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

[Boqun Feng]

On Sat, Jun 21, 2025 at 12:37:53PM +0100, Gary Guo wrote:
[...]
> > +    /// 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.
> > +    ///
> > +    /// 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.
> > +    ///
> > +    /// # 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"
> > +    ))]
> > +    #[inline(always)]
> > +    pub fn cmpxchg<Ordering: All>(&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.
> > +        if self.try_cmpxchg(&mut old, new, o) {
> > +            Ok(old)
> > +        } else {
> > +            Err(old)
> > +        }
> > +    }
> > +
> > +    /// Atomic compare and exchange and returns whether the operation succeeds.
> > +    ///
> > +    /// "Compare" and "Ordering" part are the same as [`Atomic::cmpxchg()`].
> > +    ///
> > +    /// Returns `true` means the cmpxchg succeeds otherwise returns `false` with `old` updated to
> > +    /// the value of the atomic variable when cmpxchg was happening.
> > +    #[inline(always)]
> > +    fn try_cmpxchg<Ordering: All>(&self, old: &mut T, new: T, _: Ordering) -> bool {
> > +        let old = (old as *mut T).cast::<T::Repr>();
> > +        let new = T::into_repr(new);
> > +        let a = self.0.get().cast::<T::Repr>();
> > +
> > +        // SAFETY:
> > +        // - For calling the atomic_try_cmpchg*() function:
> > +        //   - `self.as_ptr()` is a valid pointer, and per the safety requirement of `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.
> > +        //   - `old` is a valid pointer to write because it comes from a mutable reference.
> > +        // - For extra safety requirement of usage on pointers returned by `self.as_ptr():
> > +        //   - atomic operations are used here.
> > +        unsafe {
> > +            match Ordering::TYPE {
> > +                OrderingType::Full => T::Repr::atomic_try_cmpxchg(a, old, new),
> > +                OrderingType::Acquire => T::Repr::atomic_try_cmpxchg_acquire(a, old, new),
> > +                OrderingType::Release => T::Repr::atomic_try_cmpxchg_release(a, old, new),
> > +                OrderingType::Relaxed => T::Repr::atomic_try_cmpxchg_relaxed(a, old, new),
> > +            }
> > +        }
> 
> Again this function is only using `T::into_repr`, bypassing
> `T::from_repr` and just use pointer casting.
> 
> BTW, any reason that this is a separate function, and it couldn't just
> be in `cmpxchg` function?
> 

It's a non-public function, I feel it's easier to see that Rust's
cmpxchg() is implemented via a try_cmpxchg() that is a wrapper of
`atomic_try_cmpxchg*()`.

Regards,
Boqun

> 
> > +    }
> > +}
>