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Darwin 0.2 Driver Kit

disk I/O thread - qlock conditions

work to do (ALL threads should wake up and run main loop once) when
	anything is added to either q (q_disk, q_nodisk), or
	-diskPresent needs to wakeup I/O thread to tell it "disk present"
		
another way:
work to do when 
	q_nodisk non-empty, or
	((!queue_empty(q_disk)) &&
		(lastReadyState != RS_NODISK) &&
		(lastReadyState != RS_EJECTING) &&
      		(!ejectPending)) 
		
threads sleep when
	q_disk and q_nodisk empty
	q_nodisk empty && no disk present
	
Update condition variable (WORK_TO_DO/NO_WORK_TO_DO):
	-- enqueueIoBuf - set uncond to WORK_TO_DO
	-- ioThreadWakeup - set uncond to WORK_TO_DO
	-- in I/O thread - before sleeping (to end of loop, at final unlock).
	
@interface NXCondition
{

}

- signal;
- broadcast;
- wait;

...IOThread:
{
	while(1) {
		service q_nodisk;
		service q_disk if appropriate;
		cal volCheckRequest if appropriate;
		[NXcondition wait];
	}
}

enqueueIoBuf:
{
	[qlock lock];
	enqueue IOBuf;
	[NXCondition broadcast];		// need a lock for this too?
	[qlock unlock];
}
..............

How about NXConditionLock..
in IOQueue:
	int workToDo;	
	
This is the "thread-work quanta" to do. Each IOBuf enqueued 

enqueueIoBuf
{
	[qlock lock];
	enqueue IOBuf;
	workToDo += numThreads;
	[qlock unlockWith:WORK_TO_DO];
}

IOThread
{
	while(1) {
		[qlock lockWhen:WORK_TO_DO];
		workToDo--;
		if(workToDo == 0)
			[qlock unlockWith:NO_WORK_TO_DO];
		else
			[qlock unlock];
		service queues;
		// all other unlocks leave condition variable alone.
	}
}

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