[concurrency-interest] Problem with ReentrantReadWriteLock

[Jon Rowland]

Hi,

I'm using an executor service with a fixed thread pool size of 2. Both tasks
use a custom-cache to which access is protected with a
ReentrantReadWriteLock. If I create the lock with fairness set to false
(default), everything works ok. However, if I create the lock with fairness
set to true, processing hangs (CPU idle) very quickly as if it is
deadlocked. Breaking into the debugger shows the thread details below.

It looks to me like:

Thread 1 has just entered the cache, and has no locks. It is waiting for the
Read lock:
	rwl.readLock().lock();

Thread 2 has just updated the cache. It has the write lock, and is trying to
downgrade to a Read lock with:
	rwl.readLock().lock();
	rwl.writeLock().unlock();

It looks to me like Thread 2 is not able to get the read lock - even though
it currently has the write lock.

Any ideas? Is there something that I am misunderstanding about the
'fairness' functionality? Something I'm doing wrong, or possible 'feature'?

Incidentally, with beta 1 - when it hung, it would spin on 100% CPU. With
beta 2 - it hangs but with the CPU idle.

Thanks and regards,
Jon Rowland

pool-1-thread-2@ad6 prio=5, in group "main", status: WAIT
	  park():-1, Unsafe.java
	  park():118, LockSupport.java
	  parkAndCheckInterrupt():681, AbstractQueuedSynchronizer.java
	  doAcquireShared():809, AbstractQueuedSynchronizer.java
	  acquireShared():1124, AbstractQueuedSynchronizer.java
	  lock():421, ReentrantReadWriteLock.java
	  getItem():70, DateCacheKeyOnDemand.java
	  getItem():30, DateCache.java
	  getRoute():30, SwtIdRutMatchStrRouteDateCache.java
	  matchCommon():160, PreMatch.java
	  matchCdrEvent():156, ServiceHelper.java
	  run():259, MatchAndRateTask.java
	  runTask():650, ThreadPoolExecutor.java
	  run():675, ThreadPoolExecutor.java
	  run():595, Thread.java

pool-1-thread-1@add prio=5, in group "main", status: WAIT
	  park():-1, Unsafe.java
	  park():118, LockSupport.java
	  parkAndCheckInterrupt():681, AbstractQueuedSynchronizer.java
	  doAcquireShared():809, AbstractQueuedSynchronizer.java
	  acquireShared():1124, AbstractQueuedSynchronizer.java
	  lock():421, ReentrantReadWriteLock.java
	  getItem():153, DateCacheKeyOnDemand.java
	  getItem():30, DateCache.java
	  getRoute():30, SwtIdRutMatchStrRouteDateCache.java
	  matchCommon():160, PreMatch.java
	  matchCdrEvent():156, ServiceHelper.java
	  run():259, MatchAndRateTask.java
	  runTask():650, ThreadPoolExecutor.java
	  run():675, ThreadPoolExecutor.java
	  run():595, Thread.java

Signal Dispatcher@ae1 daemon prio=10, in group "system", status: RUNNING

Finalizer@ae2 daemon prio=8, in group "system", status: WAIT
	  wait():-1, Object.java
	  remove():116, ReferenceQueue.java
	  remove():132, ReferenceQueue.java
	  run():159, Finalizer.java

Reference Handler@ae3 daemon prio=10, in group "system", status: WAIT
	  wait():-1, Object.java
	  wait():474, Object.java
	  run():116, Reference.java

main@1 prio=5, in group "main", status: WAIT
	  park():-1, Unsafe.java
	  parkNanos():146, LockSupport.java
	  awaitNanos():1808, AbstractQueuedSynchronizer.java
	  awaitTermination():1024, ThreadPoolExecutor.java
	  processFile():174, MatchAndRateTask.java
	  runTask():118, MatchAndRateTask.java
	  run():59, TestMatchAndRate.java
	  main():26, TestMatchAndRate.java


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