[
https://issues.apache.org/jira/browse/DERBY-4279?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12879337#action_12879337
]
Brett Wooldridge commented on DERBY-4279:
-----------------------------------------
The original still had a synchronized block that synchronized on the prepared
statement. Was it unnecessary? I'm not so sure...
One concern is the statement above "GenericActivationHolder instances are
private to a transaction". Private to a transaction, and private to a thread
are two different things. We run in an XA environment, and theoretically (as
far as I understand XA), a transaction can span both connections and threads.
Another issue, is that, when running with the above patch I am seeing
intermittent failures in our server application. There are multiple threads
doing "DELETE FROM" and "INSERT INTO" the same table, and I am seeing an
occasional insert failure stating that the insert would cause a primary key
violation.
At this point, I am not sure it is related to the patch or not because our
server code has been shifting about as well. However, a short run using
10.6.1.0 without the patch did not see this particular error. Because the
issue is intermittent, and I unsure whether the cause is the patch or
application level code. Further testing tomorrow should clarify.
Note that running the derby test suite did not result in any errors.
> Statement cache deadlock
> ------------------------
>
> Key: DERBY-4279
> URL: https://issues.apache.org/jira/browse/DERBY-4279
> Project: Derby
> Issue Type: Bug
> Components: SQL
> Affects Versions: 10.0.2.1, 10.1.3.1, 10.2.2.0, 10.3.3.0, 10.4.2.0,
> 10.5.1.1
> Environment: Windows Vista
> Reporter: Jeff Stuckman
> Assignee: Brett Wooldridge
> Attachments: Derby4279.java, patch4279.txt, stacktrace.txt
>
>
> Due to a design flaw in the statement cache, a deadlock can occur if a
> prepared statement becomes out-of-date.
> I will illustrate this with the following example:
> The application is using the embedded Derby driver. The application has two
> threads, and each thread uses its own connection.
> There is a table named MYTABLE with column MYCOLUMN.
> 1. A thread prepares and executes the query SELECT MYCOLUMN FROM MYTABLE. The
> prepared statement is stored in the statement cache (see
> org.apache.derby.impl.sql.GenericStatement for this logic)
> 2. After some time, the prepared statement becomes invalid or out-of-date for
> some reason (see org.apache.derby.impl.sql.GenericPreparedStatement)
> 3. Thread 1 begins a transaction and executes LOCK TABLE MYTABLE IN EXCLUSIVE
> MODE
> 4. Thread 2 begins a transaction and executes SELECT MYCOLUMN FROM MYTABLE.
> The statement is in the statement cache but it is out-of-date. The thread
> begins to recompile the statement. To compile the statement, the thread needs
> a shared lock on MYTABLE. Thread 1 already has an exclusive lock on MYTABLE.
> Thread 2 waits.
> 5. Thread 1 executes SELECT MYCOLUMN FROM MYTABLE. The statement is in the
> statement cache but it is being compiled. Thread 1 waits on the statement's
> monitor.
> 6. We have a deadlock. Derby eventually detects a lock timeout, but the error
> message is not descriptive. The stacks at the time of the deadlock are:
> This deadlock is unique because it can still occur in a properly designed
> database. You are only safe if all of your transactions are very simple and
> cannot be interleaved in a sequence that causes the deadlock, or if your
> particular statements do not require a table lock to compile. (For the sake
> of simplicity, I used LOCK TABLE in my example, but any UPDATE statement
> would fit.)
--
This message is automatically generated by JIRA.
-
You can reply to this email to add a comment to the issue online.
