Re: [PATCH v2 0/4] rust: Add i8/i16 atomic xchg helpers

[Gary Guo <gary@garyguo.net>]

On Tue, 23 Dec 2025 15:21:36 +0900
FUJITA Tomonori <fujita.tomonori@gmail.com> wrote:

> This adds atomic xchg helpers with full, acquire, release, and relaxed
> orderings in preparation for i8/i16 atomic xchg support.


Any reason this series is not sent together with the user (i.e.
Rust support) that uses it?

> 
> The architectures supporting Rust, implement atomic xchg families
> using architecture-specific instructions. They work for i8/i16 too so
> the helpers just call them.
> 
> Tested on QEMU (86_64, arm64, riscv, loongarch, and armv7).
> 
> Note the architectures that support Rust handle xchg differently:
> 
> - arm64 and riscv support xchg with all the orderings.
> 
> - x86_64 and loongarch support only full-ordering xchg. They calls the
>   full-ordering xchg for any orderings.

Maybe it's just that I'm reading this differently, but I think this is a
bit confusing, as if there's an optimisation opportunity.

x86 is TSO, so even a relaxed xchg is a full xchg. So in this sense x86
has implemented all orderings.

Looking at loongarch ISA manual it's suggested that apart from load/store,
all other atomic memory instructions are also always full ordering.

The change themselves LGTM, so

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

Best,
Gary

> 
> - arm v7 supports only relaxed-odering xchg. It uses __atomic_op_
>  macros to add barriers properly.


> 
> v2:
> - Add comment about ARCH_SUPPORTS_ATOMIC_RMW dependency
> - Add Alice's Reviewed-by
> v1: https://lore.kernel.org/rust-for-linux/20251217213742.639812-1-fujita.tomonori@gmail.com/
> 
> FUJITA Tomonori (4):
>   rust: helpers: Add i8/i16 atomic xchg helpers
>   rust: helpers: Add i8/i16 atomic xchg_acquire helpers
>   rust: helpers: Add i8/i16 atomic xchg_release helpers
>   rust: helpers: Add i8/i16 atomic xchg_relaxed helpers
> 
>  rust/helpers/atomic_ext.c | 48 +++++++++++++++++++++++++++++++++++++++
>  1 file changed, 48 insertions(+)
> 
> 
> base-commit: cae418334e4c402e410d3ee234935da33c7f904b