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

[kabutz]

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()

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Commit messages:
 - Made code consistent with JavaDoc for only creating nodes if there is space in the LBD.

Changes: https://git.openjdk.org/jdk/pull/24521/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24521&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8354076
  Stats: 47 lines in 1 file changed: 18 ins; 16 del; 13 mod
  Patch: https://git.openjdk.org/jdk/pull/24521.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24521/head:pull/24521

PR: https://git.openjdk.org/jdk/pull/24521