Re: [PATCH v2 1/2] rust: sync: atomic: Clarify the need of CONFIG_ARCH_SUPPORTS_ATOMIC_RMW

["Gary Guo" <gary@garyguo.net>]

On Tue Jan 20, 2026 at 2:05 PM GMT, Boqun Feng wrote:
> Currently, since all the architectures that support Rust all have
> CONFIG_ARCH_SUPPORTS_ATOMIC_RMW selected, the helpers of atomic
> load/store on i8 and i16 relies on CONFIG_ARCH_SUPPORTS_ATOMIC_RMW=y.
> It's generally fine since most of architectures support that.
>
> The plan for CONFIG_ARCH_SUPPORTS_ATOMIC_RMW=n architectures is adding
> their (probably lock-based) atomic load/store for i8 and i16 as their
> atomic_{read,set}() and atomic64_{read,set}() counterpart when they
> plans to support Rust.
>
> Hence use a statis_assert!() to check this and remind the future us the
> need of the helpers. This is more clear than the #[cfg] on impl blocks
> of i8 and i16.
>
> Suggested-by: Dirk Behme <dirk.behme@gmail.com>
> Suggested-by: Benno Lossin <lossin@kernel.org>
> Signed-off-by: Boqun Feng <boqun.feng@gmail.com>

Reviewed-by: Gary Guo <gary@garyguo.net>

> ---
>  rust/kernel/sync/atomic/internal.rs | 19 +++++++++++++------
>  1 file changed, 13 insertions(+), 6 deletions(-)
>
> diff --git a/rust/kernel/sync/atomic/internal.rs b/rust/kernel/sync/atomic/internal.rs
> index 0dac58bca2b3..ef516bcb02ee 100644
> --- a/rust/kernel/sync/atomic/internal.rs
> +++ b/rust/kernel/sync/atomic/internal.rs
> @@ -37,16 +37,23 @@ pub trait AtomicImpl: Sized + Send + Copy + private::Sealed {
>      type Delta;
>  }
>  
> -// The current helpers of load/store uses `{WRITE,READ}_ONCE()` hence the atomicity is only
> -// guaranteed against read-modify-write operations if the architecture supports native atomic RmW.
> -#[cfg(CONFIG_ARCH_SUPPORTS_ATOMIC_RMW)]
> +// The current helpers of load/store of atomic `i8` and `i16` use `{WRITE,READ}_ONCE()` hence the
> +// atomicity is only guaranteed against read-modify-write operations if the architecture supports
> +// native atomic RmW.
> +//
> +// In the future when a CONFIG_ARCH_SUPPORTS_ATOMIC_RMW=n architecture plans to support Rust, the
> +// load/store helpers that guarantee atomicity against RmW operations (usually via a lock) need to
> +// be added.
> +crate::static_assert!(
> +    cfg!(CONFIG_ARCH_SUPPORTS_ATOMIC_RMW),
> +    "The current implementation of atomic i8/i16/ptr relies on the architecure being \
> +    ARCH_SUPPORTS_ATOMIC_RMW"

The printed string when assertion fails will have 5 spaces between "being" and
"ARCH", although it probably doesn't matter..

Best,
Gary

> +);
> +
>  impl AtomicImpl for i8 {
>      type Delta = Self;
>  }
>  
> -// The current helpers of load/store uses `{WRITE,READ}_ONCE()` hence the atomicity is only
> -// guaranteed against read-modify-write operations if the architecture supports native atomic RmW.
> -#[cfg(CONFIG_ARCH_SUPPORTS_ATOMIC_RMW)]
>  impl AtomicImpl for i16 {
>      type Delta = Self;
>  }