parking lot perf improvements
parking lot perf improvements

Summary:
Linux's futex checks the waiter count atomically before grabbing the lock.  This results in much better perf if there are no waiters and you call futexWake().
Add the same improvement to ParkingLot.

Reviewed By: yfeldblum

Differential Revision: D6639337

fbshipit-source-id: 6e03eff07e4c9ce327bc66d0aabd8cec051a5b19

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly