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Date:   Thu, 22 Dec 2022 14:16:28 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Boqun Feng <boqun.feng@gmail.com>
Cc:     torvalds@linux-foundation.org, corbet@lwn.net, will@kernel.org,
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Subject: Re: [RFC][PATCH 05/12] arch: Introduce
 arch_{,try_}_cmpxchg128{,_local}()
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 <20221219154119.154045458@infradead.org>
 <Y6OyAL2epKPHj+tr@boqun-archlinux>
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On Wed, Dec 21, 2022 at 05:25:20PM -0800, Boqun Feng wrote:

> > +#define __CMPXCHG128(name, mb, cl...)					\
> > +static __always_inline u128						\
> > +__lse__cmpxchg128##name(volatile u128 *ptr, u128 old, u128 new)		\
> > +{									\
> > +	union __u128_halves r, o = { .full = (old) },			\
> > +			       n = { .full = (new) };			\
> > +	register unsigned long x0 asm ("x0") = o.low;			\
> > +	register unsigned long x1 asm ("x1") = o.high;			\
> > +	register unsigned long x2 asm ("x2") = n.low;			\
> > +	register unsigned long x3 asm ("x3") = n.high;			\
> > +	register unsigned long x4 asm ("x4") = (unsigned long)ptr;	\
> > +									\
> > +	asm volatile(							\
> > +	__LSE_PREAMBLE							\
> > +	"	casp" #mb "\t%[old1], %[old2], %[new1], %[new2], %[v]\n"\
> > +	: [old1] "+&r" (x0), [old2] "+&r" (x1),				\
> > +	  [v] "+Q" (*(unsigned long *)ptr)				\
> > +	: [new1] "r" (x2), [new2] "r" (x3), [ptr] "r" (x4),		\
> 
> Issue #1: the line below can be removed, otherwise..
> 
> > +	  [oldval1] "r" (r.low), [oldval2] "r" (r.high)			\
> 
> warning:
> 
> 	./arch/arm64/include/asm/atomic_lse.h: In function '__lse__cmpxchg128_mb':
> 	./arch/arm64/include/asm/atomic_lse.h:309:27: warning: 'r.<U97b8>.low' is used uninitialized [-Wuninitialized]
> 	  309 |           [oldval1] "r" (r.low), [oldval2] "r" (r.high)
> 
> 
> > +	: cl);								\
> > +									\
> > +	r.low = x0; r.high = x1;					\
> > +									\
> > +	return r.full;							\
> > +}
> > +
> > +__CMPXCHG128(   ,   )
> > +__CMPXCHG128(_mb, al, "memory")
> > +
> > +#undef __CMPXCHG128
> > +
> >  #endif	/* __ASM_ATOMIC_LSE_H */
> > --- a/arch/arm64/include/asm/cmpxchg.h
> > +++ b/arch/arm64/include/asm/cmpxchg.h
> > @@ -147,6 +147,19 @@ __CMPXCHG_DBL(_mb)
> >  
> >  #undef __CMPXCHG_DBL
> >  
> > +#define __CMPXCHG128(name)						\
> > +static inline long __cmpxchg128##name(volatile u128 *ptr,		\
> 
> Issue #2: this should be
> 
> static inline u128 __cmpxchg128##name(..)
> 
> because cmpxchg* needs to return the old value.

Duh.. fixed both. Pushed out to queue/core/wip-u128. I'll probably
continue all this in two weeks (yay xmas break!).