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

[Boqun Feng <boqun.feng@gmail.com>]

On Tue, Jul 15, 2025 at 01:21:20PM +0200, Benno Lossin wrote:
> 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.
> 

Let's use `wrapping_add` then.

    /// `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 AllowAtomicAdd<Rhs = Self>: AllowAtomic {
> 
> Why `Allow*`? I think `AtomicAdd` is better?
> 

To be consistent with `AllowAtomic` (the super trait), if we use
`AtomicAdd` here, should we change `AllowAtomic` to `AtomicBase`?

> > +    /// 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`.
> 

Hmm.. the CAST comment should explain why a pointer of `T` can be a
valid pointer of `T::Repr` because the atomic_add() below is going to
read through the pointer and write value back. The comment starting with
"`*self`" explains the value written is a valid `T`, therefore
conceptually atomic_add() below writes a valid `T` in form of `T::Repr`
into `a`.

Basically

// CAST
let a = ..

^ explains what `a` is a valid for and why it's valid.

// `*self` remains

^ explains that we write a valid value to `a`.


So I don't think we need to move?

Regards,
Boqun

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