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From "Przemek Bruski (JIRA)" <>
Subject [jira] [Created] (AMQ-3319) A possible browsing concurrency issue in
Date Thu, 12 May 2011 21:58:47 GMT
A possible browsing concurrency issue in

                 Key: AMQ-3319
             Project: ActiveMQ
          Issue Type: Bug
            Reporter: Przemek Bruski

Some time ago clients of our ActiveMQ instance locked up while browsing. Analysis of the log
files showed a large amount of:
	at java.util.LinkedList.remove(
	at java.util.LinkedList.removeFirst(
	at org.apache.activemq.thread.PooledTaskRunner.runTask(
	at org.apache.activemq.thread.PooledTaskRunner$
	at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(
	at java.util.concurrent.ThreadPoolExecutor$
Happening before lockup.

It seems there is a problem in Queue class, which uses non thread safe LinkedList collection.
Additions and removals to/from this collection are wrapped by a shared readLock, which means
there is no guard against concurrent modification and there is also a possibility of a race
condition between isEmpty and removeFirst call during concurrent usages of getNextBrowserDispatch
(if they are possible).

I think the easiest fix is to switch from LinkedList to ConcurrentLinkedQueue and make use
of Queue methods to access the collection (because they allow single step isEmpty/remove call).
I am attaching a patch that does it. I've left the readLocks in case they are used to block
writes someplace else, but they are not needed anymore for concurrency control over the new

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