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

[Vineet Gupta]

On 9/22/26 11:59, Paul E. McKenney wrote:
> On Tue, Sep 22, 2026 at 07:37:54PM +0100, Bradley Morgan wrote:
>> On 22 September 2026 19:29:32 BST, "Paul E. McKenney" <paulmck at kernel.org>
>> wrote:
>>> 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 at kernel.org>
>>>
>>> But I could of course easily be missing subtle arch-specific bugs.
>> There are, according to sashiko, but I can't seem to make that thing happy
>> no matter what I do
> Hahahahaha!!!
>
> We forgot to CC the architecture maintainers.

No you didn't - they were CC'ed all along.
I'll give this is a test run and report back later.

Both the cover letter and 1/5 explain what is being done and not clearly 
why ?
Is there an existing or proposed user for this API.
Otherwise it will just bit-rot. No big deal from arch code pov but just 
saying...

Thx,
-Vineet

>
> For the changes to lib/cmpxchg-emu.c:
>
> 	Cc: Andrew Morton <akpm at linux-foundation.org>
> 	Cc: Arnd Bergmann <arnd at kernel.org>
>
> For ARC:
>
> 	Cc: Vineet Gupta <vgupta at kernel.org>
> 	linux-snps-arc at lists.infradead.org
>
> For sh:
>
> 	Cc: Yoshinori Sato <ysato at users.sourceforge.jp>
> 	Cc: Rich Felker <dalias at libc.org>
> 	Cc: John Paul Adrian Glaubitz <glaubitz at physik.fu-berlin.de>
> 	Cc: <linux-sh at vger.kernel.org>
>
> For csky:
>
> 	Cc: Guo Ren <guoren at kernel.org>
> 	Cc: <linux-csky at vger.kernel.org>
> 	Cc: <linux-kernel at vger.kernel.org>
>
> For xtensa:
>
> 	Cc: Chris Zankel <chris at zankel.net>
> 	Cc: Max Filippov <jcmvbkbc at gmail.com>
>
> They might be willing to help.  ;-)
>
> 							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
>> --- Thanks!
>> "I'm not a very positive person" - Linus torvalds