From 7c249cf0ca71fe9e288e504ead014a71d91a9906 Mon Sep 17 00:00:00 2001
From: Ruihan Li <lrh2000@pku.edu.cn>
Date: Wed, 25 Dec 2024 13:54:20 +0800
Subject: [PATCH] Fix the memory order in `Subject`

---
 kernel/src/events/subject.rs | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/kernel/src/events/subject.rs b/kernel/src/events/subject.rs
index 8628dda69..fd9b2ad47 100644
--- a/kernel/src/events/subject.rs
+++ b/kernel/src/events/subject.rs
@@ -23,6 +23,7 @@ impl<E: Events, F: EventsFilter<E>> Subject<E, F> {
             num_observers: AtomicUsize::new(0),
         }
     }
+
     /// Register an observer.
     ///
     /// A registered observer will get notified through its `on_events` method.
@@ -37,7 +38,8 @@ impl<E: Events, F: EventsFilter<E>> Subject<E, F> {
             observers.insert(observer, filter).is_none()
         };
         if is_new {
-            self.num_observers.fetch_add(1, Ordering::Relaxed);
+            // This `Acquire` pairs with the `Release` in `notify_observers`.
+            self.num_observers.fetch_add(1, Ordering::Acquire);
         }
     }
 
@@ -66,7 +68,11 @@ impl<E: Events, F: EventsFilter<E>> Subject<E, F> {
     /// It will remove the observers which have been freed.
     pub fn notify_observers(&self, events: &E) {
         // Fast path.
-        if self.num_observers.load(Ordering::Relaxed) == 0 {
+        //
+        // Note: This must use `Release`, which pairs with `Acquire` in `register_observer`, to
+        // ensure that even if this fast path is used, a concurrently registered observer will see
+        // the event we want to notify.
+        if self.num_observers.fetch_add(0, Ordering::Release) == 0 {
             return;
         }