Re: [PATCH v6 5/9] rust: sync: atomic: Add atomic {cmp,}xchg operations

["Benno Lossin" <lossin@kernel.org>]

On Thu Jul 10, 2025 at 8:00 AM CEST, Boqun Feng wrote:
> 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.
>
> Reviewed-by: Alice Ryhl <aliceryhl@google.com>
> Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
> ---
>  rust/kernel/sync/atomic/generic.rs | 170 +++++++++++++++++++++++++++++
>  1 file changed, 170 insertions(+)
>
> diff --git a/rust/kernel/sync/atomic/generic.rs b/rust/kernel/sync/atomic/generic.rs
> index e044fe21b128..1beb802843ee 100644
> --- a/rust/kernel/sync/atomic/generic.rs
> +++ b/rust/kernel/sync/atomic/generic.rs
> @@ -287,3 +287,173 @@ pub fn store<Ordering: ReleaseOrRelaxed>(&self, v: T, _: Ordering) {
>          };
>      }
>  }
> +
> +impl<T: AllowAtomic> Atomic<T>
> +where
> +    T::Repr: AtomicHasXchgOps,
> +{
> +    /// Atomic exchange.

Please also give a longer sentence describing the operation.

> +    ///
> +    /// # 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", "swap"))]
> +    #[inline(always)]
> +    pub fn xchg<Ordering: Any>(&self, v: T, _: Ordering) -> T {
> +        let v = into_repr(v);
> +        // CAST: Per the safety requirement of `AllowAtomic`, a valid pointer of `T` is also a
> +        // valid pointer of `T::Repr`.

Ditto as the last patch (I'm not going to mention the others).

> +        let a = self.as_ptr().cast::<T::Repr>();
> +
> +        // SAFETY:
> +        // - For calling the atomic_xchg*() function:
> +        //   - `a` is a valid pointer for the function per the CAST justification above.
> +        //   - Per the type guarantees, 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.
> +        // - For the bit validity of `Atomic<T>`:
> +        //   - `v` is a valid bit pattern of `T`, so it's sound to store it in an `Atomic<T>`.
> +        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),
> +            }
> +        };
> +
> +        // SAFETY: The atomic variable holds a valid `T`, so `ret` is a valid bit pattern of `T`,
> +        // therefore it's safe to call `from_repr()`.
> +        unsafe { from_repr(ret) }
> +    }
> +
> +    /// Atomic compare and exchange.

Also longer description for this function.

> +    ///
> +    /// 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.

Why did you chose to say "should be treated" can't you say it is a
relaxed read? What would the difference be between those two be?

> +    ///
> +    /// Returns `Ok(value)` if cmpxchg succeeds, and `value` is guaranteed to be equal to `old`,
> +    /// otherwise returns `Err(value)`, and `value` is the value of the atomic variable when
> +    /// cmpxchg was happening.

s/cmpxchg was happening/`cmpxchg` was executed/

> +    ///
> +    /// # Examples
> +    ///
> +    /// ```rust
> +    /// use kernel::sync::atomic::{Atomic, Full, Relaxed};
> +    ///
> +    /// let x = Atomic::new(42);
> +    ///
> +    /// // Checks whether cmpxchg succeeded.
> +    /// let success = x.cmpxchg(52, 64, Relaxed).is_ok();
> +    /// # assert!(!success);
> +    ///
> +    /// // Checks whether cmpxchg failed.
> +    /// let failure = x.cmpxchg(52, 64, Relaxed).is_err();
> +    /// # assert!(failure);
> +    ///
> +    /// // Uses the old value if failed, probably re-try cmpxchg.
> +    /// match x.cmpxchg(52, 64, Relaxed) {
> +    ///     Ok(_) => { },
> +    ///     Err(old) => {
> +    ///         // do something with `old`.
> +    ///         # assert_eq!(old, 42);
> +    ///     }
> +    /// }
> +    ///
> +    /// // Uses the latest value regardlessly, same as atomic_cmpxchg() in C.
> +    /// let latest = x.cmpxchg(42, 64, Full).unwrap_or_else(|old| old);
> +    /// # assert_eq!(42, latest);
> +    /// assert_eq!(64, x.load(Relaxed));
> +    /// ```
> +    #[doc(alias(
> +        "atomic_cmpxchg",
> +        "atomic64_cmpxchg",
> +        "atomic_try_cmpxchg",
> +        "atomic64_try_cmpxchg",
> +        "compare_exchange"
> +    ))]
> +    #[inline(always)]
> +    pub fn cmpxchg<Ordering: Any>(&self, mut old: T, new: T, o: Ordering) -> Result<T, T> {
> +        // Note on code generation:
> +        //
> +        // try_cmpxchg() is used to implement cmpxchg(), and if the helper functions are inlined,
> +        // the compiler is able to figure out that branch is not needed if the users don't care
> +        // about whether the operation succeeds or not. One exception is on x86, due to commit
> +        // 44fe84459faf ("locking/atomic: Fix atomic_try_cmpxchg() semantics"), the
> +        // atomic_try_cmpxchg() on x86 has a branch even if the caller doesn't care about the
> +        // success of cmpxchg and only wants to use the old value. For example, for code like:
> +        //
> +        //     let latest = x.cmpxchg(42, 64, Full).unwrap_or_else(|old| old);
> +        //
> +        // It will still generate code:
> +        //
> +        //     movl    $0x40, %ecx
> +        //     movl    $0x34, %eax
> +        //     lock
> +        //     cmpxchgl        %ecx, 0x4(%rsp)
> +        //     jne     1f
> +        //     2:
> +        //     ...
> +        //     1:  movl    %eax, %ecx
> +        //     jmp 2b
> +        //
> +        // This might be "fixed" by introducing a try_cmpxchg_exclusive() that knows the "*old"
> +        // location in the C function is always safe to write.

Oh wow the mentioned commit was an interesting read...

---
Cheers,
Benno

> +        if self.try_cmpxchg(&mut old, new, o) {
> +            Ok(old)
> +        } else {
> +            Err(old)
> +        }
> +    }