[PATCH 04/14] rust: sync: atomic: Add generic atomics

Boqun Feng
2025-09-05T04:42:00Z

To provide using LKMM atomics for Rust code, a generic `Atomic<T>` is
added, currently `T` needs to be Send + Copy because these are the
straightforward usages and all basic types support this.

Implement `AtomicType` for `i32` and `i64`, and so far only basic
operations load() and store() are introduced.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Benno Lossin <lossin@kernel.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Link: https://lore.kernel.org/all/20250719030827.61357-5-boqun.feng@gmail.com/
---
rust/kernel/sync/atomic.rs | 271 +++++++++++++++++++++++++++
rust/kernel/sync/atomic/predefine.rs | 15 ++
2 files changed, 286 insertions(+)
create mode 100644 rust/kernel/sync/atomic/predefine.rs

diff --git a/rust/kernel/sync/atomic.rs b/rust/kernel/sync/atomic.rs
index 2302e6d51fe2..ea5782b6ee95 100644
--- a/rust/kernel/sync/atomic.rs
+++ b/rust/kernel/sync/atomic.rs
@@ -19,6 +19,277 @@
#[allow(dead_code, unreachable_pub)]
mod internal;
pub mod ordering;
+mod predefine;

pub use internal::AtomicImpl;
pub use ordering::{Acquire, Full, Relaxed, Release};
+
+use crate::build_error;
+use internal::{AtomicBasicOps, AtomicRepr};
+use ordering::OrderingType;
+
+/// A memory location which can be safely modified from multiple execution contexts.
+///
+/// This has the same size, alignment and bit validity as the underlying type `T`. And it disables
+/// niche optimization for the same reason as [`UnsafeCell`].
+///
+/// The atomic operations are implemented in a way that is fully compatible with the [Linux Kernel
+/// Memory (Consistency) Model][LKMM], hence they should be modeled as the corresponding
+/// [`LKMM`][LKMM] atomic primitives. With the help of [`Atomic::from_ptr()`] and
+/// [`Atomic::as_ptr()`], this provides a way to interact with [C-side atomic operations]
+/// (including those without the `atomic` prefix, e.g. `READ_ONCE()`, `WRITE_ONCE()`,
+/// `smp_load_acquire()` and `smp_store_release()`).
+///
+/// # Invariants
+///
+/// `self.0` is a valid `T`.
+///
+/// [`UnsafeCell`]: core::cell::UnsafeCell
+/// [LKMM]: srctree/tools/memory-model/
+/// [C-side atomic operations]: srctree/Documentation/atomic_t.txt
+#[repr(transparent)]
+pub struct Atomic<T: AtomicType>(AtomicRepr<T::Repr>);
+
+// SAFETY: `Atomic<T>` is safe to share among execution contexts because all accesses are atomic.
+unsafe impl<T: AtomicType> Sync for Atomic<T> {}
+
+/// Types that support basic atomic operations.
+///
+/// # Round-trip transmutability
+///
+/// `T` is round-trip transmutable to `U` if and only if both of these properties hold:
+///
+/// - Any valid bit pattern for `T` is also a valid bit pattern for `U`.
+/// - Transmuting (e.g. using [`transmute()`]) a value of type `T` to `U` and then to `T` again
+/// yields a value that is in all aspects equivalent to the original value.
+///
+/// # Safety
+///
+/// - [`Self`] must have the same size and alignment as [`Self::Repr`].
+/// - [`Self`] must be [round-trip transmutable] to [`Self::Repr`].
+///
+/// Note that this is more relaxed than requiring the bi-directional transmutability (i.e.
+/// [`transmute()`] is always sound between `U` and `T`) because of the support for atomic
+/// variables over unit-only enums, see [Examples].
+///
+/// # Limitations
+///
+/// Because C primitives are used to implement the atomic operations, and a C function requires a
+/// valid object of a type to operate on (i.e. no `MaybeUninit<_>`), hence at the Rust <-> C
+/// surface, only types with all the bits initialized can be passed. As a result, types like `(u8,
+/// u16)` (padding bytes are uninitialized) are currently not supported.
+///
+/// # Examples
+///
+/// A unit-only enum that implements [`AtomicType`]:
