[PATCH v6 6/9] rust: sync: atomic: Add the framework of arithmetic operations

[Boqun Feng <boqun.feng@gmail.com>]

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 `AllowAtomic` 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 add a subtrait of `AllowAtomic` describing
which types have and can do atomic arithemtic operations.

Trait `AllowAtomicArithmetic` has an associate type `Delta` instead of
using `AllowAllowAtomic::Repr` because, a `Bar(u32)` (whose `Repr` is
`i32`) may not wants an `add(&self, i32)`, but an `add(&self, u32)`.

Only add() and fetch_add() are added. The rest will be added in the
future.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
---
 rust/kernel/sync/atomic.rs         |  18 +++++
 rust/kernel/sync/atomic/generic.rs | 108 +++++++++++++++++++++++++++++
 2 files changed, 126 insertions(+)

diff --git a/rust/kernel/sync/atomic.rs b/rust/kernel/sync/atomic.rs
index c5193c1c90fe..26f66cccd4e0 100644
--- a/rust/kernel/sync/atomic.rs
+++ b/rust/kernel/sync/atomic.rs
@@ -29,8 +29,26 @@ unsafe impl generic::AllowAtomic for i32 {
     type Repr = i32;
 }
 
+// SAFETY: `i32` is always sound to transmute back to itself.
+unsafe impl generic::AllowAtomicArithmetic for i32 {
+    type Delta = i32;
+
+    fn delta_into_repr(d: Self::Delta) -> Self::Repr {
+        d
+    }
+}
+
 // SAFETY: `i64` has the same size and alignment with itself, and is round-trip transmutable to
 // itself.
 unsafe impl generic::AllowAtomic for i64 {
     type Repr = i64;
 }
+
+// SAFETY: `i64` is always sound to transmute back to itself.
+unsafe impl generic::AllowAtomicArithmetic for i64 {
+    type Delta = i64;
+
+    fn delta_into_repr(d: Self::Delta) -> Self::Repr {
+        d
+    }
+}
diff --git a/rust/kernel/sync/atomic/generic.rs b/rust/kernel/sync/atomic/generic.rs
index 1beb802843ee..412a2c811c3d 100644
--- a/rust/kernel/sync/atomic/generic.rs
+++ b/rust/kernel/sync/atomic/generic.rs
@@ -111,6 +111,20 @@ const fn into_repr<T: AllowAtomic>(v: T) -> T::Repr {
     unsafe { core::mem::transmute_copy(&r) }
 }
 
+/// Atomics that allows arithmetic operations with an integer type.
+///
+/// # Safety
+///
+/// Implementers must guarantee [`Self::Repr`] can always soundly [`transmute()`] to [`Self`] after
+/// arithmetic operations.
+pub unsafe trait AllowAtomicArithmetic: AllowAtomic {
+    /// The delta types for arithmetic operations.
+    type Delta;
+
+    /// Converts [`Self::Delta`] into the representation of the atomic type.
+    fn delta_into_repr(d: Self::Delta) -> Self::Repr;
+}
+
 impl<T: AllowAtomic> Atomic<T> {
     /// Creates a new atomic.
     pub const fn new(v: T) -> Self {
@@ -457,3 +471,97 @@ fn try_cmpxchg<Ordering: Any>(&self, old: &mut T, new: T, _: Ordering) -> bool {
         ret
     }
 }
+
+impl<T: AllowAtomicArithmetic> Atomic<T>
+where
+    T::Repr: AtomicHasArithmeticOps,
+{
+    /// Atomic add.
+    ///
+    /// The addition is a wrapping addition.
+    ///
+    /// # Examples
+    ///
+    /// ```rust
+    /// 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<Ordering: RelaxedOnly>(&self, v: T::Delta, _: Ordering) {
+        let v = T::delta_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_add() 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>`:
+        //   - `T: AllowAtomicArithmetic` guarantees the arithmetic operation result is sound to
+        //     stored in an `Atomic<T>`.
+        unsafe {
+            T::Repr::atomic_add(a, v);
+        }
+    }
+
+    /// Atomic fetch and add.
+    ///
+    /// The addition is a wrapping addition.
+    ///
+    /// # Examples
+    ///
+    /// ```rust
+    /// 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<Ordering: Any>(&self, v: T::Delta, _: Ordering) -> T {
+        let v = T::delta_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_fetch_add*() 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>`:
+        //   - `T: AllowAtomicArithmetic` guarantees the arithmetic operation result is sound to
+        //     stored in an `Atomic<T>`.
+        let ret = unsafe {
+            match Ordering::TYPE {
+                OrderingType::Full => T::Repr::atomic_fetch_add(a, v),
+                OrderingType::Acquire => T::Repr::atomic_fetch_add_acquire(a, v),
+                OrderingType::Release => T::Repr::atomic_fetch_add_release(a, v),
+                OrderingType::Relaxed => T::Repr::atomic_fetch_add_relaxed(a, v),
+            }
+        };
+
+        // SAFETY: Per safety requirement of `AllowAtomicArithmetic`, `ret` is a valid bit pattern
+        // of `T`.
+        unsafe { from_repr(ret) }
+    }
+}
-- 
2.39.5 (Apple Git-154)