Fix racy access of TaskHeader::executor

embassy-executor/src/raw/mod.rs

@@ -14,7 +14,58 @@ mod run_queue;
 #[cfg_attr(all(cortex_m, target_has_atomic = "8"), path = "state_atomics_arm.rs")]
 #[cfg_attr(all(not(cortex_m), target_has_atomic = "8"), path = "state_atomics.rs")]
 #[cfg_attr(not(target_has_atomic = "8"), path = "state_critical_section.rs")]
-mod state;
+pub(crate) mod state;
+
+#[cfg(target_has_atomic = "ptr")]
+mod owner {
+    use core::sync::atomic::{AtomicPtr, Ordering};
+
+    use super::{state::Token, SyncExecutor};
+
+    pub(crate) struct ExecutorRef(AtomicPtr<SyncExecutor>);
+
+    impl ExecutorRef {
+        pub const fn new() -> Self {
+            Self(AtomicPtr::new(core::ptr::null_mut()))
+        }
+
+        pub fn set(&self, executor: Option<&'static SyncExecutor>, _: Token) {
+            let ptr = executor.map(|e| e as *const SyncExecutor).unwrap_or(core::ptr::null());
+            self.0.store(ptr.cast_mut(), Ordering::Release);
+        }
+
+        pub fn get(&self, _: Token) -> *const SyncExecutor {
+            self.0.load(Ordering::Acquire).cast_const()
+        }
+    }
+}
+#[cfg(not(target_has_atomic = "ptr"))]
+mod owner {
+    use super::{state::Token, SyncExecutor};
+    use core::cell::Cell;
+
+    use critical_section::Mutex;
+
+    pub(crate) struct ExecutorRef(Mutex<Cell<*const SyncExecutor>>);
+
+    unsafe impl Send for ExecutorRef {}
+    unsafe impl Sync for ExecutorRef {}
+
+    impl ExecutorRef {
+        pub const fn new() -> Self {
+            Self(Mutex::new(Cell::new(core::ptr::null())))
+        }
+
+        pub fn set(&self, executor: Option<&'static SyncExecutor>, cs: Token) {
+            let ptr = executor.map(|e| e as *const SyncExecutor).unwrap_or(core::ptr::null());
+            self.0.borrow(cs).set(ptr);
+        }
+
+        pub fn get(&self, cs: Token) -> *const SyncExecutor {
+            self.0.borrow(cs).get()
+        }
+    }
+}
 
 pub mod timer_queue;
 #[cfg(feature = "trace")]
@@ -30,6 +81,8 @@ use core::pin::Pin;
 use core::ptr::NonNull;
 use core::task::{Context, Poll};
 
+use crate::raw::owner::ExecutorRef;
+
 use self::run_queue::{RunQueue, RunQueueItem};
 use self::state::State;
 use self::util::{SyncUnsafeCell, UninitCell};
@@ -40,7 +93,7 @@ use super::SpawnToken;
 pub(crate) struct TaskHeader {
     pub(crate) state: State,
     pub(crate) run_queue_item: RunQueueItem,
-    pub(crate) executor: SyncUnsafeCell<Option<&'static SyncExecutor>>,
+    pub(crate) executor: ExecutorRef,
     poll_fn: SyncUnsafeCell<Option<unsafe fn(TaskRef)>>,
 
     /// Integrated timer queue storage. This field should not be accessed outside of the timer queue.
@@ -86,7 +139,8 @@ impl TaskRef {
 
     /// Returns a reference to the executor that the task is currently running on.
     pub unsafe fn executor(self) -> Option<&'static Executor> {
-        self.header().executor.get().map(|e| Executor::wrap(e))
+        let executor = state::locked(|token| self.header().executor.get(token));
+        executor.as_ref().map(|e| Executor::wrap(e))
     }
 
