Re: [PATCH v1 1/4] rust: helpers: Add i8/i16 atomic try_cmpxchg helpers

[Boqun Feng <boqun.feng@gmail.com>]

On Sat, Dec 27, 2025 at 08:59:48PM +0900, FUJITA Tomonori wrote:
> Add i8/i16 atomic try_cmpxchg helpers that call raw_try_cmpxchg()
> macro implementing atomic try_cmpxchg using architecture-specific
> instructions.
> 
> x86_64 implements try_cmpxchg() with full ordering.
> 
> On other architectures, try_cmpxchg() isn't implemented; so calling
> try_cmpxchg() ends up using cmpxchg() implementation.
> 
> loongarch, arm64, and riscv implement cmpxchg with full ordering.
> 
> arm v7 only supports relaxed-ordering cmpxchg; __atomic_op_fence()
> macro is used to add barriers before and after the relaxed cmpxchg.
> 
> Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
> ---

One more thing, we should add this to comments in atomic_ext.c as well:

diff --git a/rust/helpers/atomic_ext.c b/rust/helpers/atomic_ext.c
index 10733bb4a75e..5ef81d2b47cf 100644
--- a/rust/helpers/atomic_ext.c
+++ b/rust/helpers/atomic_ext.c
@@ -91,6 +91,13 @@ __rust_helper s16 rust_helper_atomic_i16_xchg_relaxed(s16 *ptr, s16 new)
        return raw_xchg_relaxed(ptr, new);
 }

+/*
+ * try_cmpxchg helpers depend on ARCH_SUPPORTS_ATOMIC_RMW and on the
+ * architecture provding try_cmpxchg() support for i8 and i16.
+ *
+ * The architectures that currently support Rust (x86_64, armv7,
+ * arm64, riscv, and loongarch) satisfy these requirements.
+ */
 __rust_helper bool rust_helper_atomic_i8_try_cmpxchg(s8 *ptr, s8 *old, s8 new)
 {
        return raw_try_cmpxchg(ptr, old, new);

I can add it myself if it works for you.

Regards,
Boqun

>  rust/helpers/atomic_ext.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/rust/helpers/atomic_ext.c b/rust/helpers/atomic_ext.c
> index 089f31bc8de8..9d62f659c8d2 100644
> --- a/rust/helpers/atomic_ext.c
> +++ b/rust/helpers/atomic_ext.c
> @@ -90,3 +90,13 @@ __rust_helper s16 rust_helper_atomic_i16_xchg_relaxed(s16 *ptr, s16 new)
>  {
>  	return raw_xchg_relaxed(ptr, new);
>  }
> +
> +__rust_helper bool rust_helper_atomic_i8_try_cmpxchg(s8 *ptr, s8 *old, s8 new)
> +{
> +	return raw_try_cmpxchg(ptr, old, new);
> +}
> +
> +__rust_helper bool rust_helper_atomic_i16_try_cmpxchg(s16 *ptr, s16 *old, s16 new)
> +{
> +	return raw_try_cmpxchg(ptr, old, new);
> +}
> -- 
> 2.43.0
>