[PATCH v7 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 the plan is to add a few subtraits of
`AllowAtomic` describing which types have and can do atomic arithemtic
operations.

One trait `AllowAtomicAdd` is added, and 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         |  14 ++++
 rust/kernel/sync/atomic/generic.rs | 111 ++++++++++++++++++++++++++++-
 2 files changed, 124 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/sync/atomic.rs b/rust/kernel/sync/atomic.rs
index c5193c1c90fe..54f5b4618337 100644
--- a/rust/kernel/sync/atomic.rs
+++ b/rust/kernel/sync/atomic.rs
@@ -29,8 +29,22 @@ unsafe impl generic::AllowAtomic for i32 {
     type Repr = i32;
 }
 
+// SAFETY: The wrapping add result of two `i32`s is a valid `i32`.
+unsafe impl generic::AllowAtomicAdd<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 generic::AllowAtomic for i64 {
     type Repr = i64;
 }
+
+// SAFETY: The wrapping add result of two `i64`s is a valid `i64`.
+unsafe impl generic::AllowAtomicAdd<i64> for i64 {
+    fn rhs_into_delta(rhs: i64) -> i64 {
+        rhs
+    }
+}
diff --git a/rust/kernel/sync/atomic/generic.rs b/rust/kernel/sync/atomic/generic.rs
index 4e45d594d8ef..9e2394017202 100644
--- a/rust/kernel/sync/atomic/generic.rs
+++ b/rust/kernel/sync/atomic/generic.rs
@@ -2,7 +2,7 @@
 
 //! Generic atomic primitives.
 
-use super::ops::{AtomicHasBasicOps, AtomicHasXchgOps, AtomicImpl};
+use super::ops::{AtomicHasArithmeticOps, AtomicHasBasicOps, AtomicHasXchgOps, AtomicImpl};
 use super::{ordering, ordering::OrderingType};
 use crate::build_error;
 use core::cell::UnsafeCell;
@@ -104,6 +104,18 @@ const fn into_repr<T: AllowAtomic>(v: T) -> T::Repr {
     unsafe { core::mem::transmute_copy(&r) }
 }
 
+/// Types that support atomic add operations.
+///
+/// # Safety
+///
+/// Wrapping adding 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 AllowAtomicAdd<Rhs = Self>: AllowAtomic {
+    /// Converts `Rhs` into the `Delta` type of the atomic implementation.
+    fn rhs_into_delta(rhs: Rhs) -> <Self::Repr as AtomicImpl>::Delta;
+}
+
 impl<T: AllowAtomic> Atomic<T> {
     /// Creates a new atomic `T`.
     pub const fn new(v: T) -> Self {
@@ -462,3 +474,100 @@ fn try_cmpxchg<Ordering: ordering::Any>(&self, old: &mut T, new: T, _: Ordering)
         ret
     }
 }
+
+impl<T: AllowAtomic> Atomic<T>
+where
+    T::Repr: AtomicHasArithmeticOps,
+{
+    /// 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, Ordering: ordering::RelaxedOnly>(&self, v: Rhs, _: Ordering)
+    where
+        T: AllowAtomicAdd<Rhs>,
+    {
+        let v = T::rhs_into_delta(v);
+        // CAST: Per the safety requirement of `AllowAtomic`, a valid pointer of `T` is a valid
+        // pointer of `T::Repr` for reads and valid for writes of values transmutable to `T`.
+        let a = self.as_ptr().cast::<T::Repr>();
+
+        // `*self` remains valid after `atomic_add()` because of the safety requirement of
+        // `AllowAtomicAdd`.
+        //
+        // SAFETY:
+        // - `a` is aligned to `align_of::<T::Repr>()` because of the safety requirement of
+        //   `AllowAtomic` and the guarantee of `Atomic::as_ptr()`.
+        // - `a` is a valid pointer per the CAST justification above.
+        unsafe {
+            T::Repr::atomic_add(a, 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::Any>(&self, v: Rhs, _: Ordering) -> T
+    where
+        T: AllowAtomicAdd<Rhs>,
+    {
+        let v = T::rhs_into_delta(v);
+        // CAST: Per the safety requirement of `AllowAtomic`, a valid pointer of `T` is a valid
+        // pointer of `T::Repr` for reads and valid for writes of values transmutable to `T`.
+        let a = self.as_ptr().cast::<T::Repr>();
+
+        // `*self` remains valid after `atomic_fetch_add*()` because of the safety requirement of
+        // `AllowAtomicAdd`.
+        //
+        // SAFETY:
+        // - `a` is aligned to `align_of::<T::Repr>()` because of the safety requirement of
+        //   `AllowAtomic` and the guarantee of `Atomic::as_ptr()`.
+        // - `a` is a valid pointer per the CAST justification above.
+        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: `ret` comes from reading `a` which was derived from `self.as_ptr()` which points
+        // at a valid `T`.
+        unsafe { from_repr(ret) }
+    }
+}
-- 
2.39.5 (Apple Git-154)