Deadlock in broker can occur when optimised dispatch is true
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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
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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