Re: [PATCH 05/14] rust: sync: atomic: Add atomic {cmp,}xchg operations

Elle Rhumsaa
2025-09-06T04:23:35Z

On Thu, Sep 04, 2025 at 09:41:32PM -0700, 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>
> Reviewed-by: Benno Lossin <lossin@kernel.org>
> Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
> Link: https://lore.kernel.org/all/20250719030827.61357-6-boqun.feng@gmail.com/
> ---
> rust/kernel/sync/atomic.rs | 168 ++++++++++++++++++++++++++++++++++++-
> 1 file changed, 167 insertions(+), 1 deletion(-)
>
> diff --git a/rust/kernel/sync/atomic.rs b/rust/kernel/sync/atomic.rs
> index ea5782b6ee95..4c32d12dc61e 100644
> --- a/rust/kernel/sync/atomic.rs
> +++ b/rust/kernel/sync/atomic.rs
> @@ -25,7 +25,7 @@
> pub use ordering::{Acquire, Full, Relaxed, Release};
>
> use crate::build_error;
> -use internal::{AtomicBasicOps, AtomicRepr};
> +use internal::{AtomicBasicOps, AtomicExchangeOps, AtomicRepr};
> use ordering::OrderingType;
>
> /// A memory location which can be safely modified from multiple execution contexts.
> @@ -293,3 +293,169 @@ pub fn store<Ordering: ordering::ReleaseOrRelaxed>(&self, v: T, _: Ordering) {
> }
> }
> }
> +
> +impl<T: AtomicType> Atomic<T>
> +where
> + T::Repr: AtomicExchangeOps,
> +{
> + /// Atomic exchange.
> + ///
> + /// Atomically updates `*self` to `v` and returns the old value of `*self`.
> + ///
> + /// # Examples
> + ///
> + /// ```
> + /// 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: ordering::Ordering>(&self, v: T, _: Ordering) -> T {
> + let v = into_repr(v);
> +
> + // INVARIANT: `self.0` is a valid `T` after `atomic_xchg*()` because `v` is transmutable to
> + // `T`.
> + let ret = {
> + match Ordering::TYPE {
> + OrderingType::Full => T::Repr::atomic_xchg(&self.0, v),
> + OrderingType::Acquire => T::Repr::atomic_xchg_acquire(&self.0, v),
> + OrderingType::Release => T::Repr::atomic_xchg_release(&self.0, v),
> + OrderingType::Relaxed => T::Repr::atomic_xchg_relaxed(&self.0, v),
> + }
> + };
> +
> + // SAFETY: `ret` comes from reading `*self`, which is a valid `T` per type invariants.
> + unsafe { from_repr(ret) }
> + }
> +
> + /// Atomic compare and exchange.
> + ///
> + /// If `*self` == `old`, atomically updates `*self` to `new`. Otherwise, `*self` is not
> + /// modified.
> + ///
> + /// Compare: The comparison is done via the byte level comparison between `*self` and `old`.
> + ///
> + /// Ordering: When succeeds, provides the corresponding ordering as the `Ordering` type
> + /// parameter indicates, and a failed one doesn't provide any ordering, the load part of a
> + /// failed cmpxchg is a [`Relaxed`] load.
> + ///
> + /// Returns `Ok(value)` if cmpxchg succeeds, and `value` is guaranteed to be equal to `old`,
> + /// otherwise returns `Err(value)`, and `value` is the current value of `*self`.
> + ///
> + /// # Examples
> + ///
> + /// ```
> + /// 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));
> + /// ```
> + ///
> + /// [`Relaxed`]: ordering::Relaxed
> + #[doc(alias(
> + "atomic_cmpxchg",
> + "atomic64_cmpxchg",
> + "atomic_try_cmpxchg",
> + "atomic64_try_cmpxchg",
> + "compare_exchange"
> + ))]
> + #[inline(always)]
> + pub fn cmpxchg<Ordering: ordering::Ordering>(
> + &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.
> + ///
> + /// If `*self` == `old`, atomically updates `*self` to `new`. Otherwise, `*self` is not
> + /// modified, `*old` is updated to the current value of `*self`.
> + ///
> + /// "Compare" and "Ordering" part are the same as [`Atomic::cmpxchg()`].
> + ///
> + /// Returns `true` means the cmpxchg succeeds otherwise returns `false`.
> + #[inline(always)]
> + fn try_cmpxchg<Ordering: ordering::Ordering>(&self, old: &mut T, new: T, _: Ordering) -> bool {
> + let mut tmp = into_repr(*old);
> + let new = into_repr(new);
> +
> + // INVARIANT: `self.0` is a valid `T` after `atomic_try_cmpxchg*()` because `new` is
> + // transmutable to `T`.
> + let ret = {
> + match Ordering::TYPE {
> + OrderingType::Full => T::Repr::atomic_try_cmpxchg(&self.0, &mut tmp, new),
> + OrderingType::Acquire => {
> + T::Repr::atomic_try_cmpxchg_acquire(&self.0, &mut tmp, new)
> + }
> + OrderingType::Release => {
> + T::Repr::atomic_try_cmpxchg_release(&self.0, &mut tmp, new)
> + }
> + OrderingType::Relaxed => {
> + T::Repr::atomic_try_cmpxchg_relaxed(&self.0, &mut tmp, new)
> + }
> + }
> + };
> +
> + // SAFETY: `tmp` comes from reading `*self`, which is a valid `T` per type invariants.
> + *old = unsafe { from_repr(tmp) };
> +
> + ret
> + }
> +}
> --
> 2.51.0
>
>

Reviewed-by: Elle Rhumsaa <elle@weathered-steel.dev>