[jbosscache-dev] OwnableReentrantLock

[Manik Surtani]

In implementing MVCC, I found that the existing  
ReadWriteWithUpgradeLock in JBoss Cache was hugely inefficient (lots  
of unnecessary syncs) and overkill in the support for readers which I  
did not need.  Looking around for something a little more "standard",  
I considered the j.u.c.locks.ReentrantLock, but found this useless as  
well since I need lock owners to be either Threads (if no tx is  
running) or a GlobalTransaction instance.  Furthermore, due to  
replication of events, lock owners should not be compared using  
identity (==) but equals() instead (important in the case of  
GlobalTransactions owning locks).

So I went about implementing my own lock.  After a few attempts using  
a concurrent queue and LockSupport.park(), and then fiddling with  
o.j.c.locks.AbstractQueuedSynchronizer (and several brainstorming  
sessions with Jason), I finally settled on an implementation that does  
look good enough.  Note that this is *not* a general-purpose lock and  
is tied to JBoss Cache, to be able to identify whether the lock to be  
acquired needs to be on a Thread or a GlobalTransaction.

Have a look at the OwnableReentrantLock [1].  Apart from containing  
minimal syncs and an efficient queueing algorithm thanks to AQS, one  
of the other upsells is that it implements Lock, a nice familiar  
interface.  One benefit here is that if the cache is configured not to  
use transactions, we can fall back to a j.u.c.locks.ReentrantLock  
instead.

Please have a look at the implementation, comments appreciated.

Cheers
Manik

[1] http://anonsvn.jboss.org/repos/jbosscache/core/trunk/src/main/java/org/jboss/cache/util/concurrent/locks/OwnableReentrantLock.java


--
Manik Surtani
Lead, JBoss Cache
manik at jboss.org