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From "A Rahman (JIRA)" <ibatis-...@incubator.apache.org>
Subject [jira] Commented: (IBATIS-249) Race conditions in Throttle lead to thread blockage popping items from ThrottledPools under stress
Date Thu, 20 Mar 2008 10:25:24 GMT

    [ https://issues.apache.org/jira/browse/IBATIS-249?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12580720#action_12580720

A Rahman commented on IBATIS-249:

I took the latest code from SVN which has no Throttle, tested. It works GREAT, no deadlocks.
THANK YOU CLINTON! By the way iBATIS is an excellent tool, THANK YOU! 

I was in a big trouble with my application in production giving this deadlock problem, its
resolved, i will have a peacefull sleep.....

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
> 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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