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

Boqun Feng
2025-06-23T05:23:22Z

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

>
> > + }
> > +}
>