executor: Replace NonNull<TaskHeader> with TaskRef

embassy-executor/src/raw/mod.rs

@@ -43,14 +43,11 @@ pub(crate) const STATE_RUN_QUEUED: u32 = 1 << 1;
 pub(crate) const STATE_TIMER_QUEUED: u32 = 1 << 2;
 
 /// Raw task header for use in task pointers.
-///
-/// This is an opaque struct, used for raw pointers to tasks, for use
-/// with funtions like [`wake_task`] and [`task_from_waker`].
-pub struct TaskHeader {
+pub(crate) struct TaskHeader {
     pub(crate) state: AtomicU32,
     pub(crate) run_queue_item: RunQueueItem,
-    pub(crate) executor: Cell<*const Executor>, // Valid if state != 0
-    pub(crate) poll_fn: UninitCell<unsafe fn(NonNull<TaskHeader>)>, // Valid if STATE_SPAWNED
+    pub(crate) executor: Cell<*const Executor>,         // Valid if state != 0
+    pub(crate) poll_fn: UninitCell<unsafe fn(TaskRef)>, // Valid if STATE_SPAWNED
 
     #[cfg(feature = "integrated-timers")]
     pub(crate) expires_at: Cell<Instant>,
@@ -59,7 +56,7 @@ pub struct TaskHeader {
 }
 
 impl TaskHeader {
-    pub(crate) const fn new() -> Self {
+    const fn new() -> Self {
         Self {
             state: AtomicU32::new(0),
             run_queue_item: RunQueueItem::new(),
@@ -74,6 +71,36 @@ impl TaskHeader {
     }
 }
 
+/// This is essentially a `&'static TaskStorage<F>` where the type of the future has been erased.
+#[derive(Clone, Copy)]
+pub struct TaskRef {
+    ptr: NonNull<TaskHeader>,
+}
+
+impl TaskRef {
+    fn new<F: Future + 'static>(task: &'static TaskStorage<F>) -> Self {
+        Self {
+            ptr: NonNull::from(task).cast(),
+        }
+    }
+
+    /// Safety: The pointer must have been obtained with `Task::as_ptr`
+    pub(crate) unsafe fn from_ptr(ptr: *const TaskHeader) -> Self {
+        Self {
+            ptr: NonNull::new_unchecked(ptr as *mut TaskHeader),
+        }
+    }
+
+    pub(crate) fn header(self) -> &'static TaskHeader {
+        unsafe { self.ptr.as_ref() }
+    }
+
+    /// The returned pointer is valid for the entire TaskStorage.
+    pub(crate) fn as_ptr(self) -> *const TaskHeader {
+        self.ptr.as_ptr()
+    }
+}
+
 /// Raw storage in which a task can be spawned.
 ///
 /// This struct holds the necessary memory to spawn one task whose future is `F`.
@@ -135,14 +162,14 @@ impl<F: Future + 'static> TaskStorage<F> {
             .is_ok()
     }
 
-    unsafe fn spawn_initialize(&'static self, future: impl FnOnce() -> F) -> NonNull<TaskHeader> {
+    unsafe fn spawn_initialize(&'static self, future: impl FnOnce() -> F) -> TaskRef {
         // Initialize the task
         self.raw.poll_fn.write(Self::poll);
         self.future.write(future());
-        NonNull::new_unchecked(self as *const TaskStorage<F> as *const TaskHeader as *mut TaskHeader)
+        TaskRef::new(self)
     }
 
-    unsafe fn poll(p: NonNull<TaskHeader>) {
+    unsafe fn poll(p: TaskRef) {
         let this = &*(p.as_ptr() as *const TaskStorage<F>);
 
         let future = Pin::new_unchecked(this.future.as_mut());
@@ -307,7 +334,7 @@ impl Executor {
     /// - `task` must be set up to run in this executor.
     /// - `task` must NOT be already enqueued (in this executor or another one).
     #[inline(always)]
-    unsafe fn enqueue(&self, cs: CriticalSection, task: NonNull<TaskHeader>) {
+    unsafe fn enqueue(&self, cs: CriticalSection, task: TaskRef) {
         #[cfg(feature = "rtos-trace")]
         trace::task_ready_begin(task.as_ptr() as u32);
 
