Use atomics to share state instead of a `RefCell`

1 files changed, 83 insertions, 57 deletions

diff --git a/embassy-nrf/src/rng.rs b/embassy-nrf/src/rng.rs
index 0ec87eff..80282c4b 100644
--- a/embassy-nrf/src/rng.rs
+++ b/embassy-nrf/src/rng.rs
@@ -1,14 +1,14 @@
-use core::cell::RefCell;
 use core::convert::Infallible;
 use core::future::Future;
 use core::marker::PhantomData;
-use core::ptr::NonNull;
+use core::ptr;
+use core::sync::atomic::AtomicPtr;
+use core::sync::atomic::Ordering;
 use core::task::Poll;
-use core::task::Waker;
 
 use embassy::interrupt::InterruptExt;
 use embassy::traits;
-use embassy::util::CriticalSectionMutex;
+use embassy::util::AtomicWaker;
 use embassy::util::OnDrop;
 use embassy::util::Unborrow;
 use embassy_extras::unborrow;
@@ -25,25 +25,18 @@ impl RNG {
     }
 }
 
-static STATE: CriticalSectionMutex<RefCell<State>> =
-    CriticalSectionMutex::new(RefCell::new(State {
-        buffer: None,
-        waker: None,
-        index: 0,
-    }));
+static STATE: State = State {
+    ptr: AtomicPtr::new(ptr::null_mut()),
+    end: AtomicPtr::new(ptr::null_mut()),
+    waker: AtomicWaker::new(),
+};
 
 struct State {
-    buffer: Option<NonNull<[u8]>>,
-    waker: Option<Waker>,
-    index: usize,
+    ptr: AtomicPtr<u8>,
+    end: AtomicPtr<u8>,
+    waker: AtomicWaker,
 }
 
-// SAFETY: `NonNull` is `!Send` because of the possibility of it being aliased.
-// However, `buffer` is only used within `on_interrupt`,
-// and the original `&mut` passed to `fill_bytes` cannot be used because the safety contract of `Rng::new`
-// means that it must still be borrowed by `RngFuture`, and so `rustc` will not let it be accessed.
-unsafe impl Send for State {}
-
 /// A wrapper around an nRF RNG peripheral.
 ///
 /// It has a non-blocking API, through `embassy::traits::Rng`, and a blocking api through `rand`.
@@ -70,7 +63,7 @@ impl<'d> Rng<'d> {
             phantom: PhantomData,
         };
 
-        Self::stop();
+        this.stop();
         this.disable_irq();
 
         this.irq.set_handler(Self::on_interrupt);
@@ -81,25 +74,54 @@ impl<'d> Rng<'d> {
     }
 
     fn on_interrupt(_: *mut ()) {
-        critical_section::with(|cs| {
-            let mut state = STATE.borrow(cs).borrow_mut();
-            // SAFETY: the safety requirements on `Rng::new` make sure that the original `&mut`'s lifetime is still valid,
-            // meaning it can't be aliased and is a valid pointer.
-            let buffer = unsafe { state.buffer.unwrap().as_mut() };
-            buffer[state.index] = RNG::regs().value.read().value().bits();
-            state.index += 1;
-            if state.index == buffer.len() {
-                // Stop the RNG within the interrupt so that it doesn't get triggered again on the way to waking the future.
-                Self::stop();
-                if let Some(waker) = state.waker.take() {
-                    waker.wake();
+        // Clear the event.
+        RNG::regs().events_valrdy.reset();
+
+        // Mutate the slice within a critical section,
+        // so that the future isn't dropped in between us loading the pointer and actually dereferencing it.
+        let (ptr, end) = critical_section::with(|_| {
+            let ptr = STATE.ptr.load(Ordering::Relaxed);
+            // We need to make sure we haven't already filled the whole slice,
+            // in case the interrupt fired again before the executor got back to the future.
+            let end = STATE.end.load(Ordering::Relaxed);
+            if !ptr.is_null() && ptr != end {
+                // If the future was dropped, the pointer would have been set to null,
+                // so we're still good to mutate the slice.
+                // The safety contract of `Rng::new` means that the future can't have been dropped
+                // without calling its destructor.
+                unsafe {
+                    *ptr = RNG::regs().value.read().value().bits();
                 }
             }
-            RNG::regs().events_valrdy.reset();
+            (ptr, end)
         });
+
+        if ptr.is_null() || ptr == end {
+            // If the future was dropped, there's nothing to do.
+            // If `ptr == end`, we were called by mistake, so return.
+            return;
+        }
+
+        let new_ptr = unsafe { ptr.add(1) };
+        match STATE
+            .ptr
+            .compare_exchange(ptr, new_ptr, Ordering::Relaxed, Ordering::Relaxed)
+        {
+            Ok(ptr) => {
+                let end = STATE.end.load(Ordering::Relaxed);
+                // It doesn't matter if `end` was changed under our feet, because then this will just be false.
+                if ptr == end {
+                    STATE.waker.wake();
+                }
+            }
+            Err(_) => {
+                // If the future was dropped or finished, there's no point trying to wake it.
+                // It will have already stopped the RNG, so there's no need to do that either.
+            }
+        }
     }
 
