[concurrency-interest] ReentrantReadWriteLock in inconsistent state

Phil Harvey phil at philharveyonline.com
Wed Aug 1 09:20:52 EDT 2012


We had a deadlock-like failure of our application recently.

I initially reported it on the BDB JE forum (
https://forums.oracle.com/forums/thread.jspa?messageID=10480988) but
further analysis of the heap and thread dumps has exposed a problem that
looks like a Java locking bug. I'm hoping you can offer advice on whether
this is the case.

We’re using Oracle JVM 1.6.0_25-b06, running on Linux version:
2.6.18-194.32.1.el5.

We are launching Java as follows: java -server -XX:+UseConcMarkSweepGC
-XX:+HeapDumpOnOutOfMemoryError -Xmx1024m ...

Several consecutive thread dumps showed that Thread t at 41101 was blocked
indefinitely in ReentrantReadWriteLock. writeLock().lock().

We know from code inspection that nothing ever takes a read lock on this
ReentrantReadWriteLock, so started trying to find out what has got its
write lock.

The output of "jstack -l" should list which thread holds this exclusive
lock in the "locked ownable synchronizers" section but does not.

Our first theory was that the owning thread might have terminated.

We wrote a simple test program to explore this. We found from heap dump
analysis that even if the owning thread terminates, the lock itself still
refers to it via the ReentrantReadWriteLock.WriteLock.sync.
exclusiveOwnerThread field. Looking in the java.util.concurrent source
code, it seems that this field only gets null'ed when the lock is released.

However, looking in the heap dump taken following our "deadlock", we were
surprised to find that the lock in question has a null
sync.exclusiveOwnerThread field.

Surely a write lock should be in one of two states (except possibly for a
tiny instant when its state is being non-atomically switched):

1) The lock is available, and sync.exclusiveOwnerThread is null 2) The lock
is unavailable, and sync.exclusiveOwnerThread is populated

But our lock was indefinitely in this state:

3) The lock is unavailable and sync.exclusiveOwnerThread is null

Does anyone know whether this represents a bug? If not, can you explain
what it means for a lock to be in this counterintuitive state?

Thanks, Phil
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