[RFC v2 06/13] rust: sync: atomic: Add the framework of arithmetic operations

[Boqun Feng]

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.

A few things about this `AllowAtomicArithmetic` trait:

* It 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)`.

* `AtomicImpl` types already implement an `AtomicHasArithmeticOps`
  trait, so add blanket implementation for them. In the future, `i8` and
  `i16` may impl `AtomicImpl` but not `AtomicHasArithmeticOps` if
  arithemtic operations are not available.

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

Signed-off-by: Boqun Feng <boqun.feng at gmail.com>
---
 rust/kernel/sync/atomic/generic.rs | 102 +++++++++++++++++++++++++++++
 1 file changed, 102 insertions(+)

diff --git a/rust/kernel/sync/atomic/generic.rs b/rust/kernel/sync/atomic/generic.rs
index bfccc4336c75..a75c3e9f4c89 100644
--- a/rust/kernel/sync/atomic/generic.rs
+++ b/rust/kernel/sync/atomic/generic.rs
@@ -3,6 +3,7 @@
 //! Generic atomic primitives.
 
 use super::ops::*;
+use super::ordering;
 use super::ordering::*;
 use crate::types::Opaque;
 
@@ -54,6 +55,23 @@ fn from_repr(repr: Self::Repr) -> Self {
     }
 }
 
+/// Atomics that allows arithmetic operations with an integer type.
+pub 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: AtomicImpl + AtomicHasArithmeticOps> AllowAtomicArithmetic for T {
+    type Delta = Self;
+
+    fn delta_into_repr(d: Self::Delta) -> Self::Repr {
+        d
+    }
+}
+
 impl<T: AllowAtomic> Atomic<T> {
     /// Creates a new atomic.
     pub const fn new(v: T) -> Self {
@@ -402,3 +420,87 @@ pub fn compare_exchange<Ordering: All>(&self, mut old: T, new: T, o: Ordering) -
         }
     }
 }
+
+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);
+        let a = self.as_ptr().cast::<T::Repr>();
+
+        // SAFETY:
+        // - For calling the atomic_add() function:
+        //   - `self.as_ptr()` is a valid pointer, and per the safety requirement of `AllocAtomic`,
+        //      a `*mut T` is a valid `*mut T::Repr`. Therefore `a` is a valid pointer,
+        //   - per the type invariants, 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.
+        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: All>(&self, v: T::Delta, _: Ordering) -> T {
+        let v = T::delta_into_repr(v);
+        let a = self.as_ptr().cast::<T::Repr>();
+
+        // SAFETY:
+        // - For calling the atomic_fetch_add*() function:
+        //   - `self.as_ptr()` is a valid pointer, and per the safety requirement of `AllocAtomic`,
+        //      a `*mut T` is a valid `*mut T::Repr`. Therefore `a` is a valid pointer,
+        //   - per the type invariants, 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.
+        let ret = unsafe {
+            match Ordering::ORDER {
+                ordering::OrderingDesc::Full => T::Repr::atomic_fetch_add(a, v),
+                ordering::OrderingDesc::Acquire => T::Repr::atomic_fetch_add_acquire(a, v),
+                ordering::OrderingDesc::Release => T::Repr::atomic_fetch_add_release(a, v),
+                ordering::OrderingDesc::Relaxed => T::Repr::atomic_fetch_add_relaxed(a, v),
+            }
+        };
+
+        T::from_repr(ret)
+    }
+}
-- 
2.45.2