rust: sync: atomic: Add atomic {cmp,}xchg operations

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.

Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Benno Lossin <lossin@kernel.org>
Reviewed-by: Elle Rhumsaa <elle@weathered-steel.dev>
Link: https://lore.kernel.org/all/20250719030827.61357-6-boqun.feng@gmail.com/

Author: fbq

rust/kernel/sync/atomic.rs

@@ -25,7 +25,7 @@ pub use internal::AtomicImpl;
 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 @@ where
         }
     }
 }
+
+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
+    }
+}