This is v3 of the two byte cmpxchg emulation series, wiring
cmpxchg_emu_u16() into arc, csky, sh and xtensa.

v2 tried u16 as the parameter type. David Laight pointed out that
this does not compile warning free when exchanging a pointer type,
because the switch statements in the architecture macros instantiate
every size case, so a pointer cmpxchg() type checks the two byte
case, and the (u16) casts there warn. v3 takes the old and new
values as unsigned long and casts to u16 inside the function, so the
call sites need no narrowing casts and pointer exchanges compile
clean. The function still compares and returns exactly the 16 bits
of the emulated halfword, which matches hardware cmpxchg r16
behaviour, and a host test of 972 cases across both halfword offsets
against a byte level reference model still passes.

David also noted the missing pointer to integer type check in some
of the macros, so cmpxchg(&p, 4, 5) compiled silently. The csky
macros typed __old and __new through __typeof__(old), which skips
the check, and sh had none either. Both now type check through
(unsigned long)(0 ? *(ptr) : (old)), the idiom David suggested,
which keeps the pointer to integer conversion explicit while making
the compiler reject mismatched types. Both of his test cases now
fail to compile on every macro shape.

The ARMv6 wiring stays dropped from v1, per Arnd Bergmann's offer to
take the INTEGRATOR_CM1136JFS cleanup in his platform removal series.

Thank you, Paul McKenney for suggesting this, and David Laight and
Arnd Bergmann for the review that shaped all three revisions.

Bradley Morgan (5):
  lib: Add two-byte cmpxchg emulation function
  ARC: Emulate two-byte cmpxchg
  csky: Emulate two-byte cmpxchg
  sh: Emulate two-byte cmpxchg
  xtensa: Emulate two-byte cmpxchg
-- 
2.47.3

On Thu, 17 Sep 2026 16:38:25 +0000
Bradley Morgan <brads@mainlining.org> wrote:

> This is v3 of the two byte cmpxchg emulation series, wiring
> cmpxchg_emu_u16() into arc, csky, sh and xtensa.
> 
> v2 tried u16 as the parameter type. David Laight pointed out that
> this does not compile warning free when exchanging a pointer type,
> because the switch statements in the architecture macros instantiate
> every size case, so a pointer cmpxchg() type checks the two byte
> case, and the (u16) casts there warn. v3 takes the old and new
> values as unsigned long and casts to u16 inside the function, so the
> call sites need no narrowing casts and pointer exchanges compile
> clean. The function still compares and returns exactly the 16 bits
> of the emulated halfword, which matches hardware cmpxchg r16
> behaviour, and a host test of 972 cases across both halfword offsets
> against a byte level reference model still passes.
> 
> David also noted the missing pointer to integer type check in some
> of the macros, so cmpxchg(&p, 4, 5) compiled silently. The csky
> macros typed __old and __new through __typeof__(old), which skips
> the check, and sh had none either. Both now type check through
> (unsigned long)(0 ? *(ptr) : (old)), the idiom David suggested,
> which keeps the pointer to integer conversion explicit while making
> the compiler reject mismatched types. Both of his test cases now
> fail to compile on every macro shape.

You fixed csky but not the others.
I guess that could be a separate patch (maybe series).

David

> 
> The ARMv6 wiring stays dropped from v1, per Arnd Bergmann's offer to
> take the INTEGRATOR_CM1136JFS cleanup in his platform removal series.
> 
> Thank you, Paul McKenney for suggesting this, and David Laight and
> Arnd Bergmann for the review that shaped all three revisions.
> 
> Bradley Morgan (5):
>   lib: Add two-byte cmpxchg emulation function
>   ARC: Emulate two-byte cmpxchg
>   csky: Emulate two-byte cmpxchg
>   sh: Emulate two-byte cmpxchg
>   xtensa: Emulate two-byte cmpxchg