@@ -325,8 +352,8 @@ impl Executor {
     /// It is OK to use `unsafe` to call this from a thread that's not the executor thread.
     /// In this case, the task's Future must be Send. This is because this is effectively
     /// sending the task to the executor thread.
-    pub(super) unsafe fn spawn(&'static self, task: NonNull<TaskHeader>) {
-        task.as_ref().executor.set(self);
+    pub(super) unsafe fn spawn(&'static self, task: TaskRef) {
+        task.header().executor.set(self);
 
         #[cfg(feature = "rtos-trace")]
         trace::task_new(task.as_ptr() as u32);
@@ -359,7 +386,7 @@ impl Executor {
             self.timer_queue.dequeue_expired(Instant::now(), |task| wake_task(task));
 
             self.run_queue.dequeue_all(|p| {
-                let task = p.as_ref();
+                let task = p.header();
 
                 #[cfg(feature = "integrated-timers")]
                 task.expires_at.set(Instant::MAX);
@@ -378,7 +405,7 @@ impl Executor {
                 trace::task_exec_begin(p.as_ptr() as u32);
 
                 // Run the task
-                task.poll_fn.read()(p as _);
+                task.poll_fn.read()(p);
 
                 #[cfg(feature = "rtos-trace")]
                 trace::task_exec_end();
@@ -424,9 +451,9 @@ impl Executor {
 /// # Safety
 ///
 /// `task` must be a valid task pointer obtained from [`task_from_waker`].
-pub unsafe fn wake_task(task: NonNull<TaskHeader>) {
+pub unsafe fn wake_task(task: TaskRef) {
     critical_section::with(|cs| {
-        let header = task.as_ref();
+        let header = task.header();
         let state = header.state.load(Ordering::Relaxed);
 
         // If already scheduled, or if not started,
@@ -450,7 +477,7 @@ struct TimerQueue;
 impl embassy_time::queue::TimerQueue for TimerQueue {
     fn schedule_wake(&'static self, at: Instant, waker: &core::task::Waker) {
         let task = waker::task_from_waker(waker);
-        let task = unsafe { task.as_ref() };
+        let task = task.header();
         let expires_at = task.expires_at.get();
         task.expires_at.set(expires_at.min(at));
     }

embassy-executor/src/raw/run_queue.rs

@@ -4,7 +4,7 @@ use core::ptr::NonNull;
 use atomic_polyfill::{AtomicPtr, Ordering};
 use critical_section::CriticalSection;
 
-use super::TaskHeader;
+use super::{TaskHeader, TaskRef};
 
 pub(crate) struct RunQueueItem {
     next: AtomicPtr<TaskHeader>,
@@ -46,25 +46,26 @@ impl RunQueue {
     ///
     /// `item` must NOT be already enqueued in any queue.
     #[inline(always)]
-    pub(crate) unsafe fn enqueue(&self, _cs: CriticalSection, task: NonNull<TaskHeader>) -> bool {
+    pub(crate) unsafe fn enqueue(&self, _cs: CriticalSection, task: TaskRef) -> bool {
         let prev = self.head.load(Ordering::Relaxed);
-        task.as_ref().run_queue_item.next.store(prev, Ordering::Relaxed);
-        self.head.store(task.as_ptr(), Ordering::Relaxed);
+        task.header().run_queue_item.next.store(prev, Ordering::Relaxed);
+        self.head.store(task.as_ptr() as _, Ordering::Relaxed);
         prev.is_null()
     }
 
     /// Empty the queue, then call `on_task` for each task that was in the queue.
     /// NOTE: It is OK for `on_task` to enqueue more tasks. In this case they're left in the queue
     /// and will be processed by the *next* call to `dequeue_all`, *not* the current one.
-    pub(crate) fn dequeue_all(&self, on_task: impl Fn(NonNull<TaskHeader>)) {
+    pub(crate) fn dequeue_all(&self, on_task: impl Fn(TaskRef)) {
         // Atomically empty the queue.
         let mut ptr = self.head.swap(ptr::null_mut(), Ordering::AcqRel);
 
