embassy/embassy-executor/src/raw/run_queue.rs

use core::ptr;
use core::ptr::NonNull;

use atomic_polyfill::{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);
        }
    }
}