+///
+/// ```
+/// use kernel::sync::atomic::{AtomicType, Atomic, Relaxed};
+///
+/// #[derive(Clone, Copy, PartialEq, Eq)]
+/// #[repr(i32)]
+/// enum State {
+/// Uninit = 0,
+/// Working = 1,
+/// Done = 2,
+/// };
+///
+/// // SAFETY: `State` and `i32` has the same size and alignment, and it's round-trip
+/// // transmutable to `i32`.
+/// unsafe impl AtomicType for State {
+/// type Repr = i32;
+/// }
+///
+/// let s = Atomic::new(State::Uninit);
+///
+/// assert_eq!(State::Uninit, s.load(Relaxed));
+/// ```
+/// [`transmute()`]: core::mem::transmute
+/// [round-trip transmutable]: AtomicType#round-trip-transmutability
+/// [Examples]: AtomicType#examples
+pub unsafe trait AtomicType: Sized + Send + Copy {
+ /// The backing atomic implementation type.
+ type Repr: AtomicImpl;
+}
+
+#[inline(always)]
+const fn into_repr<T: AtomicType>(v: T) -> T::Repr {
+ // SAFETY: Per the safety requirement of `AtomicType`, `T` is round-trip transmutable to
+ // `T::Repr`, therefore the transmute operation is sound.
+ unsafe { core::mem::transmute_copy(&v) }
+}
+
+/// # Safety
+///
+/// `r` must be a valid bit pattern of `T`.
+#[inline(always)]
+const unsafe fn from_repr<T: AtomicType>(r: T::Repr) -> T {
+ // SAFETY: Per the safety requirement of the function, the transmute operation is sound.
+ unsafe { core::mem::transmute_copy(&r) }
+}
+
+impl<T: AtomicType> Atomic<T> {
+ /// Creates a new atomic `T`.
+ pub const fn new(v: T) -> Self {
+ // INVARIANT: Per the safety requirement of `AtomicType`, `into_repr(v)` is a valid `T`.
+ Self(AtomicRepr::new(into_repr(v)))
+ }
+
+ /// Creates a reference to an atomic `T` from a pointer of `T`.
+ ///
+ /// This usually is used when communicating with C side or manipulating a C struct, see
+ /// examples below.
+ ///
+ /// # Safety
+ ///
+ /// - `ptr` is aligned to `align_of::<T>()`.
+ /// - `ptr` is valid for reads and writes for `'a`.
+ /// - For the duration of `'a`, other accesses to `*ptr` must not cause data races (defined
+ /// by [`LKMM`]) against atomic operations on the returned reference. Note that if all other
+ /// accesses are atomic, then this safety requirement is trivially fulfilled.
+ ///
+ /// [`LKMM`]: srctree/tools/memory-model
+ ///
+ /// # Examples
+ ///
+ /// Using [`Atomic::from_ptr()`] combined with [`Atomic::load()`] or [`Atomic::store()`] can
+ /// achieve the same functionality as `READ_ONCE()`/`smp_load_acquire()` or
+ /// `WRITE_ONCE()`/`smp_store_release()` in C side:
+ ///
+ /// ```
+ /// # use kernel::types::Opaque;
+ /// use kernel::sync::atomic::{Atomic, Relaxed, Release};
+ ///
+ /// // Assume there is a C struct `foo`.
+ /// mod cbindings {
+ /// #[repr(C)]
+ /// pub(crate) struct foo {
+ /// pub(crate) a: i32,
+ /// pub(crate) b: i32
+ /// }
+ /// }
+ ///
+ /// let tmp = Opaque::new(cbindings::foo { a: 1, b: 2 });
+ ///
+ /// // struct foo *foo_ptr = ..;
+ /// let foo_ptr = tmp.get();
+ ///
+ /// // SAFETY: `foo_ptr` is valid, and `.a` is in bounds.
+ /// let foo_a_ptr = unsafe { &raw mut (*foo_ptr).a };
+ ///
+ /// // a = READ_ONCE(foo_ptr->a);
+ /// //
+ /// // SAFETY: `foo_a_ptr` is valid for read, and all other accesses on it is atomic, so no
+ /// // data race.
+ /// let a = unsafe { Atomic::from_ptr(foo_a_ptr) }.load(Relaxed);
+ /// # assert_eq!(a, 1);
+ ///
+ /// // smp_store_release(&foo_ptr->a, 2);
+ /// //
+ /// // SAFETY: `foo_a_ptr` is valid for writes, and all other accesses on it is atomic, so
+ /// // no data race.
+ /// unsafe { Atomic::from_ptr(foo_a_ptr) }.store(2, Release);
+ /// ```
+ pub unsafe fn from_ptr<'a>(ptr: *mut T) -> &'a Self
+ where
+ T: Sync,
+ {
+ // CAST: `T` and `Atomic<T>` have the same size, alignment and bit validity.
+ // SAFETY: Per function safety requirement, `ptr` is a valid pointer and the object will
