notes for abstractqueuesynchronizer

##Note for AbstractQueueSynchronizer

• Synchronizer process two kind of operation:
  1. acquire
  2. release

• operation of acquire as:

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if(synchronization state does not allow acquire){
	enqueue current thread if not already queued;
	block current thread
}
dequeue current thread if it was queued;








operation of release as:





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update synchronization state;
if(state may permit a blocked thread to acquire){
	unblock one or more queued threads;
}







Support for these operation require the coordination of these basic components:

Atomically managing synchronization state;
Blocking and unblocking thread;
Maintaing queue;



AbstractQueuedSynchronizer maintains synchronization state using only a single (32bit) int.


LockSupport.park operation issued the problem of Thread.suspend while multiple resume called before suspend through not counted.


Queue



There are two candicates to implemtent queue:

MCS(Mellor-Crummey and Scott).In fact,I don’t know what it is.
CLH,a low-level locks that is used only in spinlock.



The main additional modification needed to use CLH queues for blocking synchronizer is to provide an efficiency way for one node to locate its successor.

Once it need change,in spinlocks,the node will be notified on next spin by its successor.
But in blocking queue,the node need to explicitly wake up its successor by Lock.unpark.



To provide an atomically insertion node opertion.Because of the speciality of locate opertion,the node only need guarantee entail is atomic. 


Usage



Concrete subclass need to implement tryAcquire and tryRelease through by inspecting and updating state.


Shared-mode,ReentrantReadWriteLock,will wake up multiple thread by cascading singals.



			
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