libasync: replace executor Mutexes with RefCells

this will not run on multi-core.

libasync/src/executor.rs

@@ -1,17 +1,16 @@
 use core::{
-    cell::{Cell, UnsafeCell},
+    cell::{RefCell, UnsafeCell},
     future::Future,
     mem::MaybeUninit,
     pin::Pin,
-    sync::atomic::{self, AtomicBool, Ordering},
+    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;
-use libcortex_a9::mutex::Mutex;
 // TODO: delete
-use libboard_zynq::println;
+//use libboard_zynq::println;
 
 // NOTE `*const ()` is &AtomicBool
 static VTABLE: RawWakerVTable = {
@@ -35,16 +34,16 @@ static VTABLE: RawWakerVTable = {
 ///
 /// This is a singleton
 pub struct Executor {
-    in_block_on: Mutex<bool>,
+    in_block_on: RefCell<bool>,
-    tasks: Mutex<Deque<Task>>,
+    tasks: RefCell<Deque<Task>>,
 }
 
 impl Executor {
     /// Creates a new instance of the executor
     pub fn new() -> Self {
         Self {
-            in_block_on: Mutex::new(false),
+            in_block_on: RefCell::new(false),
-            tasks: Mutex::new(Deque::new()),
+            tasks: RefCell::new(Deque::new()),
         }
     }
 
@@ -54,7 +53,7 @@ impl Executor {
         // application will only call `block_on` once on an infinite task
         // (`Future<Output = !>`)
         {
-            let mut in_block_on = self.in_block_on.lock();
+            let mut in_block_on = self.in_block_on.borrow_mut();
             if *in_block_on {
                 panic!("nested `block_on`");
             }
@@ -70,14 +69,17 @@ impl Executor {
             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.lock().pop_front();
+            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
@@ -94,14 +96,16 @@ impl Executor {
                     };
                     let mut cx = Context::from_waker(&waker);
                     // this points into a `static` memory so it's already pinned
-                    let r = unsafe {
+                    // println!("run task");
+                    let ready = unsafe {
                         Pin::new_unchecked(&mut *task.f)
                             .poll(&mut cx)
                             .is_ready()
                     };
-                    if !r {
+                    // println!("ran task, ready={:?}", r);
+                    if !ready {
                         // Task is not finished, requeue
-                        self.tasks.lock().push_back(task);
+                        self.tasks.borrow_mut().push_back(task);
                     }
                 }
             }
@@ -110,7 +114,7 @@ impl Executor {
             // // interrupt ran (regardless of whether it generated a wake-up or not)
             // asm::wfe();
         };
-        *self.in_block_on.lock() = false;
+        self.in_block_on.replace(false);
         val
     }
 
@@ -120,13 +124,13 @@ impl Executor {
     pub fn spawn(&self, f: impl Future + 'static) {
         // NOTE(unsafe) only safe as long as `spawn` is never re-entered and this does not overlap
         // with operation `(A)` (see `Task::block_on`)
-        self.tasks.lock().push_back(Task::new(f));
+        self.tasks.borrow_mut().push_back(Task::new(f));
     }
 }
 
 pub struct Task {
     ready: AtomicBool,
-    f: Box<Future<Output = ()>>,
+    f: Box<dyn Future<Output = ()>>,
 }
 
 impl Task {