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From "Brett Wooldridge (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (DERBY-4279) Statement cache deadlock
Date Wed, 27 Jun 2012 12:43:44 GMT

    [ https://issues.apache.org/jira/browse/DERBY-4279?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13402191#comment-13402191
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Brett Wooldridge commented on DERBY-4279:
-----------------------------------------

Kristian,

You might be right, I could be mistaken about a SELECT not blocking DDL.  Be that as it may,
this defect here is only tangential to DDL, and as the testcase shows, the deadlock occurs
without any DDL per se.  Just simple table-locking.  As to whether this change could adversely
affect DDL that is run concurrently with queries -- such as it used to block it but now doesn't,
I am not sure.  I don't have a test that runs DDL concurrently with queries, do you know if
any of the standard tests or stress tests do that?

                
> 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, 10.8.1.2
>         Environment: Windows Vista, OS X 10.5+
>            Reporter: Jeff Stuckman
>              Labels: derby_triage10_5_2
>         Attachments: Derby4279.java, client_stacktrace_activation_closed.txt, 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becomes out-of-date.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 After some time, the prepared statement becomes invalid or out-of-date for some reason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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