RFR: 8354076: LinkedBlockingDeque offer() creates nodes even if capacity has been reached

[Chen Liang]

On Tue, 8 Apr 2025 18:39:30 GMT, kabutz <duke at openjdk.org> wrote:

> In the JavaDoc of LinkedBlockingDeque, it states: "Linked nodes are dynamically created upon each insertion unless this would bring the deque above capacity." However, in the current implementation, nodes are always created, even if the deque is full. This is because count is non-volatile, and we only check inside the linkFirst/Last() methods whether the queue is full. At this point we have already locked and have created the Node. Instead, the count could be volatile, and we could check before locking.
> 
> In the current version, calling offer() on a full LinkedBlockingDeque creates unnecessary objects and contention. Similarly for poll() and peek(), we could exit prior to locking by checking the count field.
> 
> Our suggestion is to make count volatile, and then exiting early from poll() and offer()

If we strictly adhere to this interpretation of "create nodes", then we must only call a constructor after the locked capacity check has passed in places like `offerLast`, and this patch has not accomplished that yet.

I think the correct way to interpret the spec here is that "create node" means a node is created in the linked list, so we can interpret that as a change in collection membership reflected in accessor methods.

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PR Comment: https://git.openjdk.org/jdk/pull/24521#issuecomment-2803675483