Re: [RFC][PATCH 05/12] arch: Introduce arch_{,try_}_cmpxchg128{,_local}()

Peter Zijlstra
2022-12-22T13:17:34Z

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!).