         // Iterate the linked list of tasks that were previously in the queue.
         while let Some(task) = NonNull::new(ptr) {
+            let task = unsafe { TaskRef::from_ptr(task.as_ptr()) };
             // If the task re-enqueues itself, the `next` pointer will get overwritten.
             // Therefore, first read the next pointer, and only then process the task.
-            let next = unsafe { task.as_ref() }.run_queue_item.next.load(Ordering::Relaxed);
+            let next = task.header().run_queue_item.next.load(Ordering::Relaxed);
 
             on_task(task);
 

embassy-executor/src/raw/timer_queue.rs

@@ -1,45 +1,39 @@
 use core::cell::Cell;
 use core::cmp::min;
-use core::ptr;
-use core::ptr::NonNull;
 
 use atomic_polyfill::Ordering;
 use embassy_time::Instant;
 
-use super::{TaskHeader, STATE_TIMER_QUEUED};
+use super::{TaskRef, STATE_TIMER_QUEUED};
 
 pub(crate) struct TimerQueueItem {
-    next: Cell<*mut TaskHeader>,
+    next: Cell<Option<TaskRef>>,
 }
 
 impl TimerQueueItem {
     pub const fn new() -> Self {
-        Self {
-            next: Cell::new(ptr::null_mut()),
-        }
+        Self { next: Cell::new(None) }
     }
 }
 
 pub(crate) struct TimerQueue {
-    head: Cell<*mut TaskHeader>,
+    head: Cell<Option<TaskRef>>,
 }
 
 impl TimerQueue {
     pub const fn new() -> Self {
-        Self {
-            head: Cell::new(ptr::null_mut()),
-        }
+        Self { head: Cell::new(None) }
     }
 
-    pub(crate) unsafe fn update(&self, p: NonNull<TaskHeader>) {
-        let task = p.as_ref();
+    pub(crate) unsafe fn update(&self, p: TaskRef) {
+        let task = p.header();
         if task.expires_at.get() != Instant::MAX {
             let old_state = task.state.fetch_or(STATE_TIMER_QUEUED, Ordering::AcqRel);
             let is_new = old_state & STATE_TIMER_QUEUED == 0;
 
             if is_new {
                 task.timer_queue_item.next.set(self.head.get());
-                self.head.set(p.as_ptr());
+                self.head.set(Some(p));
             }
         }
     }
@@ -47,7 +41,7 @@ impl TimerQueue {
     pub(crate) unsafe fn next_expiration(&self) -> Instant {
         let mut res = Instant::MAX;
         self.retain(|p| {
-            let task = p.as_ref();
+            let task = p.header();
             let expires = task.expires_at.get();
             res = min(res, expires);
             expires != Instant::MAX
@@ -55,9 +49,9 @@ impl TimerQueue {
         res
     }
 
-    pub(crate) unsafe fn dequeue_expired(&self, now: Instant, on_task: impl Fn(NonNull<TaskHeader>)) {
+    pub(crate) unsafe fn dequeue_expired(&self, now: Instant, on_task: impl Fn(TaskRef)) {
         self.retain(|p| {
-            let task = p.as_ref();
+            let task = p.header();
             if task.expires_at.get() <= now {
                 on_task(p);
                 false
@@ -67,11 +61,10 @@ impl TimerQueue {
         });
     }
 
-    pub(crate) unsafe fn retain(&self, mut f: impl FnMut(NonNull<TaskHeader>) -> bool) {
+    pub(crate) unsafe fn retain(&self, mut f: impl FnMut(TaskRef) -> bool) {
         let mut prev = &self.head;
-        while !prev.get().is_null() {
-            let p = NonNull::new_unchecked(prev.get());
-            let task = &*p.as_ptr();
+        while let Some(p) = prev.get() {
+            let task = p.header();
             if f(p) {
                 // Skip to next
                 prev = &task.timer_queue_item.next;

embassy-executor/src/raw/waker.rs

@@ -1,8 +1,7 @@
 use core::mem;
-use core::ptr::NonNull;
 use core::task::{RawWaker, RawWakerVTable, Waker};
 
