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Date: Wed, 19 Mar 2008 23:08:24 -0700 (PDT)
From: "A Rahman (JIRA)" <ibatis-dev@incubator.apache.org>
To: dev@ibatis.apache.org
Subject: [jira] Commented: (IBATIS-249) Race conditions in Throttle lead to
 thread blockage popping items from ThrottledPools under stress
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    [ https://issues.apache.org/jira/browse/IBATIS-249?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12580678#action_12580678 ] 

A Rahman commented on IBATIS-249:
---------------------------------

Clinton, Thanks a lot for your quick reply. I will dig in my code. But here are my results. Please let me know when the next release will be available for download

When I test my application manually as a single user for more than an hour continuously doing lots of retrieval, insert, update and deletes, I don't see this problem, also I monitor the database at the same time no open connections left.

I ran a simple load test with 5 concurrent users, doing simple retrieval of records, the deadlock occurs and I also see some open connections waiting in the database to be closed and application is dead.

Abdur Rahman

> Race conditions in Throttle lead to thread blockage popping items from ThrottledPools under stress
> --------------------------------------------------------------------------------------------------
>
>                 Key: IBATIS-249
>                 URL: https://issues.apache.org/jira/browse/IBATIS-249
>             Project: iBatis for Java
>          Issue Type: Bug
>          Components: SQL Maps
>    Affects Versions: 2.1.7
>            Reporter: Jonathan Burstein
>            Assignee: Sven Boden
>             Fix For: 2.2.0
>
>         Attachments: IBATIS-249.diff
>
>
> com.ibatis.common.util.Throttle.increment contains a synchronization error. Currently, when a waiting thread returns from LOCK.wait it will increment count and return without checking that count is below limit. There are a number of situations where LOCK.wait can complete but the count will not be less than the limit:
> 1) The wait was interrupted
> 2) There was a spurious thread wakeup
> 3) Another thread obtained the lock between the time LOCK.notify was called (by a thread calling decrement) and the wait returned.
> The fix here is to re-check the value of count after exiting the wait (using a while loop). A small amount of extra logic is necessary to satisfy maxWait properly.
> ThrottledPool.pop attempts to work around these race conditions by catching swallowing all exceptions from throttle.increment and pool.remove(0) and looping until an item is obtained. Since the increment call is within the loop this logic can lead to multiple increments with no corresponding decrements. Note that an IndexOutOfBound exception from pool.remove(0) is an artifact of the bug in Throttle.increment -- this could never occur if Throttle behaved correctly.
> This routine should simple call throttle.increment and pool.remove(0). It should most certainly not swallow exceptions.
> Finally, ThrottledPool.push incorrectly calls throttle.decrement if the parameter is invalid (null or of an incorrect type).

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