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 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