-use super::{wake_task, TaskHeader};
+use super::{wake_task, TaskHeader, TaskRef};
 
 const VTABLE: RawWakerVTable = RawWakerVTable::new(clone, wake, wake, drop);
 
@@ -11,14 +10,14 @@ unsafe fn clone(p: *const ()) -> RawWaker {
 }
 
 unsafe fn wake(p: *const ()) {
-    wake_task(NonNull::new_unchecked(p as *mut TaskHeader))
+    wake_task(TaskRef::from_ptr(p as *const TaskHeader))
 }
 
 unsafe fn drop(_: *const ()) {
     // nop
 }
 
-pub(crate) unsafe fn from_task(p: NonNull<TaskHeader>) -> Waker {
+pub(crate) unsafe fn from_task(p: TaskRef) -> Waker {
     Waker::from_raw(RawWaker::new(p.as_ptr() as _, &VTABLE))
 }
 
@@ -33,7 +32,7 @@ pub(crate) unsafe fn from_task(p: NonNull<TaskHeader>) -> Waker {
 /// # Panics
 ///
 /// Panics if the waker is not created by the Embassy executor.
-pub fn task_from_waker(waker: &Waker) -> NonNull<TaskHeader> {
+pub fn task_from_waker(waker: &Waker) -> TaskRef {
     // safety: OK because WakerHack has the same layout as Waker.
     // This is not really guaranteed because the structs are `repr(Rust)`, it is
     // indeed the case in the current implementation.
@@ -43,8 +42,8 @@ pub fn task_from_waker(waker: &Waker) -> NonNull<TaskHeader> {
         panic!("Found waker not created by the Embassy executor. `embassy_time::Timer` only works with the Embassy executor.")
     }
 
-    // safety: we never create a waker with a null data pointer.
-    unsafe { NonNull::new_unchecked(hack.data as *mut TaskHeader) }
+    // safety: our wakers are always created with `TaskRef::as_ptr`
+    unsafe { TaskRef::from_ptr(hack.data as *const TaskHeader) }
 }
 
 struct WakerHack {

embassy-executor/src/spawner.rs

@@ -1,7 +1,6 @@
 use core::future::poll_fn;
 use core::marker::PhantomData;
 use core::mem;
-use core::ptr::NonNull;
 use core::task::Poll;
 
 use super::raw;
@@ -22,12 +21,12 @@ use super::raw;
 /// Once you've invoked a task function and obtained a SpawnToken, you *must* spawn it.
 #[must_use = "Calling a task function does nothing on its own. You must spawn the returned SpawnToken, typically with Spawner::spawn()"]
 pub struct SpawnToken<S> {
-    raw_task: Option<NonNull<raw::TaskHeader>>,
+    raw_task: Option<raw::TaskRef>,
     phantom: PhantomData<*mut S>,
 }
 
 impl<S> SpawnToken<S> {
-    pub(crate) unsafe fn new(raw_task: NonNull<raw::TaskHeader>) -> Self {
+    pub(crate) unsafe fn new(raw_task: raw::TaskRef) -> Self {
         Self {
             raw_task: Some(raw_task),
             phantom: PhantomData,
@@ -92,7 +91,7 @@ impl Spawner {
     pub async fn for_current_executor() -> Self {
         poll_fn(|cx| unsafe {
             let task = raw::task_from_waker(cx.waker());
-            let executor = (*task.as_ptr()).executor.get();
+            let executor = task.header().executor.get();
             Poll::Ready(Self::new(&*executor))
         })
         .await
@@ -168,7 +167,7 @@ impl SendSpawner {
     pub async fn for_current_executor() -> Self {
         poll_fn(|cx| unsafe {
             let task = raw::task_from_waker(cx.waker());
-            let executor = (*task.as_ptr()).executor.get();
+            let executor = task.header().executor.get();
             Poll::Ready(Self::new(&*executor))
         })
         .await