sync: fix racy UnsafeCell access on a closed oneshot (#4226)

tokio/src/sync/oneshot.rs

@@ -852,11 +852,38 @@ impl State {
     }
 
     fn set_complete(cell: &AtomicUsize) -> State {
-        // TODO: This could be `Release`, followed by an `Acquire` fence *if*
-        // the `RX_TASK_SET` flag is set. However, `loom` does not support
-        // fences yet.
-        let val = cell.fetch_or(VALUE_SENT, AcqRel);
-        State(val)
+        // This method is a compare-and-swap loop rather than a fetch-or like
+        // other `set_$WHATEVER` methods on `State`. This is because we must
+        // check if the state has been closed before setting the `VALUE_SENT`
+        // bit.
+        //
+        // We don't want to set both the `VALUE_SENT` bit if the `CLOSED`
+        // bit is already set, because `VALUE_SENT` will tell the receiver that
+        // it's okay to access the inner `UnsafeCell`. Immediately after calling
+        // `set_complete`, if the channel was closed, the sender will _also_
+        // access the `UnsafeCell` to take the value back out, so if a
+        // `poll_recv` or `try_recv` call is occurring concurrently, both
+        // threads may try to access the `UnsafeCell` if we were to set the
+        // `VALUE_SENT` bit on a closed channel.
+        let mut state = cell.load(Ordering::Relaxed);
+        loop {
+            if State(state).is_closed() {
+                break;
+            }
+            // TODO: This could be `Release`, followed by an `Acquire` fence *if*
+            // the `RX_TASK_SET` flag is set. However, `loom` does not support
+            // fences yet.
+            match cell.compare_exchange_weak(
+                state,
+                state | VALUE_SENT,
+                Ordering::AcqRel,
+                Ordering::Acquire,
+            ) {
+                Ok(_) => break,
+                Err(actual) => state = actual,
+            }
+        }
+        State(state)
     }
 
     fn is_rx_task_set(self) -> bool {

tokio/src/sync/tests/loom_oneshot.rs

@@ -55,6 +55,35 @@ fn changing_rx_task() {
     });
 }
 
+#[test]
+fn try_recv_close() {
+    // reproduces https://github.com/tokio-rs/tokio/issues/4225
+    loom::model(|| {
+        let (tx, mut rx) = oneshot::channel();
+        thread::spawn(move || {
+            let _ = tx.send(());
+        });
+
+        rx.close();
+        let _ = rx.try_recv();
+    })
+}
+
+#[test]
+fn recv_closed() {
+    // reproduces https://github.com/tokio-rs/tokio/issues/4225
+    loom::model(|| {
+        let (tx, mut rx) = oneshot::channel();
+
+        thread::spawn(move || {
+            let _ = tx.send(1);
+        });
+
+        rx.close();
+        let _ = block_on(rx);
+    });
+}
+
 // TODO: Move this into `oneshot` proper.
 
 use std::future::Future;