[PATCH v5 06/10] rust: sync: atomic: Add the framework of arithmetic operations

Boqun Feng
2025-06-18T16:49:47Z

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 `AllowAtomic` 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 add a subtrait of `AllowAtomic` describing
which types have and can do atomic arithemtic operations.

A few things about this `AllowAtomicArithmetic` trait:

* It has an associate type `Delta` instead of using
`AllowAllowAtomic::Repr` because, a `Bar(u32)` (whose `Repr` is `i32`)
may not wants an `add(&self, i32)`, but an `add(&self, u32)`.

* `AtomicImpl` types already implement an `AtomicHasArithmeticOps`
trait, so add blanket implementation for them. In the future, `i8` and
`i16` may impl `AtomicImpl` but not `AtomicHasArithmeticOps` if
arithemtic operations are not available.

Only add() and fetch_add() are added. The rest will be added in the
future.

Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
---
rust/kernel/sync/atomic/generic.rs | 101 +++++++++++++++++++++++++++++
1 file changed, 101 insertions(+)

diff --git a/rust/kernel/sync/atomic/generic.rs b/rust/kernel/sync/atomic/generic.rs
index bcdbeea45dd8..8c5bd90b2619 100644
--- a/rust/kernel/sync/atomic/generic.rs
+++ b/rust/kernel/sync/atomic/generic.rs
@@ -57,6 +57,23 @@ fn from_repr(repr: Self::Repr) -> Self {
}
}

+/// Atomics that allows arithmetic operations with an integer type.
+pub trait AllowAtomicArithmetic: AllowAtomic {
+ /// The delta types for arithmetic operations.
+ type Delta;
+
+ /// Converts [`Self::Delta`] into the representation of the atomic type.
+ fn delta_into_repr(d: Self::Delta) -> Self::Repr;
+}
+
+impl<T: AtomicImpl + AtomicHasArithmeticOps> AllowAtomicArithmetic for T {
+ type Delta = Self;
+
+ fn delta_into_repr(d: Self::Delta) -> Self::Repr {
+ d
+ }
+}
+
impl<T: AllowAtomic> Atomic<T> {
/// Creates a new atomic.
pub const fn new(v: T) -> Self {
@@ -410,3 +427,87 @@ fn try_cmpxchg<Ordering: All>(&self, old: &mut T, new: T, _: Ordering) -> bool {
}
}
}
+
+impl<T: AllowAtomicArithmetic> Atomic<T>
+where
+ T::Repr: AtomicHasArithmeticOps,
+{
+ /// Atomic add.
+ ///
+ /// The addition is a wrapping addition.
+ ///
+ /// # Examples
+ ///
+ /// ```rust
+ /// 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<Ordering: RelaxedOnly>(&self, v: T::Delta, _: Ordering) {
+ let v = T::delta_into_repr(v);
+ let a = self.as_ptr().cast::<T::Repr>();
+
+ // SAFETY:
+ // - For calling the atomic_add() function:
+ // - `self.as_ptr()` is a valid pointer, and per the safety requirement of `AllocAtomic`,
+ // a `*mut T` is a valid `*mut T::Repr`. Therefore `a` is a valid pointer,
+ // - per the type invariants, the following atomic operation won't cause data races.
+ // - For extra safety requirement of usage on pointers returned by `self.as_ptr():
+ // - atomic operations are used here.
+ unsafe {
+ T::Repr::atomic_add(a, v);
+ }
+ }
+
+ /// Atomic fetch and add.
+ ///
+ /// The addition is a wrapping addition.
+ ///
+ /// # Examples
+ ///
+ /// ```rust
+ /// 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<Ordering: All>(&self, v: T::Delta, _: Ordering) -> T {
+ let v = T::delta_into_repr(v);
+ let a = self.as_ptr().cast::<T::Repr>();
+
+ // SAFETY:
+ // - For calling the atomic_fetch_add*() function:
+ // - `self.as_ptr()` is a valid pointer, and per the safety requirement of `AllocAtomic`,
+ // a `*mut T` is a valid `*mut T::Repr`. Therefore `a` is a valid pointer,
+ // - per the type invariants, the following atomic operation won't cause data races.
+ // - For extra safety requirement of usage on pointers returned by `self.as_ptr():
+ // - atomic operations are used here.
+ let ret = unsafe {
+ match Ordering::TYPE {
+ OrderingType::Full => T::Repr::atomic_fetch_add(a, v),
+ OrderingType::Acquire => T::Repr::atomic_fetch_add_acquire(a, v),
+ OrderingType::Release => T::Repr::atomic_fetch_add_release(a, v),
+ OrderingType::Relaxed => T::Repr::atomic_fetch_add_relaxed(a, v),
+ }
+ };
+
+ T::from_repr(ret)
+ }
+}
--
2.39.5 (Apple Git-154)