Re: [PATCH 3/4] s390/percpu: Drop unused output register from arch_this_cpu_to_op()

[Heiko Carstens <hca@linux.ibm.com>]

On Mon, Oct 12, 2026 at 11:54:14AM +0000, sashiko-bot@kernel.org wrote:
> Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
> - [High] s390/percpu: unpredictable LAN/LAO behavior from R1==R3
> --
> 
> commit 657f51d5e5b9873bdbdaca586024d213577edd7b
> Author: Heiko Carstens <hca@linux.ibm.com>
> 
> s390/percpu: Drop unused output register from arch_this_cpu_to_op()
> 
> The lan/lang/lao/laog instructions return the old memory content, but
> arch_this_cpu_to_op() does not need it. This commit changes the val__
> constraint to use it as an input/output register instead of a dedicated
> old__ register, aiming to reduce register pressure.
> 
> > diff --git a/arch/s390/include/asm/percpu.h b/arch/s390/include/asm/percpu.h
> > index fd0a3a9951ce7..7d87e0a262c37 100644
> > --- a/arch/s390/include/asm/percpu.h
> > +++ b/arch/s390/include/asm/percpu.h
> > @@ -257,18 +257,18 @@ do {									\
> >  	typedef typeof(pcp) pcp_op_T__; 				\
> >  	union register_pair rp__;					\
> >  	pcp_op_T__ val__ = (val);					\
> > -	pcp_op_T__ old__, *ptr__;					\
> > +	pcp_op_T__ *ptr__;						\
> >  									\
> >  	ptr__ = PERCPU_PTR(&(pcp));					\
> >  	asm_inline volatile(						\
> >  		__PCPU_BEGIN("%[lcreg]","%[lcoff]","%[ptr__]",		\
> >  			     "%[pair__]","%N[pair__]")			\
> > -		op "    %[old__],%[val__],0(%N[pair__])\n"		\
> > +		op "    %[val__],%[val__],0(%N[pair__])\n"		\
> 
> [Severity: High]
> Does this inline assembly string force the compiler to emit the atomic
> instruction using the identical register for both the r1 and r3 operands?
> 
> According to the z/Architecture Principles of Operation, if r1 and r3
> designate the same register in laa, lan, and lao instructions, the results
> are unpredictable.

Not true. According to the PoP this behaviour is well defined.
See "Programming Notes" on page 7-306 in SA22-7832-14.