sync: avoid deadlocks in broadcast with custom wakers

tokio/src/sync/broadcast.rs

@@ -118,8 +118,9 @@
 
 use crate::loom::cell::UnsafeCell;
 use crate::loom::sync::atomic::AtomicUsize;
-use crate::loom::sync::{Arc, Mutex, RwLock, RwLockReadGuard};
+use crate::loom::sync::{Arc, Mutex, MutexGuard, RwLock, RwLockReadGuard};
 use crate::util::linked_list::{self, LinkedList};
+use crate::util::WakeList;
 
 use std::fmt;
 use std::future::Future;
@@ -569,12 +570,10 @@ impl<T> Sender<T> {
         // Release the slot lock before notifying the receivers.
         drop(slot);
 
-        tail.notify_rx();
-
-        // Release the mutex. This must happen after the slot lock is released,
-        // otherwise the writer lock bit could be cleared while another thread
-        // is in the critical section.
-        drop(tail);
+        // Notify and release the mutex. This must happen after the slot lock is
+        // released, otherwise the writer lock bit could be cleared while another
+        // thread is in the critical section.
+        self.shared.notify_rx(tail);
 
         Ok(rem)
     }
@@ -743,7 +742,7 @@ impl<T> Sender<T> {
         let mut tail = self.shared.tail.lock();
         tail.closed = true;
 
-        tail.notify_rx();
+        self.shared.notify_rx(tail);
     }
 }
 
@@ -764,18 +763,47 @@ fn new_receiver<T>(shared: Arc<Shared<T>>) -> Receiver<T> {
     Receiver { shared, next }
 }
 
-impl Tail {
-    fn notify_rx(&mut self) {
-        while let Some(mut waiter) = self.waiters.pop_back() {
-            // Safety: `waiters` lock is still held.
-            let waiter = unsafe { waiter.as_mut() };
+impl<T> Shared<T> {
+    fn notify_rx<'a, 'b: 'a>(&'b self, mut tail: MutexGuard<'a, Tail>) {
+        let mut wakers = WakeList::new();
+        'outer: loop {
+            while wakers.can_push() {
+                match tail.waiters.pop_back() {
+                    Some(mut waiter) => {
+                        // Safety: `tail` lock is still held.
+                        let waiter = unsafe { waiter.as_mut() };
+
+                        assert!(waiter.queued);
+                        waiter.queued = false;
+
+                        if let Some(waker) = waiter.waker.take() {
+                            wakers.push(waker);
+                        }
+                    }
+                    None => {
+                        break 'outer;
+                    }
+                }
+            }
+
+            // Release the lock before waking.
+            drop(tail);
+
+            // Before we acquire the lock again all sorts of things can happen:
+            // some waiters may remove themselves from the list and new waiters
+            // may be added. This is fine since at worst we will unnecessarily
+            // wake up waiters which will then queue themselves again.
 
-            assert!(waiter.queued);
-            waiter.queued = false;
+            wakers.wake_all();
 
-            let waker = waiter.waker.take().unwrap();
-            waker.wake();
+            // Acquire the lock again.
+            tail = self.tail.lock();
         }
+
+        // Release the lock before waking.
+        drop(tail);
+
+        wakers.wake_all();
     }
 }
 
@@ -884,6 +912,8 @@ impl<T> Receiver<T> {
             // the slot lock.
             drop(slot);
 
+            let mut old_waker = None;
+
             let mut tail = self.shared.tail.lock();
 
             // Acquire slot lock again
@@ -916,7 +946,10 @@ impl<T> Receiver<T> {
                                 match (*ptr).waker {
                                     Some(ref w) if w.will_wake(waker) => {}
                                     _ => {
-                                        (*ptr).waker = Some(waker.clone());
+                                        old_waker = std::mem::replace(
+                                            &mut (*ptr).waker,
+                                            Some(waker.clone()),
+                                        );
                                     }
                                 }
 
@@ -928,6 +961,11 @@ impl<T> Receiver<T> {
                         }
                     }
 
+                    // Drop the old waker after releasing the locks.
+                    drop(slot);
+                    drop(tail);
+                    drop(old_waker);
+
                     return Err(TryRecvError::Empty);
                 }
 

tokio/tests/sync_broadcast.rs

@@ -587,3 +587,57 @@ fn sender_len_random() {
         assert_eq!(tx.len(), expected_len);
     }
 }
+
+#[test]
+fn send_in_waker_drop() {
+    use futures::task::ArcWake;
+    use std::future::Future;
+    use std::task::Context;
+
+    struct SendOnDrop(broadcast::Sender<()>);
+
+    impl Drop for SendOnDrop {
+        fn drop(&mut self) {
+            let _ = self.0.send(());
+        }
+    }
+
+    impl ArcWake for SendOnDrop {
+        fn wake_by_ref(_arc_self: &Arc<Self>) {}
+    }
+
+    // Test if there is no deadlock when replacing the old waker.
+
+    let (tx, mut rx) = broadcast::channel(16);
+
+    let mut fut = Box::pin(async {
+        let _ = rx.recv().await;
+    });
+
+    // Store our special waker in the receiving future.
+    let waker = futures::task::waker(Arc::new(SendOnDrop(tx)));
+    let mut cx = Context::from_waker(&waker);
+    assert!(fut.as_mut().poll(&mut cx).is_pending());
+    drop(waker);
+
+    // Second poll shouldn't deadlock.
+    let mut cx = Context::from_waker(futures::task::noop_waker_ref());
+    let _ = fut.as_mut().poll(&mut cx);
+
+    // Test if there is no deadlock when calling waker.wake().
+
+    let (tx, mut rx) = broadcast::channel(16);
+
+    let mut fut = Box::pin(async {
+        let _ = rx.recv().await;
+    });
+
+    // Store our special waker in the receiving future.
+    let waker = futures::task::waker(Arc::new(SendOnDrop(tx.clone())));
+    let mut cx = Context::from_waker(&waker);
+    assert!(fut.as_mut().poll(&mut cx).is_pending());
+    drop(waker);
+
+    // Shouldn't deadlock.
+    let _ = tx.send(());
+}