+ // live long enough. It's safe to return a `&Atomic<T>` because function safety requirement
+ // guarantees other accesses won't cause data races.
+ unsafe { &*ptr.cast::<Self>() }
+ }
+
+ /// Returns a pointer to the underlying atomic `T`.
+ ///
+ /// Note that use of the return pointer must not cause data races defined by [`LKMM`].
+ ///
+ /// # Guarantees
+ ///
+ /// The returned pointer is valid and properly aligned (i.e. aligned to [`align_of::<T>()`]).
+ ///
+ /// [`LKMM`]: srctree/tools/memory-model
+ /// [`align_of::<T>()`]: core::mem::align_of
+ pub const fn as_ptr(&self) -> *mut T {
+ // GUARANTEE: Per the function guarantee of `AtomicRepr::as_ptr()`, the `self.0.as_ptr()`
+ // must be a valid and properly aligned pointer for `T::Repr`, and per the safety guarantee
+ // of `AtomicType`, it's a valid and properly aligned pointer of `T`.
+ self.0.as_ptr().cast()
+ }
+
+ /// Returns a mutable reference to the underlying atomic `T`.
+ ///
+ /// This is safe because the mutable reference of the atomic `T` guarantees exclusive access.
+ pub fn get_mut(&mut self) -> &mut T {
+ // CAST: `T` and `T::Repr` has the same size and alignment per the safety requirement of
+ // `AtomicType`, and per the type invariants `self.0` is a valid `T`, therefore the casting
+ // result is a valid pointer of `T`.
+ // SAFETY: The pointer is valid per the CAST comment above, and the mutable reference
+ // guarantees exclusive access.
+ unsafe { &mut *self.0.as_ptr().cast() }
+ }
+}
+
+impl<T: AtomicType> Atomic<T>
+where
+ T::Repr: AtomicBasicOps,
+{
+ /// Loads the value from the atomic `T`.
+ ///
+ /// # Examples
+ ///
+ /// ```
+ /// use kernel::sync::atomic::{Atomic, Relaxed};
+ ///
+ /// let x = Atomic::new(42i32);
+ ///
+ /// assert_eq!(42, x.load(Relaxed));
+ ///
+ /// let x = Atomic::new(42i64);
+ ///
+ /// assert_eq!(42, x.load(Relaxed));
+ /// ```
+ #[doc(alias("atomic_read", "atomic64_read"))]
+ #[inline(always)]
+ pub fn load<Ordering: ordering::AcquireOrRelaxed>(&self, _: Ordering) -> T {
+ let v = {
+ match Ordering::TYPE {
+ OrderingType::Relaxed => T::Repr::atomic_read(&self.0),
+ OrderingType::Acquire => T::Repr::atomic_read_acquire(&self.0),
+ _ => build_error!("Wrong ordering"),
+ }
+ };
+
+ // SAFETY: `v` comes from reading `self.0`, which is a valid `T` per the type invariants.
+ unsafe { from_repr(v) }
+ }
+
+ /// Stores a value to the atomic `T`.
+ ///
+ /// # Examples
+ ///
+ /// ```
+ /// use kernel::sync::atomic::{Atomic, Relaxed};
+ ///
+ /// let x = Atomic::new(42i32);
+ ///
+ /// assert_eq!(42, x.load(Relaxed));
+ ///
+ /// x.store(43, Relaxed);
+ ///
+ /// assert_eq!(43, x.load(Relaxed));
+ /// ```
+ #[doc(alias("atomic_set", "atomic64_set"))]
+ #[inline(always)]
+ pub fn store<Ordering: ordering::ReleaseOrRelaxed>(&self, v: T, _: Ordering) {
+ let v = into_repr(v);
+
+ // INVARIANT: `v` is a valid `T`, and is stored to `self.0` by `atomic_set*()`.
+ match Ordering::TYPE {
+ OrderingType::Relaxed => T::Repr::atomic_set(&self.0, v),
+ OrderingType::Release => T::Repr::atomic_set_release(&self.0, v),
+ _ => build_error!("Wrong ordering"),
+ }
+ }
+}
diff --git a/rust/kernel/sync/atomic/predefine.rs b/rust/kernel/sync/atomic/predefine.rs
new file mode 100644
index 000000000000..33356deee952
--- /dev/null
+++ b/rust/kernel/sync/atomic/predefine.rs
@@ -0,0 +1,15 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Pre-defined atomic types
+
+// SAFETY: `i32` has the same size and alignment with itself, and is round-trip transmutable to
+// itself.
+unsafe impl super::AtomicType for i32 {
+ type Repr = i32;
+}
+
+// SAFETY: `i64` has the same size and alignment with itself, and is round-trip transmutable to
+// itself.
+unsafe impl super::AtomicType for i64 {
+ type Repr = i64;
+}
--
2.51.0