--- y/kernel/watch_queue.c
+++ w/kernel/watch_queue.c
@@ -83,11 +83,13 @@ static bool post_one_notification(struct
 	if (!pipe)
 		return false;
 
+	spin_lock_bh(&wqueue->lock);
+	if (wqueue->defunct) {
+		spin_unlock_bh(&wqueue->lock);
+		return false;
+	}
 	spin_lock_irq(&pipe->rd_wait.lock);
 
-	if (wqueue->defunct)
-		goto out;
-
 	mask = pipe->ring_size - 1;
 	head = pipe->head;
 	tail = pipe->tail;
@@ -126,6 +128,7 @@ out:
 	spin_unlock_irq(&pipe->rd_wait.lock);
 	if (done)
 		kill_fasync(&pipe->fasync_readers, SIGIO, POLL_IN);
+	spin_unlock_bh(&wqueue->lock);
 	return done;
 
 lost: