1) indeed there are a couple of barriers that could be optimized 2) in addition to what Phil already said (which is that there's no priority inversion for Safa

1) indeed there are a couple of barriers that could be optimized 2) in addition to what Phil already said (which is that there's no priority inversion for Safari/WebKit to have as all threads taking these locks have the same priority to boot), the private lock you're talking about, named the handoff lock, has terrible unlock patterns, and is why it performs so poorly when WTF::Lock benched it.