Re: [PATCH v4 0/5] Add two-byte cmpxchg emulation and wire it into the architectures

[David Laight <david.laight.linux@gmail.com>]

On Tue, 22 Sep 2026 17:33:49 +0000
Bradley Morgan <brads@mainlining.org> wrote:

> This is v4 of the two byte cmpxchg emulation series, wiring
> cmpxchg_emu_u16() into arc, csky, sh and xtensa.

Apart from a minor niggle, for the series.
Reviewed-by: David Laight <david.laight.linux@gmail.com>

> 
> v3 had changed cmpxchg_emu_u8()'s success return to (u16)old, which was
> a 16-bit mask in the 8-bit function, and a dead one at that, since the
> compare guarantees the low 8 bits of old are the byte being returned.
> David Laight asked where that cast came from. v4 returns old unmasked,
> the exact behaviour the one-byte emulator always had, so nothing that
> uses cmpxchg_emu_u8() through the widened prototypes sees a change.
> 
> David also noted v3 extended the (unsigned long)(0 ? *ptr : (old)) type
> check to csky and sh but not arc and xtensa. v4 adds it there too, so a
> cmpxchg(&p, 4, 5) fails to compile on every architecture in the series,
> verified with each architecture's macro instantiated standalone.
> 
> While adding the type check to arc, the switch subject turned out to be
> sizeof((_p_)), the pointer, not sizeof(*(_p_)), the pointee. On 32-bit
> arc the switch was always 4, so the size 1 and size 2 cases were dead
> code and every sub-word cmpxchg() went through the 32-bit llock/scond
> pair, comparing whole words against sub-word values, so the compare
> almost never succeeded. The switch now tests the pointee, and the u8
> path it was always meant to dispatch actually runs, so the one-byte
> emulation works on arc for the first time since the sizeof bug landed
> with the original cmpxchg_emu_u8() wiring.
> 
> The host test of 972 cases across both halfword offsets against a byte
> level reference model still passes, and a 20000 case randomized run
> checking the masked compare and return against a hardware cmpxchg r16
> model passes with zero mismatches.
> 
> David pointed out on v1 that a u16 prototype 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. v4 keeps taking the old and new values as unsigned long and
> casting 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 the caller asked for, which matches
> hardware cmpxchg r16 behaviour.
> 
> The ARMv6 wiring stays dropped from v1, per Arnd Bergmann's offer to
> take the INTEGRATOR_CM1136JFS cleanup in his platform removal series.
> 
> Bradley Morgan (5):
>   lib: Add two-byte cmpxchg emulation function
>   ARC: Emulate two-byte cmpxchg
>   sh: Emulate two-byte cmpxchg
>   csky: Emulate two-byte cmpxchg
>   xtensa: Emulate two-byte cmpxchg