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

[Paul E. McKenney]

On Tue, Sep 22, 2026 at 05:33:49PM +0000, Bradley Morgan wrote:
> This is v4 of the two byte cmpxchg emulation series, wiring
> cmpxchg_emu_u16() into arc, csky, sh and xtensa.
> 
> 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.

I have pulled these in, but only to expose them to things like the kernel
test robot.  My guess is that they will go in by some other path.

And to that end:

Reviewed-by: Paul E. McKenney <paulmck@xxxxxxxxxx>

But I could of course easily be missing subtle arch-specific bugs.

							Thanx, Paul

> 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