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 05:45:34PM +0200, Benno Lossin wrote:
> On Tue Jul 15, 2025 at 3:33 PM CEST, Boqun Feng wrote:
> > 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`?
> 
> Ideally, we would name that trait just `Atomic` :) But it then
> conflicts with the `Atomic<T>` struct (this would be motivation to put
> them in different modules :). I like `AtomicBase` better than

Oh, if we move `Atomic<T>` to atomic.rs and keep atomic::generic, then
we can name it atomic::generic::Atomic ;-)

> `AllowAtomic`, but maybe there is a better name, how about `AtomicType`?
> 

AtomicType may be better than AtomicBase to me.

> >> > +    /// 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`.
> 
> I see, my interpretation was that if we put it on the cast, then the
> operation that `atomic_add` does also is valid.
> 
> But I think this comment should either be part of the `CAST` or the
> `SAFETY` comment. Going by your interpretation, it would make more sense
> in the SAFETY one, since there you justify that you're actually writing
> a value of type `T`.
> 

Hmm.. you're probably right. There are two safety things about
atomic_add():

- Whether calling it is safe
- Whether the operation on `a` (a pointer to `T` essentially) is safe.

How about the following:

        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`.
        //
        // SAFETY:
        // - For calling `atomic_add()`:
        //   - `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.
        // - For accessing `*a`: the value written is transmutable to `T`
        //   due to the safety requirement of `AllowAtomicAdd`.
        unsafe { T::Repr::atomic_add(a, v) };

Regards,
Boqun

> ---
> Cheers,
> Benno
> 
> > 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?