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

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Message-Id: <DC0AKAL1LW84.MR2RFTMX1H61@kernel.org>
Date: Tue, 12 Aug 2025 10:04:12 +0200
From: "Benno Lossin" <lossin@...nel.org>
To: "Boqun Feng" <boqun.feng@...il.com>, <linux-kernel@...r.kernel.org>,
 <rust-for-linux@...r.kernel.org>, <lkmm@...ts.linux.dev>,
 <linux-arch@...r.kernel.org>
Cc: "Miguel Ojeda" <ojeda@...nel.org>, "Alex Gaynor"
 <alex.gaynor@...il.com>, "Gary Guo" <gary@...yguo.net>,
 Björn Roy Baron <bjorn3_gh@...tonmail.com>, "Andreas
 Hindborg" <a.hindborg@...nel.org>, "Alice Ryhl" <aliceryhl@...gle.com>,
 "Trevor Gross" <tmgross@...ch.edu>, "Danilo Krummrich" <dakr@...nel.org>,
 "Will Deacon" <will@...nel.org>, "Peter Zijlstra" <peterz@...radead.org>,
 "Mark Rutland" <mark.rutland@....com>, "Wedson Almeida Filho"
 <wedsonaf@...il.com>, "Viresh Kumar" <viresh.kumar@...aro.org>, "Lyude
 Paul" <lyude@...hat.com>, "Ingo Molnar" <mingo@...nel.org>, "Mitchell Levy"
 <levymitchell0@...il.com>, "Paul E. McKenney" <paulmck@...nel.org>, "Greg
 Kroah-Hartman" <gregkh@...uxfoundation.org>, "Linus Torvalds"
 <torvalds@...ux-foundation.org>, "Thomas Gleixner" <tglx@...utronix.de>,
 "Alan Stern" <stern@...land.harvard.edu>
Subject: Re: [PATCH v8 6/9] rust: sync: atomic: Add the framework of
 arithmetic operations

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@...gle.com>
> Signed-off-by: Boqun Feng <boqun.feng@...il.com>

Reviewed-by: Benno Lossin <lossin@...nel.org>

> ---
>  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.

---
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;
> +}
> +