NonblockingQueue provides a nonblocking intrusive FIFO. It was originally developed for use by the G1 post-barrier, as an internal to G1 implementation detail. It was later moved to utilities because there were plans to use it in other contexts. Those plans never came to fruition. And after JDK-8342382, it is no longer used by the G1 post-barrier. So it's now (somewhat complicated) dead code.