Although it's not the code C++ will be using the Rust implementation is a bit easier to follow:
https://doc.rust-lang.org/src/std/sys/unix/locks/futex_mutex...
Unlock is just:: self.futex.swap(0, Release) -- if the value we get back is 2 then we know at least one thread is asleep waiting on this futex, so we need a system call to wake them, but in the uncontended case we're done immediately.
