Commit d1320543 authored by Boqun Feng's avatar Boqun Feng Committed by Peter Zijlstra
Browse files

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: default avatarBoqun Feng <boqun.feng@gmail.com>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: default avatarAlice Ryhl <aliceryhl@google.com>
Reviewed-by: default avatarBenno Lossin <lossin@kernel.org>
Reviewed-by: default avatarElle Rhumsaa <elle@weathered-steel.dev>
Link: https://lore.kernel.org/all/20250719030827.61357-7-boqun.feng@gmail.com/
parent b606a532

rust/kernel/sync/atomic.rs

+92 −2
Original line number Diff line number Diff line
@@ -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 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 @@ fn try_cmpxchg<Ordering: ordering::Ordering>(&self, old: &mut T, new: T, _: Orde 
        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

+14 −0
Original line number Diff line number Diff line
@@ -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

    }

}