[binarycoffee]

Another offender is AtomicWaker [1] which does the same on contention.

[1] https://docs.rs/atomic-waker/latest/atomic_waker/

[SabrinaJewson]

AtomicWaker is much less bad, because it will only spin in the case that another thread has spuriously called `wake` – i.e. called `wake` despite that fact that the future it’s waking is in fact unable to progress.

In the common case, the waker side will operate some logic like:

    set_flag();
    atomic_waker.wake();

and the future will run:

    atomic_waker.register(cx.waker());
    if flag_is_set() {
        Poll::Ready(())
    } else {
        Poll::Pending
    }

Thus, even if `register` decides to “spin”, the flag will already be set, and so the future will not spin in reality (it may be polled unnecessarily, but this will only happen once).

I can’t immediately think of examples where support for spurious waking is necessary.

[binarycoffee]

The producers of an MPSC queue may use an AtomicWaker::wake to signal to the consumer that a new item is ready. In this case all wake-ups are necessary (not spurious).