[concurrency-interest] AbstractQueuedSynchronizer

David Holmes davidcholmes at aapt.net.au
Thu Sep 22 08:24:58 EDT 2016

Park() has a token associated with it, so if unpark() is called before the
park() the park() will return immediately and consume the token.




From: Concurrency-interest
[mailto:concurrency-interest-bounces at cs.oswego.edu] On Behalf Of Bobrowski,
Sent: Thursday, September 22, 2016 9:57 PM
To: concurrency-interest at cs.oswego.edu
Subject: [concurrency-interest] AbstractQueuedSynchronizer


I have tried to understand the inner working of abstractQueuedSynchronizer
and one thing I cannot quite explain.


When a thread decides it needs to park itself (let's say the CountDownLatch
counter > 0), it does so by calling LockSuport.park. In the meantime, of
course, the latch can be fully counted down, all awaiting threads unparked,
while this thread is just about to park itself. Obviously that would not
work, as now we have one thread parked, on a latch that is fully counted
down, so no good.


This does not happen actually (I wrote a small test and verified), and it is
down to the code of LockSupport.park()


public static void park(Object blocker) {
    Thread t = Thread.currentThread();
    setBlocker(t, blocker);
    unsafe.park(false, 0L);
    setBlocker(t, null);


Somehow, when the latch is fully counted down and unsafe.park is called, it
immediately returns, so there is some magic behind the scenes that tells the
thread not to actually park itself. Anyone could cast some light onto what
is actually happening?







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