Use load/store instead of get_mut

tokio/src/sync/broadcast.rs

@@ -1112,8 +1112,12 @@ impl<T> Receiver<T> {
                                 }
 
                                 // If the waiter is not already queued, enqueue it.
-                                if !*(*ptr).queued.get_mut() {
-                                    *(*ptr).queued.get_mut() = true;
+                                // `Relaxed` order suffices: we have synchronized with
+                                // all writers through the tail lock that we hold.
+                                if (*ptr).queued.load(Relaxed) {
+                                    // `Relaxed` order suffices: all the readers will
+                                    // synchronize with this write through the tail lock.
+                                    (*ptr).queued.store(true, Relaxed);
                                     tail.waiters.push_front(NonNull::new_unchecked(&mut *ptr));
                                 }
                             });