io: avoid unnecessary wake in registration (#2221)

See discussion in #2222. This wake/notify call has been there in one
form or another since the very early days of tokio. Currently though, it
is not clear that it is needed; the contract for polling is that you
must keep polling until you get `Pending`, so doing a wakeup when we are
about to return `Ready` is premature.

2 files changed, 4 insertions, 11 deletions

diff --git a/tokio/src/io/driver/mod.rs b/tokio/src/io/driver/mod.rs
index 8385448c..e707d3a5 100644
--- a/tokio/src/io/driver/mod.rs
+++ b/tokio/src/io/driver/mod.rs
@@ -277,20 +277,12 @@ impl Inner {
             .get(token)
             .unwrap_or_else(|| panic!("IO resource for token {:?} does not exist!", token));
 
-        let readiness = sched
-            .get_readiness(token)
-            .unwrap_or_else(|| panic!("token {:?} no longer valid!", token));
-
-        let (waker, ready) = match dir {
-            Direction::Read => (&sched.reader, !mio::Ready::writable()),
-            Direction::Write => (&sched.writer, mio::Ready::writable()),
+        let waker = match dir {
+            Direction::Read => &sched.reader,
+            Direction::Write => &sched.writer,
         };
 
         waker.register(w);
-
-        if readiness & ready.as_usize() != 0 {
-            waker.wake();
-        }
     }
 }
 
diff --git a/tokio/src/io/driver/scheduled_io.rs b/tokio/src/io/driver/scheduled_io.rs
index e26a3588..7f6446e3 100644
--- a/tokio/src/io/driver/scheduled_io.rs
+++ b/tokio/src/io/driver/scheduled_io.rs
@@ -56,6 +56,7 @@ impl Default for ScheduledIo {
 }
 
 impl ScheduledIo {
+    #[cfg(all(test, loom))]
     /// Returns the current readiness value of this `ScheduledIo`, if the
     /// provided `token` is still a valid access.
     ///