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

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

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.

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)
        }
    }

    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?

---
Cheers,
Benno