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From "Brett Wooldridge (JIRA)" <j...@apache.org>
Subject [jira] Commented: (DERBY-4279) Statement cache deadlock
Date Fri, 11 Jun 2010 14:38:14 GMT

    [ https://issues.apache.org/jira/browse/DERBY-4279?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12877821#action_12877821
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Brett Wooldridge commented on DERBY-4279:
-----------------------------------------

Hi Knut, thanks for taking a look.

1) The synchronized block in GenericActivationHolder.execute() was actually completely commented
out in the original -- but the comment regarding the synchronized block remained.  I'm actually
unsure if there is indeed multi-threaded access to GenericActivationHolder -- but I believe
there can be.  Because that method replaces several member variables, it seems unsafe to do
so outside of the context of a synchronized block.  So, basically I just restored the synchronized
block.  If it can be shown that there is no multi-threaded access -- for example because of
a lock higher up the call chain, we can certainly remove it.  But being unfamiliar with the
Derby codebase in general, I decided to err on the side of caution.

2) My thinking on removing the statement from the cache was that the new statement is an equal
candidate to be cached, and because it was just recompiled the one that does exist in the
cache is stale in a sense.

3) I thought that too.  I think it would indeed be safe to remove to calls to notifyAll(),
at least in the case of GenericStatement.


> 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
>
>
> 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becomes out-of-date.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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