feat: Support for CPU architectures with no atomic instructions

Adds a `portable-atomic` feature to support these architectures.

.github/workflows/ci.yml

@@ -51,6 +51,8 @@ jobs:
         run: cargo test -- --test-threads=1
         env:
           CARGO_TARGET_X86_64_UNKNOWN_LINUX_GNU_RUNNER: valgrind -v --error-exitcode=1 --error-limit=no --leak-check=full --show-leak-kinds=all --track-origins=yes --fair-sched=yes
+      - name: Run cargo test (with portable-atomic enabled)
+        run: cargo test --features portable-atomic
       - name: Clone async-executor
         run: git clone https://github.com/smol-rs/async-executor.git
       - name: Add patch section

Cargo.toml

@@ -18,6 +18,10 @@ exclude = ["/.*"]
 default = ["std"]
 std = []
 
+[dependencies]
+# Uses portable-atomic polyfill atomics on targets without them
+portable-atomic = { version = "1", optional = true, default-features = false }
+
 [dev-dependencies]
 atomic-waker = "1"
 easy-parallel = "3"

src/header.rs

@@ -1,8 +1,13 @@
 use core::cell::UnsafeCell;
 use core::fmt;
-use core::sync::atomic::{AtomicUsize, Ordering};
 use core::task::Waker;
 
+#[cfg(not(feature = "portable-atomic"))]
+use core::sync::atomic::AtomicUsize;
+use core::sync::atomic::Ordering;
+#[cfg(feature = "portable-atomic")]
+use portable_atomic::AtomicUsize;
+
 use crate::raw::TaskVTable;
 use crate::state::*;
 use crate::utils::abort_on_panic;

src/raw.rs

@@ -5,9 +5,14 @@ use core::marker::PhantomData;
 use core::mem::{self, ManuallyDrop};
 use core::pin::Pin;
 use core::ptr::NonNull;
-use core::sync::atomic::{AtomicUsize, Ordering};
 use core::task::{Context, Poll, RawWaker, RawWakerVTable, Waker};
 
+#[cfg(not(feature = "portable-atomic"))]
+use core::sync::atomic::AtomicUsize;
+use core::sync::atomic::Ordering;
+#[cfg(feature = "portable-atomic")]
+use portable_atomic::AtomicUsize;
+
 use crate::header::Header;
 use crate::runnable::{Schedule, ScheduleInfo};
 use crate::state::*;