RFR: 8355726: LinkedBlockingDeque fixes and improvements [v6]

[kabutz]

On Thu, 8 May 2025 14:27:07 GMT, kabutz <duke at openjdk.org> wrote:

>> test/jdk/java/util/concurrent/tck/LinkedBlockingDequeTest.java line 1958:
>> 
>>> 1956:         q.add(four);
>>> 1957:         q.add(five);
>>> 1958:         q.add(six);
>> 
>> Out of curiosity, how does `it.remove()` work under these conditions?
>
>> Out of curiosity, how does `it.remove()` work under these conditions?
> 
> If we call it.remove() on the first element, it delegates to unlinkFirst() (if we are using an ascending iterator), and unlinkLast (if we are using a descending iterator). Similarly, if we call it.remove() on the last element it will call unlinkLast() or unlinkFirst(). With unlinkFirst(), it will make f.next = f (thus linking back to itself) and with unlinkLast(), it will make l.prev = l.
> 
> If we call it.remove() on a middle element, then we simply link the p.next = n; n.prev = p; and does not do self-linking. Thus if we have an LBD with 1,2,3,4,5 with two iterators pointing onto 3, if one of them removes it, then the other will continue with 3 (cached), 4, 5, and it won't go back to the beginning and see duplicate elements.

I've added a unit test for this.

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PR Review Comment: https://git.openjdk.org/jdk/pull/24925#discussion_r2136084498