The memory order Acquire in self.state.swap(WAITING, AcqRel); is not necessary

[xmh0511]

let res = self
    .state
    .compare_exchange(REGISTERING, WAITING, AcqRel, Acquire);  // #1
match res {
   Ok(_) => {}
   Err(actual) => {
    // This branch can only be reached if at least one
    // concurrent thread called `wake`. In this
    // case, `actual` **must** be `REGISTERING |
    // `WAKING`.
     debug_assert_eq!(actual, REGISTERING | WAKING);
     let waker = (*self.waker.get()).take().unwrap();
    // We need to return to WAITING state (clear our lock and
    // concurrent WAKING flag). This needs to acquire all
    // WAKING fetch_or releases and it needs to release our
    // update to self.waker, so we need a `swap` operation.
     self.state.swap(WAITING, AcqRel);  // #2
    // ...
  }
}

As shown in the comment, the Err(actual) branch can only be reached if there is a concurrent wake invoked during the REGISTERING, since the load part of compare_exchange is Acquire, and the writing part of self.state.fetch_or(WAKING, AcqRel) is Release, that is, #1 has synchronized with self.state.fetch_or, the reading part of #2 is unnecessary to be Acquire.

IIUC, self.state.swap(WAITING, AcqRel); can be changed to self.state.swap(WAITING, Release);

[xmh0511]

Or, the purpose is to ensure #2 synchronizes with another intervening wake that occurred during the branch?