Re: [PATCH v5 1/3] ARC: Emulate two-byte cmpxchg

[Bradley Morgan <brads@mainlining.org>]

On 5 October 2026 13:05:54 BST, Bradley Morgan <brads@mainlining.org>
wrote:
>ARC has no two-byte atomic compare and swap, so the
>arch_cmpxchg_relaxed() macro switch lets case 2 fall through to
>BUILD_BUG() via default, and a two-byte cmpxchg() fails the build.
>
>Route case 2 through the new cmpxchg_emu_u16(), which narrows the
>old and new values itself, so the dispatch keeps the (uintptr_t)
>casts the case 1 path already uses.
>
>The sizeof(*(_p_)) fix that v4 folded in here landed separately
>through Vineet's f050c3e61d2a, so this patch is now only the
>case 2 wiring.
>
>Signed-off-by: Bradley Morgan <brads@mainlining.org>
>---
> arch/arc/include/asm/cmpxchg.h | 3 +++
> 1 file changed, 3 insertions(+)
>
>diff --git a/arch/arc/include/asm/cmpxchg.h b/arch/arc/include/asm/cmpxchg.h
>index 66b3743c9400..b30b20933f32 100644
>--- a/arch/arc/include/asm/cmpxchg.h
>+++ b/arch/arc/include/asm/cmpxchg.h
>@@ -50,6 +50,9 @@
> 	case 1:								\
> 		_prev_ = (__typeof__(*(ptr)))cmpxchg_emu_u8((volatile u8 *__force)_p_, (uintptr_t)_o_, (uintptr_t)_n_);	\
> 		break;							\
>+	case 2:								\
>+		_prev_ = (__typeof__(*(ptr)))cmpxchg_emu_u16((volatile u16 *__force)_p_, (unsigned long)_o_, (unsigned long)_n_);	\
>+		break;							\
> 	case 4:								\
> 		_prev_ = __cmpxchg(_p_, _o_, _n_);			\
> 		break;							\
>
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