[infinispan-dev] Semaphore vs Lock

[Mircea Markus]

On 13 Mar 2012, at 15:17, Dan Berindei wrote:

> On Tue, Mar 13, 2012 at 2:53 PM, Galder Zamarreño <galder at redhat.com> wrote:
>> 
>> On Mar 8, 2012, at 12:42 PM, Dan Berindei wrote:
>> 
>>> On Wed, Mar 7, 2012 at 2:39 PM, Sanne Grinovero <sanne at infinispan.org> wrote:
>>>> On 7 March 2012 12:05, Galder Zamarreño <galder.zamarreno at redhat.com> wrote:
>>>>> Hi,
>>>>> 
>>>>> I was reading up about Java's Semaphores (http://docs.oracle.com/javase/6/docs/api/java/util/concurrent/Semaphore.html) and a couple of ideas came to my mind:
>>>>> 
>>>>> 1. Wouldn't it make sense to use binary semaphores instead of locks in Infinispan? We're already having to override ReentrantLock in order to have locks owned by Transactions rather than threads. Initially I thought it might make easier for deadlock detection, but not so sure right now cos we're already changing things to avoid thread ownership of locks.
>>>>> 
>>> 
>>> We don't support most of the Lock operations, so I think it would be
>>> fair to remove 'implements Lock' from the OwnableReentrantLock
>>> declaration. But we can't remove the reentrant part, as we acquire the
>>> lock when we put a value in L1 in DistributionInterceptor - after we
>>> have already acquired the lock once in LockInterceptor (that's before
>>> we even consider a pessimistic transaction doing multiple puts on the
>>> same key).
>> 
>> True, but reentrant is only needed for non-transactional scenarios. For pessimistric transactions, we have ownable locks where they're owned by transactions.
>> 
> 
> I meant reentrant in a more general sense - OwnableReentrantLock is
> reentrant because it allows the same transaction to lock the same key
> twice, even if it happens on different threads.
I'm not sure we even need reentrant locks for transactions: ATM the tx helds a collection with all the locks it already acquired and reentrancy can be avoided by just not trying re-lock again a key on which we already have a lock. This would simplify the lock impl considerably.