Re: [PATCH v7 6/9] rust: sync: atomic: Add the framework of arithmetic operations

["Benno Lossin" <lossin@kernel.org>]

On Mon Jul 14, 2025 at 7:36 AM CEST, Boqun Feng wrote:
> +/// Types that support atomic add operations.
> +///
> +/// # Safety
> +///
> +/// Wrapping adding any value of type `Self::Repr::Delta` obtained by [`Self::rhs_into_delta()`] to

I don't like "wrapping adding", either we use "`wrapping_add`" or we use
some other phrasing.

> +/// 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 AllowAtomicAdd<Rhs = Self>: AllowAtomic {

Why `Allow*`? I think `AtomicAdd` is better?

> +    /// Converts `Rhs` into the `Delta` type of the atomic implementation.
> +    fn rhs_into_delta(rhs: Rhs) -> <Self::Repr as AtomicImpl>::Delta;
> +}
> +
>  impl<T: AllowAtomic> Atomic<T> {
>      /// Creates a new atomic `T`.
>      pub const fn new(v: T) -> Self {
> @@ -462,3 +474,100 @@ fn try_cmpxchg<Ordering: ordering::Any>(&self, old: &mut T, new: T, _: Ordering)
>          ret
>      }
>  }
> +
> +impl<T: AllowAtomic> Atomic<T>
> +where
> +    T::Repr: AtomicHasArithmeticOps,
> +{
> +    /// 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, Ordering: ordering::RelaxedOnly>(&self, v: Rhs, _: Ordering)
> +    where
> +        T: AllowAtomicAdd<Rhs>,
> +    {
> +        let v = T::rhs_into_delta(v);
> +        // CAST: Per the safety requirement of `AllowAtomic`, a valid pointer of `T` is a valid
> +        // pointer of `T::Repr` for reads and valid for writes of values transmutable to `T`.
> +        let a = self.as_ptr().cast::<T::Repr>();
> +
> +        // `*self` remains valid after `atomic_add()` because of the safety requirement of
> +        // `AllowAtomicAdd`.

This part should be moved to the CAST comment above, since we're not
only writing a value transmuted from `T` into `*self`.

---
Cheers,
Benno

> +        //
> +        // SAFETY:
> +        // - `a` is aligned to `align_of::<T::Repr>()` because of the safety requirement of
> +        //   `AllowAtomic` and the guarantee of `Atomic::as_ptr()`.
> +        // - `a` is a valid pointer per the CAST justification above.
> +        unsafe {
> +            T::Repr::atomic_add(a, v);
> +        }
> +    }