Re: [PATCH 1/2] ARM: cmpxchg: support 2-byte xchg() on pre-ARMv6 CPUs

[Linus Walleij]

On Sat, Oct 3, 2026 at 11:38 AM Karl Mehltretter <kmehltretter@xxxxxxxxx> wrote:

> Pre-ARMv6 CPUs have no halfword swp, so __arch_xchg() only handles 1-
> and 4-byte exchanges there and turns any other size into a link error
> via __bad_xchg().
>
> No C code needs a 16-bit xchg() on these CPUs, but the Rust atomic
> helpers in rust/helpers/atomic_ext.c provide xchg() for Atomic<i16>
> unconditionally, so building with CONFIG_RUST=y for ARMv5 fails:
>
>   ld.lld: error: undefined symbol: __bad_xchg
>   >>> referenced by helpers.c
>   >>>               rust/helpers/helpers.o:(rust_helper_atomic_i16_xchg)
>
> Implement the 2-byte case with interrupts disabled, as the pre-ARMv6
> atomic_t operations already do. This is safe because SMP requires
> ARMv6K or later.
>
> Assisted-by: LLM
> Signed-off-by: Karl Mehltretter <kmehltretter@xxxxxxxxx>

All right this looks fair.

Reviewed-by: Linus Walleij <linusw@xxxxxxxxxx>

Yours,
Linus Walleij