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

Boqun Feng
2025-07-10T06:01:06Z

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.
+ ///
+ /// # 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`.
+ 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.
+ ///
+ /// 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",
+ "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.
+ 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: Any>(&self, old: &mut T, new: T, _: Ordering) -> bool {
+ let mut old_tmp = into_repr(*old);
+ let oldp = &raw mut old_tmp;
+ let new = into_repr(new);
+ // CAST: Per the safety requirement of `AllowAtomic`, a valid pointer of `T` is also a
+ // valid pointer of `T::Repr`.
+ let a = self.0.get().cast::<T::Repr>();
+
+ // SAFETY:
+ // - For calling the atomic_try_cmpxchg*() function:
+ // - `a` is a valid pointer for the function per the CAST justification above.
+ // - `oldp` is a valid pointer for the function.
+ // - 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>`:
+ // - `new` 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_try_cmpxchg(a, oldp, new),
+ OrderingType::Acquire => T::Repr::atomic_try_cmpxchg_acquire(a, oldp, new),
+ OrderingType::Release => T::Repr::atomic_try_cmpxchg_release(a, oldp, new),
+ OrderingType::Relaxed => T::Repr::atomic_try_cmpxchg_relaxed(a, oldp, new),
+ }
+ };
+
+ // SAFETY: The atomic variable holds a valid `T`, so `old_tmp` is a valid bit pattern of
+ // `T`, therefore it's safe to call `from_repr()`.
+ *old = unsafe { from_repr(old_tmp) };
+
+ ret
+ }
+}
--
2.39.5 (Apple Git-154)