runtime: use release in wake_by_ref() even if already woken

Author: frostyplanet

tokio/src/runtime/task/state.rs

@@ -252,9 +252,15 @@ impl State {
     /// Transitions the state to `NOTIFIED`.
     pub(super) fn transition_to_notified_by_ref(&self) -> TransitionToNotifiedByRef {
         self.fetch_update_action(|mut snapshot| {
-            if snapshot.is_complete() || snapshot.is_notified() {
-                // There is nothing to do in this case.
+            if snapshot.is_complete() {
+                // The complete state is final
                 (TransitionToNotifiedByRef::DoNothing, None)
+            } else if snapshot.is_notified() {
+                // Even hough we have nothing to do in this branch,
+                // wake_by_ref() should synchronize-with the task starting execution,
+                // therefore we must use an Release store (with the same value),
+                // to pair with the Acquire in transition_to_running.
+                (TransitionToNotifiedByRef::DoNothing, Some(snapshot))
             } else if snapshot.is_running() {
                 // If the task is running, we mark it as notified, but we should
                 // not submit as the thread currently running the future is