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From: "H.J. Lu" <hjl.tools@gmail.com>
To: Noah Goldstein <goldstein.w.n@gmail.com>
Cc: Wilco Dijkstra <Wilco.Dijkstra@arm.com>,
	GNU C Library <libc-alpha@sourceware.org>
Subject: Re: [PATCH v1 2/4] nptl: Continue use arch prefered atomic exchange in spinlock loop
Date: Thu, 29 Sep 2022 11:51:39 -0700	[thread overview]
Message-ID: <CAMe9rOodmfDsHPit5+DbZLtd5V5CaqT0NknfVePdKiGBtzzXZA@mail.gmail.com> (raw)
In-Reply-To: <CAFUsyfL_WWS7coqjJzaY-D02LsowgTtSGBiK=c_m7HpPToRdiQ@mail.gmail.com>

On Thu, Sep 29, 2022 at 11:40 AM Noah Goldstein via Libc-alpha
<libc-alpha@sourceware.org> wrote:
>
> On Thu, Sep 29, 2022 at 9:35 AM Wilco Dijkstra <Wilco.Dijkstra@arm.com> wrote:
> >
> > Hi Noah,
> >
> > Did you try building it both ways? I don't think this could ever compile:
> >
> > +  if (__glibc_likely (pthread_spin_lock_grab_lock (lock, &val, 1)))
> >
> > and:
> >
> > +#  define pthread_spin_lock_grab_lock(mem, val, c) \
> > +    atomic_compare_exchange_weak_acquire (lock, &val, 1))
> >
> > The define uses 'lock' and 'mem' inconsistently and the use of the macro
> > expands into &&val...
> >
> > Apart from that there is the question whether we should keep the weird
> > ATOMIC_EXCHANGE_USES_CAS setting - I have removed it in my atomic
> > patch series since most targets appear confused as to what it means (so
> > are likely to have the wrong setting). Also there is no evidence it is actually
> > faster. So using exchange in both cases is easier (and less error prone!).
> >
> > Also you do realize that no matter how much you change this code, it
> > won't make a difference on x86, right?
>
> Why's that?

x86-64 uses sysdeps/x86_64/nptl/pthread_spin_lock.S

-- 
H.J.

  parent reply	other threads:[~2022-09-29 18:52 UTC|newest]

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-09-29 16:35 Wilco Dijkstra
2022-09-29 18:38 ` Noah Goldstein
2022-09-29 18:50   ` Wilco Dijkstra
2022-09-29 18:51   ` H.J. Lu [this message]
  -- strict thread matches above, loose matches on Subject: below --
2022-09-29  3:14 [PATCH v1 1/4] Benchtests: Add benchtests for pthread_spin_lock and mutex_trylock Noah Goldstein
2022-09-29  3:14 ` [PATCH v1 2/4] nptl: Continue use arch prefered atomic exchange in spinlock loop Noah Goldstein

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