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

[Bradley Morgan <brads@mainlining.org>]

On 23 September 2026 18:00:33 BST, "Paul E. McKenney" <paulmck@kernel.org>
wrote:
>On Wed, Sep 23, 2026 at 11:43:13AM +0100, David Laight wrote:
>> On Fri, 11 Sep 2026 19:25:34 +0000
>> Bradley Morgan <brads@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@kernel.org>
>> > Signed-off-by: Bradley Morgan <brads@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?
>
>The churn is only a few lines of code, and the potential confusion from
>the name would be forever.  I agree that if we were stuck with the old
>name, the comment would help, but why not just rip the bandage off and
>be done with it?  ;-)

true.

>
>							Thanx, Paul

--- Thanks!
"I'm not a very positive person" - Linus torvalds