I think what the parent was saying was that, for a futex, the kernel wakes you up when the condition might have been satisfied, but spurious wakes are possible

I think what the parent was saying was that, for a futex, the kernel wakes you up when the condition might have been satisfied, but spurious wakes are possible too. (I'm counting the simultaneous waking of multiple threads as spurious wakes here.) That's not the case with locks, which only wake up when the condition is satisfied.