Cherry-pick 08c21e7

    * Fix races.

    This mostly pulls in changes from rust-lang/futures-rs#881, but
    also updates Registration to be a bit more obvious as to what is going
    on.

    * Reduce spurious wakeups caused by Reactor

    This patch adds an ABA guard on token values before registering them
    with Mio. This allows catching token reuse and avoid the notification.

    This is needed for OS X as the notification is used to determine that a
    TCP connect has completed. A spurious notification can potentially cause
    write failures.

tokio-threadpool/src/shutdown.rs

@@ -34,9 +34,10 @@ impl Future for Shutdown {
     type Error = ();
 
     fn poll(&mut self) -> Poll<(), ()> {
+        use futures::task;
         trace!("Shutdown::poll");
 
-        self.inner().shutdown_task.task1.register();
+        self.inner().shutdown_task.task1.register_task(task::current());
 
         if 0 != self.inner().num_workers.load(Acquire) {
             return Ok(Async::NotReady);

tokio-threadpool/src/task.rs

@@ -223,7 +223,7 @@ impl Task {
             let actual = self.inner().state.compare_and_swap(
                 Idle.into(),
                 Scheduled.into(),
-                Relaxed).into();
+                AcqRel).into();
 
             match actual {
                 Idle => return true,