Rob Godfrey created QPID-6374:
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Summary: [Java Client] Eliminate deadlocks by acquiring locks in a
consistent order
Key: QPID-6374
URL: https://issues.apache.org/jira/browse/QPID-6374
Project: Qpid
Issue Type: Bug
Components: Java Client
Reporter: Rob Godfrey
Assignee: Rob Godfrey
Fix For: 0.31
A number of JIRAs exist relating to deadlocks in the Java client.
Deadlocks can generally be avoided if locks are always acquired in the same
order however the Java client threading model can make this hard.
Two locks which in combination cause particular issues are the
messageDeliveryLock and the failoverMutex.
The semantic purpose of the messageDeliveryLock is to meet the requirements of
JMS that session.close(), connection.close() and consumer.close() block while
the application is receiving a message in onMessage() or from receive(). The
lock should only be acquired from an application thread or from the onMessage
dispatcher thread - it should not be held by a thread acting on behalf of
processing caused by io processing (e.g. the broker closing the session or
connection). The lock is session specific.
The semantic purpose of the failover mutex is to ensure that application
actions which cause state change at the broker can are not engaged in while
failover is occurring. The lock is connection wide. In general the lock
should only be held by application threads, except for when failover is
occurring, in which case the thread managing failover will hold the lock.
Since messageDeliveryLock has a narrower scope (session) than the failoverMutex
(which is connection scoped) it makes sense to ensure that the
messageDeliveryLock is always acquired first. Changing this ordering where
necessary, and removing and path where the messageDeliveryLock can be acquired
by a non-application thread will greatly reduce the number of deadlocks seen.
Other deadlocks can be eliminated by removing the failoverMutex being acquired
by threads acting on behalf of io processing, and by only holding the
sessionCreationLock when setting the closed state or when actually creating a
session.
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