From mboxrd@z Thu Jan  1 00:00:00 1970
From: Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH v2 3/5] locking/arch: Wire up local_try_cmpxchg
Date: Wed, 12 Apr 2023 15:40:42 +0200
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To: Uros Bizjak <ubizjak@gmail.com>
Cc: linux-alpha@vger.kernel.org, loongarch@lists.linux.dev, linux-mips@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, x86@kernel.org, linux-arch@vger.kernel.org, linux-perf-users@vger.kernel.org, linux-kernel@vger.kernel.org, Richard Henderson <richard.henderson@linaro.org>, Ivan Kokshaysky <ink@jurassic.park.msu.ru>, Matt Turner <mattst88@gmail.com>, Huacai Chen <chenhuacai@kernel.org>, WANG Xuerui <kernel@xen0n.name>, Jiaxun Yang <jiaxun.yang@flygoat.com>, Jun Yi <yijun@loongson.cn>, Thomas Bogendoerfer <tsbogend@alpha.franken.de>, Michael Ellerman <mpe@ellerman.id.au>, Nicholas Piggin <npiggin@gmail.com>, Christophe Leroy <christophe.leroy@csgroup.eu>, Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>

On Wed, Apr 12, 2023 at 03:37:50PM +0200, Uros Bizjak wrote:
> On Wed, Apr 12, 2023 at 1:33=E2=80=AFPM Peter Zijlstra <peterz@infradead.=
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, l=
ong o, long n)
> > >       return t;
> > >  }
> > >
> > > +static __inline__ bool local_try_cmpxchg(local_t *l, long *po, long =
n)
> > > +{
> > > +     long o =3D *po, r;
> > > +
> > > +     r =3D local_cmpxchg(l, o, n);
> > > +     if (unlikely(r !=3D o))
> > > +             *po =3D r;
> > > +
> > > +     return likely(r =3D=3D 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?
>=20
> 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.
>=20
> PPC should be converted to use arch_cmpxchg_local (to also enable
> instrumentation), but this is not the scope of the proposed patchset.

Ah indeed. Thanks!