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

[Paul E. McKenney]

On Fri, Sep 11, 2026 at 11:10:17PM +0100, David Laight wrote:
> 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')

I suspect that Bradley is just following my cmpxchg_emu_u8() example,
which also returns uintptr_t.

I remember that *something* broke when I made this be u8, but I cannot
recall what the problem was.

Bradley, could you please try making it be u16 as David suggests just to
see what happens?  Who knows?  Maybe it was a compiler issue that has
since been fixed.  Or maybe the macros and asms using cmpxchg_emu_u8()
need that uintptr_t for some reason.

							Thanx, Paul