Re: [PATCH v6 6/9] rust: sync: atomic: Add the framework of arithmetic operations

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

On Fri, Jul 11, 2025 at 10:53:45AM +0200, Benno Lossin wrote:
> On Thu Jul 10, 2025 at 8:00 AM CEST, Boqun Feng 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.
> >
> > Trait `AllowAtomicArithmetic` 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)`.
> >
> > Only add() and fetch_add() are added. The rest will be added in the
> > future.
> >
> > Reviewed-by: Alice Ryhl <aliceryhl@google.com>
> > Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
> > ---
> >  rust/kernel/sync/atomic.rs         |  18 +++++
> >  rust/kernel/sync/atomic/generic.rs | 108 +++++++++++++++++++++++++++++
> >  2 files changed, 126 insertions(+)
> 
> I think it's better to name this trait `AtomicAdd` and make it generic:
> 
>     pub unsafe trait AtomicAdd<Rhs = Self>: AllowAtomic {
>         fn rhs_into_repr(rhs: Rhs) -> Self::Repr;
>     }
> 
> `sub` and `fetch_sub` can be added using a similar trait.
> 

Seems a good idea, I will see what I can do. Thanks!

> The generic allows you to implement it multiple times with different
> meanings, for example:
> 
>     pub struct Nanos(u64);
>     pub struct Micros(u64);
>     pub struct Millis(u64);
> 
>     impl AllowAtomic for Nanos {
>         type Repr = i64;
>     }
> 
>     impl AtomicAdd<Millis> for Nanos {
>         fn rhs_into_repr(rhs: Millis) -> i64 {
>             transmute(rhs.0 * 1000_000)

We probably want to use `as` in real code?

>         }
>     }
> 
>     impl AtomicAdd<Micros> for Nanos {
>         fn rhs_into_repr(rhs: Micros) -> i64 {
>             transmute(rhs.0 * 1000)
>         }
>     }
> 
>     impl AtomicAdd<Nanos> for Nanos {
>         fn rhs_into_repr(rhs: Nanos) -> i64 {
>             transmute(rhs.0)
>         }
>     }
> 
> For the safety requirement on the `AtomicAdd` trait, we might just
> require bi-directional transmutability... Or can you imagine a case
> where that is not guaranteed, but a weaker form is?

I have a case that I don't think it's that useful, but it's similar to
the `Micros` and `Millis` above, an `Even<T>` where `Even<i32>` is a
`i32` but it's always an even number ;-) So transmute<i32, Even<i32>>()
is not always sound. Maybe we could add a "TODO" in the safety section
of `AtomicAdd`, and revisit this later? Like:

/// (in # Safety)
/// TODO: The safety requirement may be tightened to bi-directional
/// transmutability. 

And maybe also add the `Even` example there?

Thoughts?

Regards,
Boqun

> 
> ---
> Cheers,
> Benno