netty5/buffer
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
..
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