[PATCH 06/14] rust: sync: atomic: Add the framework of arithmetic operations

Boqun Feng
2025-09-05T04:42:02Z

One important set of atomic operations is the arithmetic operations,
i.e. add(), sub(), fetch_add(), add_return(), etc. However it may not
make senses for all the types that `AtomicType` to have arithmetic
operations, for example a `Foo(u32)` may not have a reasonable add() or
sub(), plus subword types (`u8` and `u16`) currently don't have
atomic arithmetic operations even on C side and might not have them in
the future in Rust (because they are usually suboptimal on a few
architecures). Therefore the plan is to add a few subtraits of
`AtomicType` describing which types have and can do atomic arithemtic
operations.

One trait `AtomicAdd` is added, and only add() and fetch_add() are
added. The rest will be added in the future.

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/20250719030827.61357-7-boqun.feng@gmail.com/
---
rust/kernel/sync/atomic.rs | 94 +++++++++++++++++++++++++++-
rust/kernel/sync/atomic/predefine.rs | 14 +++++
2 files changed, 106 insertions(+), 2 deletions(-)

diff --git a/rust/kernel/sync/atomic.rs b/rust/kernel/sync/atomic.rs
index 4c32d12dc61e..016a6bcaf080 100644
--- a/rust/kernel/sync/atomic.rs
+++ b/rust/kernel/sync/atomic.rs
@@ -16,7 +16,6 @@
//!
//! [`LKMM`]: srctree/tools/memory-model/

-#[allow(dead_code, unreachable_pub)]
mod internal;
pub mod ordering;
mod predefine;
@@ -25,7 +24,7 @@
pub use ordering::{Acquire, Full, Relaxed, Release};

use crate::build_error;
-use internal::{AtomicBasicOps, AtomicExchangeOps, AtomicRepr};
+use internal::{AtomicArithmeticOps, AtomicBasicOps, AtomicExchangeOps, AtomicRepr};
use ordering::OrderingType;

/// A memory location which can be safely modified from multiple execution contexts.
@@ -112,6 +111,19 @@ pub unsafe trait AtomicType: Sized + Send + Copy {
type Repr: AtomicImpl;
}

+/// Types that support atomic add operations.
+///
+/// # Safety
+///
+// TODO: Properly defines `wrapping_add` in the following comment.
+/// `wrapping_add` any value of type `Self::Repr::Delta` obtained by [`Self::rhs_into_delta()`] to
+/// any value of type `Self::Repr` obtained through transmuting a value of type `Self` to must
+/// yield a value with a bit pattern also valid for `Self`.
+pub unsafe trait AtomicAdd<Rhs = Self>: AtomicType {
+ /// Converts `Rhs` into the `Delta` type of the atomic implementation.
+ fn rhs_into_delta(rhs: Rhs) -> <Self::Repr as AtomicImpl>::Delta;
+}
+
#[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
@@ -459,3 +471,81 @@ fn try_cmpxchg<Ordering: ordering::Ordering>(&self, old: &mut T, new: T, _: Orde
ret
}
}
+
+impl<T: AtomicType> Atomic<T>
+where
+ T::Repr: AtomicArithmeticOps,
+{
+ /// Atomic add.
+ ///
+ /// Atomically updates `*self` to `(*self).wrapping_add(v)`.
+ ///
+ /// # Examples
+ ///
+ /// ```
+ /// use kernel::sync::atomic::{Atomic, Relaxed};
+ ///
+ /// let x = Atomic::new(42);
+ ///
+ /// assert_eq!(42, x.load(Relaxed));
+ ///
+ /// x.add(12, Relaxed);
+ ///
+ /// assert_eq!(54, x.load(Relaxed));
+ /// ```
+ #[inline(always)]
+ pub fn add<Rhs>(&self, v: Rhs, _: ordering::Relaxed)
+ where
+ T: AtomicAdd<Rhs>,
+ {
+ let v = T::rhs_into_delta(v);
+
+ // INVARIANT: `self.0` is a valid `T` after `atomic_add()` due to safety requirement of
+ // `AtomicAdd`.
+ T::Repr::atomic_add(&self.0, v);
+ }
+
+ /// Atomic fetch and add.
+ ///
+ /// Atomically updates `*self` to `(*self).wrapping_add(v)`, and returns the value of `*self`
+ /// before the update.
+ ///
+ /// # Examples
+ ///
+ /// ```
+ /// use kernel::sync::atomic::{Atomic, Acquire, Full, Relaxed};
+ ///
+ /// let x = Atomic::new(42);
+ ///
+ /// assert_eq!(42, x.load(Relaxed));
+ ///
+ /// assert_eq!(54, { x.fetch_add(12, Acquire); x.load(Relaxed) });
+ ///
+ /// let x = Atomic::new(42);
+ ///
+ /// assert_eq!(42, x.load(Relaxed));
+ ///
+ /// assert_eq!(54, { x.fetch_add(12, Full); x.load(Relaxed) } );
+ /// ```
+ #[inline(always)]
+ pub fn fetch_add<Rhs, Ordering: ordering::Ordering>(&self, v: Rhs, _: Ordering) -> T
+ where
+ T: AtomicAdd<Rhs>,
+ {
+ let v = T::rhs_into_delta(v);
+
+ // INVARIANT: `self.0` is a valid `T` after `atomic_fetch_add*()` due to safety requirement
+ // of `AtomicAdd`.
+ let ret = {
+ match Ordering::TYPE {
+ OrderingType::Full => T::Repr::atomic_fetch_add(&self.0, v),
+ OrderingType::Acquire => T::Repr::atomic_fetch_add_acquire(&self.0, v),
+ OrderingType::Release => T::Repr::atomic_fetch_add_release(&self.0, v),
+ OrderingType::Relaxed => T::Repr::atomic_fetch_add_relaxed(&self.0, v),
+ }
+ };
+
+ // SAFETY: `ret` comes from reading `self.0`, which is a valid `T` per type invariants.
+ unsafe { from_repr(ret) }
+ }
+}
diff --git a/rust/kernel/sync/atomic/predefine.rs b/rust/kernel/sync/atomic/predefine.rs
index 33356deee952..a6e5883be7cb 100644
--- a/rust/kernel/sync/atomic/predefine.rs
+++ b/rust/kernel/sync/atomic/predefine.rs
@@ -8,8 +8,22 @@ unsafe impl super::AtomicType for i32 {
type Repr = i32;
}

+// SAFETY: The wrapping add result of two `i32`s is a valid `i32`.
+unsafe impl super::AtomicAdd<i32> for i32 {
+ fn rhs_into_delta(rhs: i32) -> i32 {
+ rhs
+ }
+}
+
// 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;
}
+
+// SAFETY: The wrapping add result of two `i64`s is a valid `i64`.
+unsafe impl super::AtomicAdd<i64> for i64 {
+ fn rhs_into_delta(rhs: i64) -> i64 {
+ rhs
+ }
+}
--
2.51.0