executor: Replace unsound critical sections with atomics

embassy-executor/src/raw/mod.rs

@@ -22,7 +22,6 @@ use core::ptr::NonNull;
 use core::task::{Context, Poll};
 
 use atomic_polyfill::{AtomicU32, Ordering};
-use critical_section::CriticalSection;
 #[cfg(feature = "integrated-timers")]
 use embassy_time::driver::{self, AlarmHandle};
 #[cfg(feature = "integrated-timers")]
@@ -373,11 +372,11 @@ impl SyncExecutor {
     /// - `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: TaskRef) {
+    unsafe fn enqueue(&self, task: TaskRef) {
         #[cfg(feature = "rtos-trace")]
         trace::task_ready_begin(task.as_ptr() as u32);
 
-        if self.run_queue.enqueue(cs, task) {
+        if self.run_queue.enqueue(task) {
             self.pender.pend();
         }
     }
@@ -394,9 +393,7 @@ impl SyncExecutor {
         #[cfg(feature = "rtos-trace")]
         trace::task_new(task.as_ptr() as u32);
 
-        critical_section::with(|cs| {
-            self.enqueue(cs, task);
-        })
+        self.enqueue(task);
     }
 
     /// # Safety
@@ -552,24 +549,25 @@ impl Executor {
 ///
 /// You can obtain a `TaskRef` from a `Waker` using [`task_from_waker`].
 pub fn wake_task(task: TaskRef) {
-    critical_section::with(|cs| {
     let header = task.header();
-        let state = header.state.load(Ordering::Relaxed);
 
+    let res = header.state.fetch_update(Ordering::SeqCst, Ordering::SeqCst, |state| {
         // If already scheduled, or if not started,
         if (state & STATE_RUN_QUEUED != 0) || (state & STATE_SPAWNED == 0) {
-            return;
-        }
-
+            None
+        } else {
             // Mark it as scheduled
-        header.state.store(state | STATE_RUN_QUEUED, Ordering::Relaxed);
+            Some(state | STATE_RUN_QUEUED)
+        }
+    });
 
+    if res.is_ok() {
         // We have just marked the task as scheduled, so enqueue it.
         unsafe {
             let executor = header.executor.get().unwrap_unchecked();
-            executor.enqueue(cs, task);
+            executor.enqueue(task);
+        }
     }
-    })
 }
 
 #[cfg(feature = "integrated-timers")]

embassy-executor/src/raw/run_queue.rs

@@ -2,7 +2,6 @@ use core::ptr;
 use core::ptr::NonNull;
 
 use atomic_polyfill::{AtomicPtr, Ordering};
-use critical_section::CriticalSection;
 
 use super::{TaskHeader, TaskRef};
 
@@ -46,11 +45,18 @@ impl RunQueue {
     ///
     /// `item` must NOT be already enqueued in any queue.
     #[inline(always)]
-    pub(crate) unsafe fn enqueue(&self, _cs: CriticalSection, task: TaskRef) -> bool {
-        let prev = self.head.load(Ordering::Relaxed);
+    pub(crate) unsafe fn enqueue(&self, task: TaskRef) -> bool {
+        let mut was_empty = false;
+
+        self.head
+            .fetch_update(Ordering::SeqCst, Ordering::SeqCst, |prev| {
+                was_empty = prev.is_null();
                 task.header().run_queue_item.next.store(prev, Ordering::Relaxed);
-        self.head.store(task.as_ptr() as _, Ordering::Relaxed);
-        prev.is_null()
+                Some(task.as_ptr() as *mut _)
+            })
+            .ok();
+
+        was_empty
     }
 
     /// Empty the queue, then call `on_task` for each task that was in the queue.