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From "Brett Wooldridge (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (DERBY-4279) Statement cache deadlock
Date Sat, 30 Jun 2012 04:52:44 GMT

    [ https://issues.apache.org/jira/browse/DERBY-4279?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13404390#comment-13404390

Brett Wooldridge commented on DERBY-4279:


Thanks for your feedback.  From an efficiency point of view, I think your suggestion (and
my initial attempt at a bug fix) is a good approach in general.  As Bryan Pendleton commented

"I think the approach of allowing each caller to prepare the statement independently is valid
and useful. In this age of multi-core computing, I think doing possibly extra work like this
in search of higher concurrency (and fewer deadlocks) is a good technique."

I would however like to do that work under another bug (enhancement), as I think the most
recent patch to fix this bug is both sufficient and correct to address this bug per se.  When
this change has been accepted, I'll open another bug under which to do work on concurrency.

> 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, OS X 10.5+
>            Reporter: Jeff Stuckman
>              Labels: derby_triage10_5_2
>         Attachments: Derby4279.java, client_stacktrace_activation_closed.txt, no-lock-experiment.diff,
patch4279.txt, patch4279_2.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becomes out-of-date.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 After some time, the prepared statement becomes invalid or out-of-date for some reason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.)

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