linux-kernel - Re: [PATCH v2 3/5] locking/arch: Wire up local_try

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Date:   Wed, 12 Apr 2023 15:37:50 +0200
From:   Uros Bizjak <ubizjak@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-alpha@...r.kernel.org, loongarch@...ts.linux.dev,
        linux-mips@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        x86@...nel.org, linux-arch@...r.kernel.org,
        linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
        Richard Henderson <richard.henderson@...aro.org>,
        Ivan Kokshaysky <ink@...assic.park.msu.ru>,
        Matt Turner <mattst88@...il.com>,
        Huacai Chen <chenhuacai@...nel.org>,
        WANG Xuerui <kernel@...0n.name>,
        Jiaxun Yang <jiaxun.yang@...goat.com>,
        Jun Yi <yijun@...ngson.cn>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Michael Ellerman <mpe@...erman.id.au>,
        Nicholas Piggin <npiggin@...il.com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH v2 3/5] locking/arch: Wire up local_try_cmpxchg

On Wed, Apr 12, 2023 at 1:33 PM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Wed, Apr 05, 2023 at 04:17:08PM +0200, Uros Bizjak wrote:
> > diff --git a/arch/powerpc/include/asm/local.h b/arch/powerpc/include/asm/local.h
> > index bc4bd19b7fc2..45492fb5bf22 100644
> > --- a/arch/powerpc/include/asm/local.h
> > +++ b/arch/powerpc/include/asm/local.h
> > @@ -90,6 +90,17 @@ static __inline__ long local_cmpxchg(local_t *l, long o, long n)
> >       return t;
> >  }
> >
> > +static __inline__ bool local_try_cmpxchg(local_t *l, long *po, long n)
> > +{
> > +     long o = *po, r;
> > +
> > +     r = local_cmpxchg(l, o, n);
> > +     if (unlikely(r != o))
> > +             *po = r;
> > +
> > +     return likely(r == o);
> > +}
> > +
>
> Why is the ppc one different from the rest? Why can't it use the
> try_cmpxchg_local() fallback and needs to have it open-coded?

Please note that ppc directly defines local_cmpxchg that bypasses
cmpxchg_local/arch_cmpxchg_local machinery. The patch takes the same
approach for local_try_cmpxchg, because fallbacks are using
arch_cmpxchg_local definitions.

PPC should be converted to use arch_cmpxchg_local (to also enable
instrumentation), but this is not the scope of the proposed patchset.

Uros.