Re: [PATCH 08/14] rust: sync: atomic: Add Atomic<{usize,isize}>

[Elle Rhumsaa <elle@weathered-steel.dev>]

On Thu, Sep 04, 2025 at 09:41:35PM -0700, Boqun Feng wrote:
> Add generic atomic support for `usize` and `isize`. Note that instead of
> mapping directly to `atomic_long_t`, the represention type
> (`AtomicType::Repr`) is selected based on CONFIG_64BIT. This reduces
> the necessity of creating `atomic_long_*` helpers, which could save
> the binary size of kernel if inline helpers are not available. To do so,
> an internal type `isize_atomic_repr` is defined, it's `i32` in 32bit
> kernel and `i64` in 64bit kernel.
> 
> Reviewed-by: Alice Ryhl <aliceryhl@google.com>
> Reviewed-by: Andreas Hindborg <a.hindborg@kernel.org>
> Reviewed-by: Benno Lossin <lossin@kernel.org>
> Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
> Link: https://lore.kernel.org/all/20250719030827.61357-9-boqun.feng@gmail.com/
> ---
>  rust/kernel/sync/atomic/predefine.rs | 53 +++++++++++++++++++++++++---
>  1 file changed, 49 insertions(+), 4 deletions(-)
> 
> diff --git a/rust/kernel/sync/atomic/predefine.rs b/rust/kernel/sync/atomic/predefine.rs
> index d0875812f6ad..45a17985cda4 100644
> --- a/rust/kernel/sync/atomic/predefine.rs
> +++ b/rust/kernel/sync/atomic/predefine.rs
> @@ -2,6 +2,9 @@
>  
>  //! Pre-defined atomic types
>  
> +use crate::static_assert;
> +use core::mem::{align_of, size_of};
> +
>  // SAFETY: `i32` has the same size and alignment with itself, and is round-trip transmutable to
>  // itself.
>  unsafe impl super::AtomicType for i32 {
> @@ -28,6 +31,35 @@ fn rhs_into_delta(rhs: i64) -> i64 {
>      }
>  }
>  
> +// Defines an internal type that always maps to the integer type which has the same size alignment
> +// as `isize` and `usize`, and `isize` and `usize` are always bi-directional transmutable to
> +// `isize_atomic_repr`, which also always implements `AtomicImpl`.
> +#[allow(non_camel_case_types)]
> +#[cfg(not(CONFIG_64BIT))]
> +type isize_atomic_repr = i32;
> +#[allow(non_camel_case_types)]
> +#[cfg(CONFIG_64BIT)]
> +type isize_atomic_repr = i64;
> +
> +// Ensure size and alignment requirements are checked.
> +static_assert!(size_of::<isize>() == size_of::<isize_atomic_repr>());
> +static_assert!(align_of::<isize>() == align_of::<isize_atomic_repr>());
> +static_assert!(size_of::<usize>() == size_of::<isize_atomic_repr>());
> +static_assert!(align_of::<usize>() == align_of::<isize_atomic_repr>());
> +
> +// SAFETY: `isize` has the same size and alignment with `isize_atomic_repr`, and is round-trip
> +// transmutable to `isize_atomic_repr`.
> +unsafe impl super::AtomicType for isize {
> +    type Repr = isize_atomic_repr;
> +}
> +
> +// SAFETY: The wrapping add result of two `isize_atomic_repr`s is a valid `usize`.
> +unsafe impl super::AtomicAdd<isize> for isize {
> +    fn rhs_into_delta(rhs: isize) -> isize_atomic_repr {
> +        rhs as isize_atomic_repr
> +    }
> +}
> +
>  // SAFETY: `u32` and `i32` has the same size and alignment, and `u32` is round-trip transmutable to
>  // `i32`.
>  unsafe impl super::AtomicType for u32 {
> @@ -54,6 +86,19 @@ fn rhs_into_delta(rhs: u64) -> i64 {
>      }
>  }
>  
> +// SAFETY: `usize` has the same size and alignment with `isize_atomic_repr`, and is round-trip
> +// transmutable to `isize_atomic_repr`.
> +unsafe impl super::AtomicType for usize {
> +    type Repr = isize_atomic_repr;
> +}
> +
> +// SAFETY: The wrapping add result of two `isize_atomic_repr`s is a valid `usize`.
> +unsafe impl super::AtomicAdd<usize> for usize {
> +    fn rhs_into_delta(rhs: usize) -> isize_atomic_repr {
> +        rhs as isize_atomic_repr
> +    }
> +}
> +
>  use crate::macros::kunit_tests;
>  
>  #[kunit_tests(rust_atomics)]
> @@ -73,7 +118,7 @@ macro_rules! for_each_type {
>  
>      #[test]
>      fn atomic_basic_tests() {
> -        for_each_type!(42 in [i32, i64, u32, u64] |v| {
> +        for_each_type!(42 in [i32, i64, u32, u64, isize, usize] |v| {
>              let x = Atomic::new(v);
>  
>              assert_eq!(v, x.load(Relaxed));
> @@ -82,7 +127,7 @@ fn atomic_basic_tests() {
>  
>      #[test]
>      fn atomic_xchg_tests() {
> -        for_each_type!(42 in [i32, i64, u32, u64] |v| {
> +        for_each_type!(42 in [i32, i64, u32, u64, isize, usize] |v| {
>              let x = Atomic::new(v);
>  
>              let old = v;
> @@ -95,7 +140,7 @@ fn atomic_xchg_tests() {
>  
>      #[test]
>      fn atomic_cmpxchg_tests() {
> -        for_each_type!(42 in [i32, i64, u32, u64] |v| {
> +        for_each_type!(42 in [i32, i64, u32, u64, isize, usize] |v| {
>              let x = Atomic::new(v);
>  
>              let old = v;
> @@ -110,7 +155,7 @@ fn atomic_cmpxchg_tests() {
>  
>      #[test]
>      fn atomic_arithmetic_tests() {
> -        for_each_type!(42 in [i32, i64, u32, u64] |v| {
> +        for_each_type!(42 in [i32, i64, u32, u64, isize, usize] |v| {
>              let x = Atomic::new(v);
>  
>              assert_eq!(v, x.fetch_add(12, Full));
> -- 
> 2.51.0
> 
> 

Reviewed-by: Elle Rhumsaa <elle@weathered-steel.dev>