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 08:39:04PM +0200, Benno Lossin wrote:
[...]
> >> > 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.
> 
> Well part of calling `T::Repr::atomic_add` is that the pointer is valid.

Here by saying "calling `T::Repr::atomic_add`", I think you mean the
whole operation, so yeah, we have to consider the validy for `T` of the
result. But what I'm trying to do is reasoning this in 2 steps:

First, let's treat it as an `atomic_add(*mut i32, i32)`, then as long as
we provide a valid `*mut i32`, it's safe to call. 

And second assume we call it with a valid pointer to `T::Repr`, and a
delta from `rhs_into_delta()`, then per the safety guarantee of
`AllowAtomicAdd`, the value written at the pointer is a valid `T`.

Based on these, we can prove the whole operation is safe for the given
input.

> But it actually isn't valid for all operations, only for the specific
> one you have here. If we want to be 100% correct, we actually need to
> change the safety comment of `atomic_add` to say that it only requires
> the result of `*a + v` to be writable... But that is most likely very
> annoying... (note that we also have this issue for `store`)
> 
> I'm not too sure on what the right way to do this is. The formal answer
> is to "just do it right", but then safety comments really just devolve
> into formally proving the correctness of the program. I think -- for now
> at least :) -- that we shouldn't do this here & now (since we also have
> a lot of other code that isn't using normal good safety comments, let
> alone formally correct ones).
> 
> > 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) };
> 
> That looks fine for now. But isn't this duplicating the sentence
> starting with `*self`?

Oh sorry, I meant to remove the sentence starting with `*self`. :(

Regards,
Boqun

> 
> ---
> Cheers,
> Benno