executor: remove atomic-polyfill.

1 files changed, 87 insertions, 0 deletions

diff --git a/embassy-executor/src/raw/run_queue_atomics.rs b/embassy-executor/src/raw/run_queue_atomics.rs
new file mode 100644
index 000000000..90907cfda
--- /dev/null
+++ b/embassy-executor/src/raw/run_queue_atomics.rs
@@ -0,0 +1,87 @@
+use core::ptr;
+use core::ptr::NonNull;
+use core::sync::atomic::{AtomicPtr, Ordering};
+
+use super::{TaskHeader, TaskRef};
+use crate::raw::util::SyncUnsafeCell;
+
+pub(crate) struct RunQueueItem {
+    next: SyncUnsafeCell<Option<TaskRef>>,
+}
+
+impl RunQueueItem {
+    pub const fn new() -> Self {
+        Self {
+            next: SyncUnsafeCell::new(None),
+        }
+    }
+}
+
+/// Atomic task queue using a very, very simple lock-free linked-list queue:
+///
+/// To enqueue a task, task.next is set to the old head, and head is atomically set to task.
+///
+/// Dequeuing is done in batches: the queue is emptied by atomically replacing head with
+/// null. Then the batch is iterated following the next pointers until null is reached.
+///
+/// Note that batches will be iterated in the reverse order as they were enqueued. This is OK
+/// for our purposes: it can't create fairness problems since the next batch won't run until the
+/// current batch is completely processed, so even if a task enqueues itself instantly (for example
+/// by waking its own waker) can't prevent other tasks from running.
+pub(crate) struct RunQueue {
+    head: AtomicPtr<TaskHeader>,
+}
+
+impl RunQueue {
+    pub const fn new() -> Self {
+        Self {
+            head: AtomicPtr::new(ptr::null_mut()),
+        }
+    }
+
+    /// Enqueues an item. Returns true if the queue was empty.
+    ///
+    /// # Safety
+    ///
+    /// `item` must NOT be already enqueued in any queue.
+    #[inline(always)]
+    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();
+                unsafe {
+                    // safety: the pointer is either null or valid
+                    let prev = NonNull::new(prev).map(|ptr| TaskRef::from_ptr(ptr.as_ptr()));
+                    // safety: there are no concurrent accesses to `next`
+                    task.header().run_queue_item.next.set(prev);
+                }
+                Some(task.as_ptr() as *mut _)
+            })
+            .ok();
+
+        was_empty
+    }
+
+    /// 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(TaskRef)) {
+        // Atomically empty the queue.
+        let ptr = self.head.swap(ptr::null_mut(), Ordering::AcqRel);
+
+        // safety: the pointer is either null or valid
+        let mut next = unsafe { NonNull::new(ptr).map(|ptr| TaskRef::from_ptr(ptr.as_ptr())) };
+
+        // Iterate the linked list of tasks that were previously in the queue.
+        while let Some(task) = next {
+            // If the task re-enqueues itself, the `next` pointer will get overwritten.
+            // Therefore, first read the next pointer, and only then process the task.
+            // safety: there are no concurrent accesses to `next`
+            next = unsafe { task.header().run_queue_item.next.get() };
+
+            on_task(task);
+        }
+    }
+}