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

On Wed, 18 Jun 2025 09:49:30 -0700
Boqun Feng <[email protected]> wrote:

> 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 <[email protected]>
> ---
>  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) {

This can be just

pub fn add(&self, v: T::Delta, _: Relaxed)

> +        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);
> +        }
> +    }
> +