rust: sync: atomic: Add the framework of arithmetic operations

One important set of atomic operations is the arithmetic operations,
i.e. add(), sub(), fetch_add(), add_return(), etc. However it may not
make senses for all the types that `AtomicType` to have arithmetic
operations, for example a `Foo(u32)` may not have a reasonable add() or
sub(), plus subword types (`u8` and `u16`) currently don't have
atomic arithmetic operations even on C side and might not have them in
the future in Rust (because they are usually suboptimal on a few
architecures). Therefore the plan is to add a few subtraits of
`AtomicType` describing which types have and can do atomic arithemtic
operations.

One trait `AtomicAdd` is added, and only add() and fetch_add() are
added. The rest will be added in the future.

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/20250719030827.61357-7-boqun.feng@gmail.com/

Author: fbq

rust/kernel/sync/atomic.rs

@@ -16,7 +16,6 @@
 //!
 //! [`LKMM`]: srctree/tools/memory-model/
 
-#[allow(dead_code, unreachable_pub)]
 mod internal;
 pub mod ordering;
 mod predefine;
@@ -25,7 +24,7 @@ pub use internal::AtomicImpl;
 pub use ordering::{Acquire, Full, Relaxed, Release};
 
 use crate::build_error;
-use internal::{AtomicBasicOps, AtomicExchangeOps, AtomicRepr};
+use internal::{AtomicArithmeticOps, AtomicBasicOps, AtomicExchangeOps, AtomicRepr};
 use ordering::OrderingType;
 
 /// A memory location which can be safely modified from multiple execution contexts.
@@ -112,6 +111,19 @@ pub unsafe trait AtomicType: Sized + Send + Copy {
     type Repr: AtomicImpl;
 }
 
+/// Types that support atomic add operations.
+///
+/// # Safety
+///
+// TODO: Properly defines `wrapping_add` in the following comment.
+/// `wrapping_add` any value of type `Self::Repr::Delta` obtained by [`Self::rhs_into_delta()`] to
+/// any value of type `Self::Repr` obtained through transmuting a value of type `Self` to must
+/// yield a value with a bit pattern also valid for `Self`.
+pub unsafe trait AtomicAdd<Rhs = Self>: AtomicType {
+    /// Converts `Rhs` into the `Delta` type of the atomic implementation.
+    fn rhs_into_delta(rhs: Rhs) -> <Self::Repr as AtomicImpl>::Delta;
+}
+
 #[inline(always)]
 const fn into_repr<T: AtomicType>(v: T) -> T::Repr {
     // SAFETY: Per the safety requirement of `AtomicType`, `T` is round-trip transmutable to
@@ -459,3 +471,81 @@ where
         ret
     }
 }
+
+impl<T: AtomicType> Atomic<T>
+where
+    T::Repr: AtomicArithmeticOps,
+{
+    /// Atomic add.
+    ///
+    /// Atomically updates `*self` to `(*self).wrapping_add(v)`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use kernel::sync::atomic::{Atomic, Relaxed};
+    ///
+    /// let x = Atomic::new(42);
+    ///
+    /// assert_eq!(42, x.load(Relaxed));
+    ///
+    /// x.add(12, Relaxed);
+    ///
+    /// assert_eq!(54, x.load(Relaxed));
+    /// ```
+    #[inline(always)]
+    pub fn add<Rhs>(&self, v: Rhs, _: ordering::Relaxed)
+    where
+        T: AtomicAdd<Rhs>,
+    {
+        let v = T::rhs_into_delta(v);
+
+        // INVARIANT: `self.0` is a valid `T` after `atomic_add()` due to safety requirement of
+        // `AtomicAdd`.
+        T::Repr::atomic_add(&self.0, v);
+    }
+
+    /// Atomic fetch and add.
+    ///
+    /// Atomically updates `*self` to `(*self).wrapping_add(v)`, and returns the value of `*self`
+    /// before the update.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use kernel::sync::atomic::{Atomic, Acquire, Full, Relaxed};
+    ///
+    /// let x = Atomic::new(42);
+    ///
+    /// assert_eq!(42, x.load(Relaxed));
+    ///
+    /// assert_eq!(54, { x.fetch_add(12, Acquire); x.load(Relaxed) });
+    ///
+    /// let x = Atomic::new(42);
+    ///
+    /// assert_eq!(42, x.load(Relaxed));
+    ///
+    /// assert_eq!(54, { x.fetch_add(12, Full); x.load(Relaxed) } );
+    /// ```
+    #[inline(always)]
+    pub fn fetch_add<Rhs, Ordering: ordering::Ordering>(&self, v: Rhs, _: Ordering) -> T
+    where
+        T: AtomicAdd<Rhs>,
+    {
+        let v = T::rhs_into_delta(v);
+
+        // INVARIANT: `self.0` is a valid `T` after `atomic_fetch_add*()` due to safety requirement
+        // of `AtomicAdd`.
+        let ret = {
+            match Ordering::TYPE {
+                OrderingType::Full => T::Repr::atomic_fetch_add(&self.0, v),
+                OrderingType::Acquire => T::Repr::atomic_fetch_add_acquire(&self.0, v),
+                OrderingType::Release => T::Repr::atomic_fetch_add_release(&self.0, v),
+                OrderingType::Relaxed => T::Repr::atomic_fetch_add_relaxed(&self.0, v),
+            }
+        };
+
+        // SAFETY: `ret` comes from reading `self.0`, which is a valid `T` per type invariants.
+        unsafe { from_repr(ret) }
+    }
+}

rust/kernel/sync/atomic/predefine.rs

@@ -8,8 +8,22 @@ unsafe impl super::AtomicType for i32 {
     type Repr = i32;
 }
 
+// SAFETY: The wrapping add result of two `i32`s is a valid `i32`.
+unsafe impl super::AtomicAdd<i32> for i32 {
+    fn rhs_into_delta(rhs: i32) -> i32 {
+        rhs
+    }
+}
+
 // SAFETY: `i64` has the same size and alignment with itself, and is round-trip transmutable to
 // itself.
 unsafe impl super::AtomicType for i64 {
     type Repr = i64;
 }
+
+// SAFETY: The wrapping add result of two `i64`s is a valid `i64`.
+unsafe impl super::AtomicAdd<i64> for i64 {
+    fn rhs_into_delta(rhs: i64) -> i64 {
+        rhs
+    }
+}