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

Boqun Feng
2025-06-28T03:03:58Z

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

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
>
>