     /// Returns a reference to the timer queue item.
@@ -153,7 +207,7 @@ impl<F: Future + 'static> TaskStorage<F> {
             raw: TaskHeader {
                 state: State::new(),
                 run_queue_item: RunQueueItem::new(),
-                executor: SyncUnsafeCell::new(None),
+                executor: ExecutorRef::new(),
                 // Note: this is lazily initialized so that a static `TaskStorage` will go in `.bss`
                 poll_fn: SyncUnsafeCell::new(None),
 
@@ -396,7 +450,9 @@ impl SyncExecutor {
     }
 
     pub(super) unsafe fn spawn(&'static self, task: TaskRef) {
-        task.header().executor.set(Some(self));
+        state::locked(|l| {
+            task.header().executor.set(Some(self), l);
+        });
 
         #[cfg(feature = "trace")]
         trace::task_new(self, &task);
@@ -549,7 +605,7 @@ pub fn wake_task(task: TaskRef) {
     header.state.run_enqueue(|l| {
         // We have just marked the task as scheduled, so enqueue it.
         unsafe {
-            let executor = header.executor.get().unwrap_unchecked();
+            let executor = header.executor.get(l).as_ref().unwrap_unchecked();
             executor.enqueue(task, l);
         }
     });
@@ -563,7 +619,7 @@ pub fn wake_task_no_pend(task: TaskRef) {
     header.state.run_enqueue(|l| {
         // We have just marked the task as scheduled, so enqueue it.
         unsafe {
-            let executor = header.executor.get().unwrap_unchecked();
+            let executor = header.executor.get(l).as_ref().unwrap_unchecked();
             executor.run_queue.enqueue(task, l);
         }
     });

embassy-executor/src/raw/state_atomics.rs

@@ -2,13 +2,14 @@ use core::sync::atomic::{AtomicU32, Ordering};
 
 use super::timer_queue::TimerEnqueueOperation;
 
+#[derive(Clone, Copy)]
 pub(crate) struct Token(());
 
 /// Creates a token and passes it to the closure.
 ///
 /// This is a no-op replacement for `CriticalSection::with` because we don't need any locking.
-pub(crate) fn locked(f: impl FnOnce(Token)) {
+pub(crate) fn locked<R>(f: impl FnOnce(Token) -> R) -> R {
-    f(Token(()));
+    f(Token(()))
 }
 
 /// Task is spawned (has a future)

embassy-executor/src/raw/state_atomics_arm.rs

@@ -3,13 +3,14 @@ use core::sync::atomic::{compiler_fence, AtomicBool, AtomicU32, Ordering};
 
 use super::timer_queue::TimerEnqueueOperation;
 
+#[derive(Clone, Copy)]
 pub(crate) struct Token(());
 
 /// Creates a token and passes it to the closure.
 ///
 /// This is a no-op replacement for `CriticalSection::with` because we don't need any locking.
-pub(crate) fn locked(f: impl FnOnce(Token)) {
+pub(crate) fn locked<R>(f: impl FnOnce(Token) -> R) -> R {
-    f(Token(()));
+    f(Token(()))
 }
 
 // Must be kept in sync with the layout of `State`!

embassy-executor/src/spawner.rs

@@ -92,7 +92,9 @@ impl Spawner {
     pub async fn for_current_executor() -> Self {
         poll_fn(|cx| {
             let task = raw::task_from_waker(cx.waker());
-            let executor = unsafe { task.header().executor.get().unwrap_unchecked() };
+            let executor = raw::state::locked(|l| {
+                unsafe { task.header().executor.get(l).as_ref().unwrap_unchecked() }
+            });
             let executor = unsafe { raw::Executor::wrap(executor) };
             Poll::Ready(Self::new(executor))
         })
@@ -164,7 +166,9 @@ impl SendSpawner {
     pub async fn for_current_executor() -> Self {
         poll_fn(|cx| {
             let task = raw::task_from_waker(cx.waker());
-            let executor = unsafe { task.header().executor.get().unwrap_unchecked() };
+            let executor = raw::state::locked(|l| {
+                unsafe { task.header().executor.get(l).as_ref().unwrap_unchecked() }
+            });
             Poll::Ready(Self::new(executor))
         })
         .await