[concurrency-interest] Reader-writer visibility inLinkedBlockingQueue

David Holmes davidcholmes at aapt.net.au
Thu Aug 15 22:50:30 EDT 2013

It is #2 - that is what the "and then reading n = count.get();" refers to.

  -----Original Message-----
  From: concurrency-interest-bounces at cs.oswego.edu
[mailto:concurrency-interest-bounces at cs.oswego.edu]On Behalf Of Ashwin
  Sent: Friday, 16 August 2013 12:36 PM
  To: concurrency-interest at cs.oswego.edu
  Subject: [concurrency-interest] Reader-writer visibility

  Hi, I have a (naive) question about the JDK 7+ LinkedBlockingQueue

  (I don't think this question has been asked before. I did search here
before deciding to use your time. I think the answer is evident but just
need a confirmation)

  In the source comment here it says:

    Visibility between writers and readers is provided as follows:

    Whenever an element is enqueued, the putLock is acquired and
    count updated.  A subsequent reader guarantees visibility to the
    enqueued Node by either acquiring the putLock (via fullyLock)
    or by acquiring the takeLock, and then reading n = count.get();
    this gives visibility to the first n items.

  My understanding of the code:

    1.. I noticed that the Node.next is not marked volatile
    2.. The dequeue thread always acquires the takeLock. The only time it
acquires, signals and quickly releases the putLock is when the queue has
room for just 1 more item before becoming full again

    3.. The enqueue thread does the opposite of the dequeue
    4.. So, except for the edge cases (recently full/recently empty) where
the reader and writer acquire and notify each other, how does the enqueue of
a new node (Node.next) become visible to the dequeue?

  Pardon my ignorance but does this mean that:

    1.. 2 threads that acquire 2 independent locks are enough to make the
"next" pointer visible?
    2.. Or is it the AtomicInteger's count.inc/dec that is providing the
desired memory effect? I think this is it, but I'm seeking clarification

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