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

[Bradley Morgan]

On 22 September 2026 19:59:39 BST, "Paul E. McKenney" <paulmck@xxxxxxxxxx>
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@xxxxxxxxxx>
>> 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@xxxxxxxxxx>
>> >
>> >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@xxxxxxxxxxxxxxxxxxxx>
>	Cc: Arnd Bergmann <arnd@xxxxxxxxxx>
>
>For ARC:
>
>	Cc: Vineet Gupta <vgupta@xxxxxxxxxx>
>	linux-snps-arc@xxxxxxxxxxxxxxxxxxx
>
>For sh:
>
>	Cc: Yoshinori Sato <ysato@xxxxxxxxxxxxxxxxxxxx>
>	Cc: Rich Felker <dalias@xxxxxxxx>
>	Cc: John Paul Adrian Glaubitz <glaubitz@xxxxxxxxxxxxxxxxxxx>
>	Cc: <linux-sh@xxxxxxxxxxxxxxx>
>
>For csky:
>
>	Cc: Guo Ren <guoren@xxxxxxxxxx>
>	Cc: <linux-csky@xxxxxxxxxxxxxxx>
>	Cc: <linux-kernel@xxxxxxxxxxxxxxx>
>
>For xtensa:
>
>	Cc: Chris Zankel <chris@xxxxxxxxxx>
>	Cc: Max Filippov <jcmvbkbc@xxxxxxxxx>
>
>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@xxxxxxxxx/