryTimeout2.out.10_1.deadlock, TryTimeout2.out.10_1.locktimeout, TryTimeout2.out.10_1.locktimeout
>
>
> The attached program TryTimeout.java should detect a deadlock but instead throws a lock
timeout exception.  The program has two threads that attempt:
> 	    
> 	    threadConnection.setAutoCommit(false);
> 	    /* set isolation level to repeatable read */
> 	    threadConnection.setTransactionIsolation(Connection.TRANSACTION_REPEATABLE_READ);
> 	    
> 	    ResultSet rs = stmt.executeQuery("select * from t where i = 456");
> 	    while (rs.next());
> 	    stmt.executeUpdate("update t set i = 456 where i = 456");
> 	    threadConnection.commit();
> This gives SQLState 40001 (deadlock) with DB2 but a lock timeout with Derby.

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