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.

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

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@kernel.org>

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

On 22 September 2026 19:29:32 BST, "Paul E. McKenney" <paulmck@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@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

>
>							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

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@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@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.

For the changes to lib/cmpxchg-emu.c:

	Cc: Andrew Morton <akpm@linux-foundation.org>
	Cc: Arnd Bergmann <arnd@kernel.org>

For ARC:

	Cc: Vineet Gupta <vgupta@kernel.org>
	linux-snps-arc@lists.infradead.org

For sh:

	Cc: Yoshinori Sato <ysato@users.sourceforge.jp>
	Cc: Rich Felker <dalias@libc.org>
	Cc: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
	Cc: <linux-sh@vger.kernel.org>

For csky:

	Cc: Guo Ren <guoren@kernel.org>
	Cc: <linux-csky@vger.kernel.org>
	Cc: <linux-kernel@vger.kernel.org>

For xtensa:

	Cc: Chris Zankel <chris@zankel.net>
	Cc: Max Filippov <jcmvbkbc@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

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@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@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@linux-foundation.org>
> 	Cc: Arnd Bergmann <arnd@kernel.org>
>
> For ARC:
>
> 	Cc: Vineet Gupta <vgupta@kernel.org>
> 	linux-snps-arc@lists.infradead.org
>
> For sh:
>
> 	Cc: Yoshinori Sato <ysato@users.sourceforge.jp>
> 	Cc: Rich Felker <dalias@libc.org>
> 	Cc: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
> 	Cc: <linux-sh@vger.kernel.org>
>
> For csky:
>
> 	Cc: Guo Ren <guoren@kernel.org>
> 	Cc: <linux-csky@vger.kernel.org>
> 	Cc: <linux-kernel@vger.kernel.org>
>
> For xtensa:
>
> 	Cc: Chris Zankel <chris@zankel.net>
> 	Cc: Max Filippov <jcmvbkbc@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

On Tue, Sep 22, 2026 at 12:13:42PM -0700, Vineet Gupta wrote:
> 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@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@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...

If I remember correctly, there are workarounds for lack of two-byte
cmpxchg() in various parts of the kernel, for example, _Q_PENDING_BITS.
So this might not be without uses for all that long.

							Thanx, Paul

> Thx,
> -Vineet
> 
> > 
> > For the changes to lib/cmpxchg-emu.c:
> > 
> > 	Cc: Andrew Morton <akpm@linux-foundation.org>
> > 	Cc: Arnd Bergmann <arnd@kernel.org>
> > 
> > For ARC:
> > 
> > 	Cc: Vineet Gupta <vgupta@kernel.org>
> > 	linux-snps-arc@lists.infradead.org
> > 
> > For sh:
> > 
> > 	Cc: Yoshinori Sato <ysato@users.sourceforge.jp>
> > 	Cc: Rich Felker <dalias@libc.org>
> > 	Cc: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
> > 	Cc: <linux-sh@vger.kernel.org>
> > 
> > For csky:
> > 
> > 	Cc: Guo Ren <guoren@kernel.org>
> > 	Cc: <linux-csky@vger.kernel.org>
> > 	Cc: <linux-kernel@vger.kernel.org>
> > 
> > For xtensa:
> > 
> > 	Cc: Chris Zankel <chris@zankel.net>
> > 	Cc: Max Filippov <jcmvbkbc@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
>

On 22 September 2026 19:59:39 BST, "Paul E. McKenney" <paulmck@kernel.org>
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@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@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.
>
>For the changes to lib/cmpxchg-emu.c:
>
>	Cc: Andrew Morton <akpm@linux-foundation.org>
>	Cc: Arnd Bergmann <arnd@kernel.org>
>
>For ARC:
>
>	Cc: Vineet Gupta <vgupta@kernel.org>
>	linux-snps-arc@lists.infradead.org
>
>For sh:
>
>	Cc: Yoshinori Sato <ysato@users.sourceforge.jp>
>	Cc: Rich Felker <dalias@libc.org>
>	Cc: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
>	Cc: <linux-sh@vger.kernel.org>
>
>For csky:
>
>	Cc: Guo Ren <guoren@kernel.org>
>	Cc: <linux-csky@vger.kernel.org>
>	Cc: <linux-kernel@vger.kernel.org>
>
>For xtensa:
>
>	Cc: Chris Zankel <chris@zankel.net>
>	Cc: Max Filippov <jcmvbkbc@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
If I have to do another 100 revisions I'll die, I'm suggesting getting the
new function in first, then converting arches later? Or does this break
arches.
--- Thanks!
https://lore.kernel.org/all/EE579805-42F2-4C58-B752-F28779EEB717@grrlz.net/

On Tue, Sep 22, 2026 at 08:03:51PM +0100, Bradley Morgan wrote:
> On 22 September 2026 19:59:39 BST, "Paul E. McKenney" <paulmck@kernel.org>
> 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@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@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.
> >
> >For the changes to lib/cmpxchg-emu.c:
> >
> >	Cc: Andrew Morton <akpm@linux-foundation.org>
> >	Cc: Arnd Bergmann <arnd@kernel.org>
> >
> >For ARC:
> >
> >	Cc: Vineet Gupta <vgupta@kernel.org>
> >	linux-snps-arc@lists.infradead.org
> >
> >For sh:
> >
> >	Cc: Yoshinori Sato <ysato@users.sourceforge.jp>
> >	Cc: Rich Felker <dalias@libc.org>
> >	Cc: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
> >	Cc: <linux-sh@vger.kernel.org>
> >
> >For csky:
> >
> >	Cc: Guo Ren <guoren@kernel.org>
> >	Cc: <linux-csky@vger.kernel.org>
> >	Cc: <linux-kernel@vger.kernel.org>
> >
> >For xtensa:
> >
> >	Cc: Chris Zankel <chris@zankel.net>
> >	Cc: Max Filippov <jcmvbkbc@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
> If I have to do another 100 revisions I'll die, I'm suggesting getting the
> new function in first, then converting arches later? Or does this break
> arches.

Well, you are only on v4.  ;-)

Adding a new function without uses is frowned upon.  After all, if it
wasn't frowned upon, there would be tons of unused code in the kernel.

But you are in luck.  John Paul Adrian Glaubitz noticed the sh patch
despite not being CCed and acked it.  And I happen to be the one who
committed the original lib/cmpxchg-emu.c commit.  Plus at least one of
the architecture-specific patches.  (Ah, yes, the problem was that RCU
had been using single-byte cmpxchg(), thus breaking the architectures
not supporting this, so I provided cmpxchg_emu_u8() as the fix.)

So what I have done is to queue the lib patch, but without the
arch/*/Kconfig pieces, and adding the CCs shown above.  I have also
queued the sh patch, with added CCs and with arch/sh/Kconfig.  The point
of moving the arch/*/Kconfig changes each to its respective patch is to
put all the arch-specific changes into the corresponding patch.

I will of course check with John Paul Adrian Glaubitz to make sure that
he is OK with relocating the arch/sh/Kconfig change to the patch that
he already acked, and I will of course adjust as needed if he isn't.

That means you only need to re-send the ARC, csky, and xtensa patches,
with CCs added as shown above.  No guarantees, but hopefully quite a
bit fewer than 100 times.  ;-)

							Thanx, Paul

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