zynq-rs/libasync/src/executor.rs

158 lines
5.2 KiB
Rust

use core::{
cell::{RefCell, UnsafeCell},
future::Future,
mem::MaybeUninit,
pin::Pin,
sync::atomic::{AtomicBool, Ordering},
task::{Context, Poll, RawWaker, RawWakerVTable, Waker},
};
use alloc::{boxed::Box, collections::VecDeque as Deque};
//use futures::future::FutureExt;
use pin_utils::pin_mut;
// NOTE `*const ()` is &AtomicBool
static VTABLE: RawWakerVTable = {
unsafe fn clone(p: *const ()) -> RawWaker {
RawWaker::new(p, &VTABLE)
}
unsafe fn wake(p: *const ()) {
wake_by_ref(p)
}
unsafe fn wake_by_ref(p: *const ()) {
(*(p as *const AtomicBool)).store(true, Ordering::Relaxed)
}
unsafe fn drop(_: *const ()) {
// no-op
}
RawWakerVTable::new(clone, wake, wake_by_ref, drop)
};
/// ready should not move as long as this waker references it. That is
/// the reason for keeping Tasks in a pinned box.
fn wrap_waker(ready: &AtomicBool) -> Waker {
unsafe { Waker::from_raw(RawWaker::new(ready as *const _ as *const _, &VTABLE)) }
}
/// A single-threaded executor
///
/// This is a singleton
pub struct Executor {
// Entered block_on() already?
in_block_on: RefCell<bool>,
/// Tasks reside on the heap, so that we just queue pointers. They
/// must also be pinned in memory because our RawWaker is a pointer
/// to their `ready` field.
tasks: RefCell<Deque<Pin<Box<Task>>>>,
}
impl Executor {
/// Creates a new instance of the executor
pub fn new() -> Self {
Self {
in_block_on: RefCell::new(false),
tasks: RefCell::new(Deque::new()),
}
}
pub fn block_on<T>(&self, f: impl Future<Output = T>) -> T {
// we want to avoid reentering `block_on` because then all the code
// below has to become more complex. It's also likely that the
// application will only call `block_on` once on an infinite task
// (`Future<Output = !>`)
{
let mut in_block_on = self.in_block_on.borrow_mut();
if *in_block_on {
panic!("nested `block_on`");
}
*in_block_on = true;
}
pin_mut!(f);
let ready = AtomicBool::new(true);
let waker = wrap_waker(&ready);
let val = loop {
// advance the main task
if ready.load(Ordering::Relaxed) {
ready.store(false, Ordering::Relaxed);
// println!("run block_on");
let mut cx = Context::from_waker(&waker);
if let Poll::Ready(val) = f.as_mut().poll(&mut cx) {
break val;
}
// println!("ran block_on");
}
// println!("tasks: {}", self.tasks.borrow().len());
// advance other tasks
let next_task = self.tasks.borrow_mut().pop_front();
if let Some(mut task) = next_task {
// NOTE we don't need a CAS operation here because `wake` invocations that come from
// interrupt handlers (the only source of 'race conditions' (!= data races)) are
// "oneshot": they'll issue a `wake` and then disable themselves to not run again
// until the woken task has made more work
if task.ready.load(Ordering::Relaxed) {
// we are about to service the task so switch the `ready` flag to `false`
task.ready.store(false, Ordering::Relaxed);
let waker = wrap_waker(&task.ready);
let mut cx = Context::from_waker(&waker);
let ready = task.f.as_mut().poll(&mut cx).is_ready();
if ready {
// Task is finished, do not requeue
continue;
}
}
// Requeue
self.tasks.borrow_mut().push_back(task);
}
// // try to sleep; this will be a no-op if any of the previous tasks generated a SEV or an
// // interrupt ran (regardless of whether it generated a wake-up or not)
// asm::wfe();
};
self.in_block_on.replace(false);
val
}
pub fn spawn(&self, f: impl Future + 'static) {
let task = Box::pin(Task::new(f));
self.tasks.borrow_mut().push_back(task);
}
}
pub struct Task {
ready: AtomicBool,
f: Pin<Box<dyn Future<Output = ()>>>,
}
impl Task {
fn new(f: impl Future + 'static) -> Self {
Task {
ready: AtomicBool::new(true),
f: Box::pin(async { f.await; }),
}
}
}
/// Returns a handle to the executor singleton
///
/// This lazily initializes the executor and allocator when first called
pub(crate) fn current() -> &'static Executor {
static INIT: AtomicBool = AtomicBool::new(false);
static mut EXECUTOR: UnsafeCell<MaybeUninit<Executor>> = UnsafeCell::new(MaybeUninit::uninit());
if INIT.load(Ordering::Relaxed) {
unsafe { &*(EXECUTOR.get() as *const Executor) }
} else {
unsafe {
let executorp = EXECUTOR.get() as *mut Executor;
executorp.write(Executor::new());
INIT.store(true, Ordering::Relaxed);
&*executorp
}
}
}