[PATCH 1/6] lib: Add two-byte cmpxchg emulation function

[David Laight]

On Fri, 11 Sep 2026 19:25:34 +0000
Bradley Morgan <brads at mainlining.org> wrote:

> cmpxchg_emu_u8() emulates one-byte cmpxchg() in terms of four-byte
> cmpxchg() for the architectures lacking native one-byte atomics.
> The same architectures also lack native two-byte cmpxchg(), where
> such an operation is not supported and either fails to compile via
> BUILD_BUG() or fails to link, because the bad pointer sentinels
> these architectures declare are never defined.
> 
> Add cmpxchg_emu_u16(), the two-byte sibling. It reads the enclosing
> word with READ_ONCE(), splices the two target bytes through a union
> and loops on cmpxchg() of the full word until the compare succeeds.
> Like cmpxchg_emu_u8() it is fully ordered.
> 
> The Kconfig symbol gating this file is renamed from
> ARCH_NEED_CMPXCHG_1_EMU to ARCH_NEED_CMPXCHG_1_2_EMU, as it now
> selects both the one-byte and the two-byte emulation.
> 
> Suggested-by: Paul E. McKenney <paulmck at kernel.org>
> Signed-off-by: Bradley Morgan <brads at mainlining.org>
> ---
>  arch/Kconfig                |  2 +-
>  arch/arc/Kconfig            |  2 +-
>  arch/arm/Kconfig            |  2 +-
>  arch/csky/Kconfig           |  2 +-
>  arch/sh/Kconfig             |  2 +-
>  arch/xtensa/Kconfig         |  2 +-
>  include/linux/cmpxchg-emu.h |  1 +
>  lib/Makefile                |  2 +-
>  lib/cmpxchg-emu.c           | 35 ++++++++++++++++++++++++++++++++---
>  9 files changed, 40 insertions(+), 10 deletions(-)
> 
...
> diff --git a/include/linux/cmpxchg-emu.h b/include/linux/cmpxchg-emu.h
> index 998deec67740..fee8171fa05e 100644
> --- a/include/linux/cmpxchg-emu.h
> +++ b/include/linux/cmpxchg-emu.h
> @@ -11,5 +11,6 @@
>  #define __LINUX_CMPXCHG_EMU_H
>  
>  uintptr_t cmpxchg_emu_u8(volatile u8 *p, uintptr_t old, uintptr_t new);
> +uintptr_t cmpxchg_emu_u16(volatile u16 *p, uintptr_t old, uintptr_t new);

Why uintptr_t? Shouldn't it just be u16?
(Which probably means the code would better if it was just 'unsigned int')

David