Nick Hill fedf3ccecb Harden ref-counting concurrency semantics (#8583) ... Motivation #8563 highlighted race conditions introduced by the prior optimistic update optimization in 83a19d5650. These were known at the time but considered acceptable given the perf benefit in high contention scenarios. This PR proposes a modified approach which provides roughly half the gains but stronger concurrency semantics. Race conditions still exist but their scope is narrowed to much less likely cases (releases coinciding with retain overflow), and even in those cases certain guarantees are still assured. Once release() returns true, all subsequent release/retains are guaranteed to throw, and in particular deallocate will be called at most once. Modifications - Use even numbers internally (including -ve) for live refcounts - "Final" release changes to odd number (equivalent to refcount 0) - Retain still uses faster getAndAdd, release uses CAS loop - First CAS attempt uses non-volatile read - Thread.yield() after a failed CAS provides a net gain Result More (though not completely) robust concurrency semantics for ref counting; increased latency under high contention, but still roughly twice as fast as the original logic. Bench results to follow		2018-11-29 08:32:32 +01:00
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src	Harden ref-counting concurrency semantics (#8583)	2018-11-29 08:32:32 +01:00
pom.xml	[maven-release-plugin] prepare for next development iteration	2018-10-29 15:38:51 +00:00