[jira] Assigned: (AMQ-1878) Deadlock in broker can occur when optimised dispatch is true

["Rob Davies (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/activemq/browse/AMQ-1878?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Rob Davies reassigned AMQ-1878:
-------------------------------

    Assignee: Rob Davies

> Deadlock in broker can occur when optimised dispatch is true
> ------------------------------------------------------------
>
>                 Key: AMQ-1878
>                 URL: https://issues.apache.org/activemq/browse/AMQ-1878
>             Project: ActiveMQ
>          Issue Type: Bug
>          Components: Broker
>    Affects Versions: 5.1.0
>            Reporter: David Sitsky
>            Assignee: Rob Davies
>            Priority: Critical
>         Attachments: deadlock.txt, queue.patch
>
>
> As an experiment in my application, I set optimised dispatch to be true for the broker and discovered during the startup of my application, a deadlock could occur.  The problem is the classic lock hierarchy issue, one thread has asked for lock A then B, another has asked for lock B then A.
> I've attached an annotated jstack output with just the deadlock information, annotated with the locks involved.  I have also added a patch which seems to resolve the issue, but I can't be sure if the change has other ramifications.
> The problem in a nutshell is one thread calls Queue.addSubscription(), which locks dispatchLock, and then calls wakeup/iterate() which blocks on iteratingMutex.
> Another thread meanwhile has called Queue.send(), which locks iteratingMutex via wakeup/iterate(), but then tries to call doPageIn() which requires dispatchLock.
> It seems to me addSubscription() doesn't need to hold on to dispatchLock while calling wakeup/iterate(), so I moved the wakeup() outside the usage of dispatchLock.  This ensures the lock heirarchy between iteratingMutex and dispatchLock is always obtained in the same order.  removeSubscription() has the same issue.

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