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From "Knut Anders Hatlen (JIRA)" <j...@apache.org>
Subject [jira] Commented: (DERBY-4279) Statement cache deadlock
Date Mon, 14 Jun 2010 09:43:13 GMT

    [ https://issues.apache.org/jira/browse/DERBY-4279?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12878516#action_12878516

Knut Anders Hatlen commented on DERBY-4279:

There's a smaller synchronized block within the block where the synchronization is commented
out (both synchronized on ps). The inner synchronization was added to address DERBY-3260.
The question was raised at that time too, why the synchronization had been commented out in
the first place. Since we didn't know why, we went for the minimum needed to fix that bug.
Unless there's some change in the patch that makes it necessary to synchronize on the entire
block, I'd be more comfortable with leaving the synchronization as it is for now. It's not
that I'm 100% convinced the current approach is correct, but if we find problems there, we
could address them in a separate issue.

As to multi-threaded access to GenericActivationHolders, I don't think that will happen. I
believe GenericActivationHolder instances are private to a transaction, and concurrent access
will be prevented by synchronization at a higher level. The instance stored in the ps field
can however be shared by multiple GenericActivationHolders, and the synchronization is there
to give a consistent view of that object's state.

Another thing that makes me think we should be careful with adding synchronization here, is
that it adds to the problem reported in DERBY-3024. I did a few runs on a 32-core machine
with the test client attached there, and saw a 10-30% performance degradation for various
multi-threaded configurations when the patch was applied.

> Statement cache deadlock
> ------------------------
>                 Key: DERBY-4279
>                 URL: https://issues.apache.org/jira/browse/DERBY-4279
>             Project: Derby
>          Issue Type: Bug
>          Components: SQL
>    Affects Versions:,,,,,
>         Environment: Windows Vista
>            Reporter: Jeff Stuckman
>            Assignee: Brett Wooldridge
>         Attachments: Derby4279.java, patch4279.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becomes out-of-date.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.)

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