-    fn stop() {
+    fn stop(&self) {
         RNG::regs().tasks_stop.write(|w| unsafe { w.bits(1) })
     }
 
@@ -140,37 +162,41 @@ impl<'d> traits::rng::Rng for Rng<'d> {
 
     fn fill_bytes<'a>(&'a mut self, dest: &'a mut [u8]) -> Self::RngFuture<'a> {
         async move {
-            critical_section::with(|cs| {
-                let mut state = STATE.borrow(cs).borrow_mut();
-                state.buffer = Some(dest.into());
-            });
+            if dest.len() == 0 {
+                return Ok(()); // Nothing to fill
+            }
+
+            let range = dest.as_mut_ptr_range();
+            // Even if we've preempted the interrupt, it can't preempt us again,
+            // so we don't need to worry about the order we write these in.
+            STATE.ptr.store(range.start, Ordering::Relaxed);
+            STATE.end.store(range.end, Ordering::Relaxed);
 
             self.enable_irq();
             self.start();
 
             let on_drop = OnDrop::new(|| {
-                Self::stop();
+                self.stop();
                 self.disable_irq();
+
+                // The interrupt is now disabled and can't preempt us anymore, so the order doesn't matter here.
+                STATE.ptr.store(ptr::null_mut(), Ordering::Relaxed);
+                STATE.end.store(ptr::null_mut(), Ordering::Relaxed);
             });
 
             poll_fn(|cx| {
-                critical_section::with(|cs| {
-                    let mut state = STATE.borrow(cs).borrow_mut();
-                    state.waker = Some(cx.waker().clone());
-                    // SAFETY: see safety message in interrupt handler.
-                    // Also, both here and in the interrupt handler, we're in a critical section,
-                    // so they can't interfere with each other.
-                    let buffer = unsafe { state.buffer.unwrap().as_ref() };
-
-                    if state.index == buffer.len() {
-                        // Reset the state for next time
-                        state.buffer = None;
-                        state.index = 0;
-                        Poll::Ready(())
-                    } else {
-                        Poll::Pending
-                    }
-                })
+                STATE.waker.register(cx.waker());
+
+                // The interrupt will never modify `end`, so load it first and then get the most up-to-date `ptr`.
+                let end = STATE.end.load(Ordering::Relaxed);
+                let ptr = STATE.ptr.load(Ordering::Relaxed);
+
+                if ptr == end {
+                    // We're done.
+                    Poll::Ready(())
+                } else {
+                    Poll::Pending
+                }
             })
             .await;
 
@@ -193,7 +219,7 @@ impl<'d> RngCore for Rng<'d> {
             *byte = regs.value.read().value().bits();
         }
 
-        Self::stop();
+        self.stop();
     }
 
     fn next_u32(&mut self) -> u32 {