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

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Message-ID: <aHSFosU8tS1Oqj4l@Mac.home>
Date: Sun, 13 Jul 2025 21:20:50 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: Benno Lossin <lossin@...nel.org>
Cc: linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org,
	lkmm@...ts.linux.dev, linux-arch@...r.kernel.org,
	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 v6 6/9] rust: sync: atomic: Add the framework of
 arithmetic operations

On Fri, Jul 11, 2025 at 02:22:13PM -0700, Boqun Feng wrote:
[...]
> > >     pub unsafe trait AtomicAdd<Rhs>: AllowAtomic {
> > >         type Delta = Self::Repr;
> > >         fn rhs_into_delta(rhs: Rhs) -> Delta;
> > >     }
> > >
> > > Note that I have to provide a `Delta` (or better named as `ReprDelta`?)
> > > because of when pointer support is added, atomic addition is between
> > > a `*mut ()` and a `isize`, not two `*mut()`.
> > 
> > Makes sense, but we don't have default associated types yet :(
> > 
> 
> Oops, we are lucky enough to only have a few types to begin with ;-)
> Maybe we can use `#[derive(AtomicAdd)] to select the default delta type
> later.
> 

Turn out I could give `AtomcImpl` an associate type:

    pub trait AtomicImpl: Sealed {
    	type Delta;
    }

and then

    pub unsafe trait AllowAtomicAdd<Rhs = Self>: AllowAtomic {
        /// Converts `Rhs` into the `Delta` type of the atomic implementation.
        fn rhs_into_delta(rhs: Rhs) -> <Self::Repr as AtomicImpl>::Delta;
    }

(Yeah, I named it `AllowAtomicAdd` ;-))

and I only need to provide `Delta` for three types (i32, i64 and *mut
()).

BTW, since atomic arithmetic is wrapping, do we want things like `impl
AllowAtomicAdd<u32> for i32`, similar to the concept of
`i32::wrapping_add_unsigned()` ;-)

Regards,
Boqun

> Regards,
> Boqun