Relax SeqCst in heph-inbox

This replaces all SeqCst atomic operations with Release, Acquire or
AcqRel.

Also see rust-lang/rust#122729.

inbox/src/lib.rs

@@ -401,7 +401,7 @@ fn try_send<T>(channel: &Channel<T>, value: T) -> Result<(), SendError<T>> {
 /// enough for most practical use cases.
 impl<T> Clone for Sender<T> {
     fn clone(&self) -> Sender<T> {
-        // For the reasoning behind this relaxed ordering see `Arc::clone`.
+        // SAFETY: for the reasoning behind this relaxed ordering see `Arc::clone`.
         let old_ref_count = self.channel().ref_count.fetch_add(1, Ordering::Relaxed);
         debug_assert!(old_ref_count & SENDER_ACCESS != 0);
         Sender {
@@ -452,7 +452,7 @@ impl<T> Drop for Sender<T> {
             return;
         }
 
-        // For the reasoning behind this ordering see `Arc::drop`.
+        // SAFETY: for the reasoning behind this ordering see `Arc::drop`.
         fence!(self.channel().ref_count, Ordering::Acquire);
 
         // Drop the memory.

inbox/src/tests.rs

@@ -22,7 +22,7 @@ struct AwokenCount {
 
 impl PartialEq<usize> for AwokenCount {
     fn eq(&self, other: &usize) -> bool {
-        self.inner.count.load(Ordering::SeqCst) == *other
+        self.inner.count.load(Ordering::Acquire) == *other
     }
 }
 
@@ -33,11 +33,11 @@ struct WakerInner {
 
 impl Wake for WakerInner {
     fn wake(self: Arc<Self>) {
-        let _ = self.count.fetch_add(1, Ordering::SeqCst);
+        let _ = self.count.fetch_add(1, Ordering::AcqRel);
     }
 
     fn wake_by_ref(self: &Arc<Self>) {
-        let _ = self.count.fetch_add(1, Ordering::SeqCst);
+        let _ = self.count.fetch_add(1, Ordering::AcqRel);
     }
 }
 

inbox/src/waker.rs

@@ -24,7 +24,7 @@ impl WakerRegistration {
         let stored_waker = self.waker.read().unwrap();
         if let Some(stored_waker) = &*stored_waker {
             if stored_waker.will_wake(waker) {
-                self.needs_wakeup.store(true, Ordering::SeqCst);
+                self.needs_wakeup.store(true, Ordering::Release);
                 return false;
             }
         }
@@ -36,26 +36,26 @@ impl WakerRegistration {
         // again.
         if let Some(stored_waker) = &*stored_waker {
             if stored_waker.will_wake(waker) {
-                self.needs_wakeup.store(true, Ordering::SeqCst);
+                self.needs_wakeup.store(true, Ordering::Release);
                 return false;
             }
         }
         *stored_waker = Some(waker.clone());
         drop(stored_waker);
 
-        self.needs_wakeup.store(true, Ordering::SeqCst);
+        self.needs_wakeup.store(true, Ordering::Release);
         true
     }
 
     /// Wake the waker registered, if required.
     pub(crate) fn wake(&self) {
-        if !self.needs_wakeup.load(Ordering::SeqCst) {
+        if !self.needs_wakeup.load(Ordering::Acquire) {
             // Receiver doesn't need a wake-up.
             return;
         }
 
         // Mark that we've woken and after actually do the waking.
-        if self.needs_wakeup.swap(false, Ordering::SeqCst) {
+        if self.needs_wakeup.swap(false, Ordering::AcqRel) {
             if let Some(waker) = &*self.waker.read().unwrap() {
                 waker.wake_by_ref();
             }

inbox/tests/util/mod.rs

@@ -22,7 +22,7 @@ pub struct AwokenCount {
 impl AwokenCount {
     /// Get the current count.
     pub fn get(&self) -> usize {
-        self.inner.count.load(Ordering::SeqCst)
+        self.inner.count.load(Ordering::Acquire)
     }
 }
 
@@ -39,11 +39,11 @@ struct WakerInner {
 
 impl Wake for WakerInner {
     fn wake(self: Arc<Self>) {
-        let _ = self.count.fetch_add(1, Ordering::SeqCst);
+        let _ = self.count.fetch_add(1, Ordering::AcqRel);
     }
 
     fn wake_by_ref(self: &Arc<Self>) {
-        let _ = self.count.fetch_add(1, Ordering::SeqCst);
+        let _ = self.count.fetch_add(1, Ordering::AcqRel);
     }
 }