Make core capq operations wait-free. #6
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… capq_cap and migrate still need to be done.
…capq_dequeue,which is currently implemented in a lock free manner on top of capq_top() and capq_pop().
… to hatrack_config.h.
…lue was a valid top() at some point after its operation started. Always being willing to return a valid, but potentially stale value simplifies the hoops we need to jump through for wait freedom, as we can bound our search to just those items that potentially were enqueued when we read the enqueue epoch, unless updates loop around the store. That second case still leads to retries, but we bound the number of times we allow that to happen, by forcing store expansion if that becomes a problem.
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I've changed the overall capq implementation, refactoring it to ensure that calls to top() are wait-free, so that one can use this abstraction to implement other wait-free algorithms.
This actually simplified the algorithm and seems to have improved performance considerably as well.
Note that it's still possible to use this abstraction in a way that destroys the progress guarantees. For example, I still provide a capq_dequeue operation built on top of top() plus compare-and-pop(), which is only lock-free (the operation mainly exists so that we can use the same code for testing capq as we do for other queues).