Re: [PATCH v8 6/9] rust: sync: atomic: Add the framework of arithmetic operations

[Boqun Feng]

On Tue, Aug 12, 2025 at 10:04:12AM +0200, Benno Lossin wrote:
> On Sat Jul 19, 2025 at 5:08 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 `AtomicType` 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 the plan is to add a few subtraits of
> > `AtomicType` describing which types have and can do atomic arithemtic
> > operations.
> >
> > One trait `AtomicAdd` is added, and only add() and fetch_add() are
> > added. The rest will be added in the future.
> >
> > Reviewed-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>
> > Signed-off-by: Boqun Feng <boqun.feng@xxxxxxxxx>
> 
> Reviewed-by: Benno Lossin <lossin@xxxxxxxxxx>
> 

Thank you!

> > ---
> >  rust/kernel/sync/atomic.rs           | 93 +++++++++++++++++++++++++++-
> >  rust/kernel/sync/atomic/predefine.rs | 14 +++++
> >  2 files changed, 105 insertions(+), 2 deletions(-)
> >
> > diff --git a/rust/kernel/sync/atomic.rs b/rust/kernel/sync/atomic.rs
> > index 793134aeaac1..e3a30b6aaee4 100644
> > --- a/rust/kernel/sync/atomic.rs
> > +++ b/rust/kernel/sync/atomic.rs
> > @@ -16,7 +16,6 @@
> >  //!
> >  //! [`LKMM`]: srctree/tools/memory-model/
> >  
> > -#[allow(dead_code, unreachable_pub)]
> >  mod internal;
> >  pub mod ordering;
> >  mod predefine;
> > @@ -25,7 +24,7 @@
> >  pub use ordering::{Acquire, Full, Relaxed, Release};
> >  
> >  use crate::build_error;
> > -use internal::{AtomicBasicOps, AtomicExchangeOps, AtomicRepr};
> > +use internal::{AtomicArithmeticOps, AtomicBasicOps, AtomicExchangeOps, AtomicRepr};
> >  use ordering::OrderingType;
> >  
> >  /// A memory location which can be safely modified from multiple execution contexts.
> > @@ -115,6 +114,18 @@ pub unsafe trait AtomicType: Sized + Send + Copy {
> >      type Repr: AtomicImpl;
> >  }
> >  
> > +/// Types that support atomic add operations.
> > +///
> > +/// # Safety
> > +///
> > +/// `wrapping_add` any value of type `Self::Repr::Delta` obtained by [`Self::rhs_into_delta()`] to
> 
> Can you add a normal comment TODO here:
> 
>     // TODO: properly define `wrapping_add` in this context.

Yeah, this sounds good to me. How do you propose we arrange the normal
comment with the doc comment, somthing like:

    // TODO: properly define `wrapping_add` in this context.
    
    /// Types that support atomic add operations.
    ///
    /// # Safety
    ///
    /// `wrapping_add` any value of type `Self::Repr::Delta` obtained by [`Self::rhs_into_delta()`] to
    ...
    pub unsafe trait AtomicAdd<...> {
        ...
    }

Regards,
Boqun

> 
> ---
> Cheers,
> Benno
> 
> > +/// 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 AtomicAdd<Rhs = Self>: AtomicType {
> > +    /// Converts `Rhs` into the `Delta` type of the atomic implementation.
> > +    fn rhs_into_delta(rhs: Rhs) -> <Self::Repr as AtomicImpl>::Delta;
> > +}
> > +
>