[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/arch/Kconfig b/arch/Kconfig
> index 3bb2e568f5b1..d56064797e09 100644
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -1817,7 +1817,7 @@ config CC_HAS_SANE_FUNCTION_ALIGNMENT
>  	# strict alignment always, even with -falign-functions.
>  	def_bool CC_HAS_MIN_FUNCTION_ALIGNMENT || CC_IS_CLANG
>  
> -config ARCH_NEED_CMPXCHG_1_EMU
> +config ARCH_NEED_CMPXCHG_1_2_EMU

Is it actually worth the churn of changing the name?
Maybe just a comment that say it adds both 1 and 2 byte emulation?

David

>  	bool
>  
...