Semaphores: Where do I learn about basic concepts such as permits, fairness, barging, etc

Question

The Semaphore class overview in developer.android.com looks pretty good - for those who are already familiar with the concepts and terminology.

I am familiar with some of the acronyms and other jargon there (e.g. FIFO, lock, etc.) but others such as permits, fairness and barging are new to me.

Can you recommend a good online source for explaining these concepts? (I can probably figure out what permits and fairness are but barging is an unknown at this point).

EDIT: After receiving the two answers below, I realized that I need a refresh on semaphores (to re-acquire() terminology). I found the following resources to be useful:

1. Semaphore_(programming)
2. Introduction to Semaphores by Dr. Richard S. Hall

Answer 1

http://download.oracle.com/javase/1.5.0/docs/api/java/util/concurrent/locks/ReentrantLock.html

http://download.oracle.com/javase/1.5.0/docs/api/java/util/concurrent/Semaphore.html

This is an excerpt from what is considered one of the seminal works in java concurrency you should check it out. http://my.safaribooksonline.com/book/programming/java/0321349601/explicit-locks/287

Answer 2

Hadn't come across these myself, but thought I'd research and summarise my findings as it's better to in-line answers than link externally (although, yes, the OP is after recommending reading):

permits are the number of concurrent accesses allowed to the semaphore-protected code. Although often semaphores are simple Mutex's, it is sometimes desirable to have more than one thread touching code. This is similar to phoning a call-centre, where there's one phone number connected to 8 lines/operators.

fairness is when a semaphore is made available to requesters in strict order of who requested first. Staying with the call-centre analogy, this means the on-hold queue is a strict FIFO.

barging is essentially an out-of-band request, that puts a thread to the top of the queue for a semaphore. The analogy is where preferred customers (or internal calls) go to the top of a call-centre queue, rather than waiting their turn.

If neither fairness nor barging are specified, then it's within spec to grant access to the most recent request, depending on timing of context switches. The 'phone analogy is a call to a company switchboard/reception, where even if calls are on hold waiting for answer, you may get lucky and ring between one call ending and another call being taken off-hold.

Let me know through comments if I've got this wrong, and I'll fix / cw my answer.