linux-kernel - Re: [PATCH v7 6/9] rust: sync: atomic: Add the framework of arithmetic operations

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Message-Id: <DBCL7YUSRMXR.22SMO1P7D5G60@kernel.org>
Date: Tue, 15 Jul 2025 13:21:20 +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 v7 6/9] rust: sync: atomic: Add the framework of
 arithmetic operations

On Mon Jul 14, 2025 at 7:36 AM CEST, Boqun Feng wrote:
> +/// Types that support atomic add operations.
> +///
> +/// # Safety
> +///
> +/// Wrapping adding any value of type `Self::Repr::Delta` obtained by [`Self::rhs_into_delta()`] to

I don't like "wrapping adding", either we use "`wrapping_add`" or we use
some other phrasing.

> +/// 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 AllowAtomicAdd<Rhs = Self>: AllowAtomic {

Why `Allow*`? I think `AtomicAdd` is better?

> +    /// Converts `Rhs` into the `Delta` type of the atomic implementation.
> +    fn rhs_into_delta(rhs: Rhs) -> <Self::Repr as AtomicImpl>::Delta;
> +}
> +
>  impl<T: AllowAtomic> Atomic<T> {
>      /// Creates a new atomic `T`.
>      pub const fn new(v: T) -> Self {
> @@ -462,3 +474,100 @@ fn try_cmpxchg<Ordering: ordering::Any>(&self, old: &mut T, new: T, _: Ordering)
>          ret
>      }
>  }
> +
> +impl<T: AllowAtomic> Atomic<T>
> +where
> +    T::Repr: AtomicHasArithmeticOps,
> +{
> +    /// Atomic add.
> +    ///
> +    /// Atomically updates `*self` to `(*self).wrapping_add(v)`.
> +    ///
> +    /// # Examples
> +    ///
> +    /// ```
> +    /// use kernel::sync::atomic::{Atomic, Relaxed};
> +    ///
> +    /// let x = Atomic::new(42);
> +    ///
> +    /// assert_eq!(42, x.load(Relaxed));
> +    ///
> +    /// x.add(12, Relaxed);
> +    ///
> +    /// assert_eq!(54, x.load(Relaxed));
> +    /// ```
> +    #[inline(always)]
> +    pub fn add<Rhs, Ordering: ordering::RelaxedOnly>(&self, v: Rhs, _: Ordering)
> +    where
> +        T: AllowAtomicAdd<Rhs>,
> +    {
> +        let v = T::rhs_into_delta(v);
> +        // CAST: Per the safety requirement of `AllowAtomic`, a valid pointer of `T` is a valid
> +        // pointer of `T::Repr` for reads and valid for writes of values transmutable to `T`.
> +        let a = self.as_ptr().cast::<T::Repr>();
> +
> +        // `*self` remains valid after `atomic_add()` because of the safety requirement of
> +        // `AllowAtomicAdd`.

This part should be moved to the CAST comment above, since we're not
only writing a value transmuted from `T` into `*self`.

---
Cheers,
Benno

> +        //
> +        // SAFETY:
> +        // - `a` is aligned to `align_of::<T::Repr>()` because of the safety requirement of
> +        //   `AllowAtomic` and the guarantee of `Atomic::as_ptr()`.
> +        // - `a` is a valid pointer per the CAST justification above.
> +        unsafe {
> +            T::Repr::atomic_add(a, v);
> +        }
> +    }