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

[Boqun Feng]

On Thu, Jun 26, 2025 at 03:12:12PM +0200, Andreas Hindborg wrote:
> "Boqun Feng" <boqun.feng@xxxxxxxxx> 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@xxxxxxxxx>
> > ---
> >  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`.
> 

Fixed.

> > +        //      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`.
> 

I see this as a different style of documenting, so in my next version,
I have the following:

//! - [`Acquire`] provides ordering between the load part of the annotated operation and all the
//!   following memory accesses.
//! - [`Release`] provides ordering between all the preceding memory accesses and the store part of
//!   the annotated operation.

in atomic/ordering.rs, I think I can extend it to:

//! - [`Acquire`] provides ordering between the load part of the annotated operation and all the
//!   following memory accesses, and if there is a store part, it has Relaxed ordering.
//! - [`Release`] provides ordering between all the preceding memory accesses and the store part of
//!   the annotated operation, and if there is load part, it has Relaxed ordering

This aligns with what we usually describe things in tool/memory-model/.

Regards,
Boqun

> 
> Best regards,
> Andreas Hindborg
> 
>