linux-kernel - Re: [PATCH v5 05/10] rust: sync: atomic: Add atomic {cmp,}xchg operations

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Message-ID: <87a55uzlxv.fsf@kernel.org>
Date: Thu, 26 Jun 2025 15:12:12 +0200
From: Andreas Hindborg <a.hindborg@...nel.org>
To: "Boqun Feng" <boqun.feng@...il.com>
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>,  "Benno
 Lossin" <lossin@...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>
Subject: Re: [PATCH v5 05/10] rust: sync: atomic: Add atomic {cmp,}xchg
 operations

"Boqun Feng" <boqun.feng@...il.com> writes:

> xchg() and cmpxchg() are basic operations on atomic. Provide these based
> on C APIs.
>
> Note that cmpxchg() use the similar function signature as
> compare_exchange() in Rust std: returning a `Result`, `Ok(old)` means
> the operation succeeds and `Err(old)` means the operation fails.
>
> Signed-off-by: Boqun Feng <boqun.feng@...il.com>
> ---
>  rust/kernel/sync/atomic/generic.rs | 154 +++++++++++++++++++++++++++++
>  1 file changed, 154 insertions(+)
>
> diff --git a/rust/kernel/sync/atomic/generic.rs b/rust/kernel/sync/atomic/generic.rs
> index 73c26f9cf6b8..bcdbeea45dd8 100644
> --- a/rust/kernel/sync/atomic/generic.rs
> +++ b/rust/kernel/sync/atomic/generic.rs
> @@ -256,3 +256,157 @@ pub fn store<Ordering: ReleaseOrRelaxed>(&self, v: T, _: Ordering) {
>          };
>      }
>  }
> +
> +impl<T: AllowAtomic> Atomic<T>
> +where
> +    T::Repr: AtomicHasXchgOps,
> +{
> +    /// Atomic exchange.
> +    ///
> +    /// # Examples
> +    ///
> +    /// ```rust
> +    /// use kernel::sync::atomic::{Atomic, Acquire, Relaxed};
> +    ///
> +    /// let x = Atomic::new(42);
> +    ///
> +    /// assert_eq!(42, x.xchg(52, Acquire));
> +    /// assert_eq!(52, x.load(Relaxed));
> +    /// ```
> +    #[doc(alias("atomic_xchg", "atomic64_xchg"))]
> +    #[inline(always)]
> +    pub fn xchg<Ordering: All>(&self, v: T, _: Ordering) -> T {
> +        let v = T::into_repr(v);
> +        let a = self.as_ptr().cast::<T::Repr>();
> +
> +        // SAFETY:
> +        // - For calling the atomic_xchg*() function:
> +        //   - `self.as_ptr()` is a valid pointer, and per the safety requirement of `AllocAtomic`,

Typo: `AllowAtomic`.

> +        //      a `*mut T` is a valid `*mut T::Repr`. Therefore `a` is a valid pointer,
> +        //   - per the type invariants, the following atomic operation won't cause data races.
> +        // - For extra safety requirement of usage on pointers returned by `self.as_ptr():
> +        //   - atomic operations are used here.
> +        let ret = unsafe {
> +            match Ordering::TYPE {
> +                OrderingType::Full => T::Repr::atomic_xchg(a, v),
> +                OrderingType::Acquire => T::Repr::atomic_xchg_acquire(a, v),
> +                OrderingType::Release => T::Repr::atomic_xchg_release(a, v),
> +                OrderingType::Relaxed => T::Repr::atomic_xchg_relaxed(a, v),
> +            }
> +        };
> +
> +        T::from_repr(ret)
> +    }
> +
> +    /// Atomic compare and exchange.
> +    ///
> +    /// Compare: The comparison is done via the byte level comparison between the atomic variables
> +    /// with the `old` value.
> +    ///
> +    /// Ordering: When succeeds, provides the corresponding ordering as the `Ordering` type
> +    /// parameter indicates, and a failed one doesn't provide any ordering, the read part of a
> +    /// failed cmpxchg should be treated as a relaxed read.

Rust `core::ptr` functions have this sentence on success ordering for
compare_exchange:

  Using Acquire as success ordering makes the store part of this
  operation Relaxed, and using Release makes the successful load
  Relaxed.

Does this translate to LKMM cmpxchg operations? If so, I think we should
include this sentence. This also applies to `Atomic::xchg`.

Best regards,
Andreas Hindborg