linux-kernel - Re: [patch] i386/x86_64: smp_call

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Message-ID: <a781481a0706071033y4bd86301v51bb0ebeb0e6f8b9@mail.gmail.com>
Date:	Thu, 7 Jun 2007 23:03:08 +0530
From:	"Satyam Sharma" <satyam.sharma@...il.com>
To:	"Avi Kivity" <avi@...ranet.com>
Cc:	"Heiko Carstens" <heiko.carstens@...ibm.com>,
	"Jan Glauber" <jan.glauber@...ibm.com>,
	"David Miller" <davem@...emloft.net>, akpm@...l.org, mingo@...e.hu,
	ak@...e.de, schwidefsky@...ibm.com, linux-kernel@...r.kernel.org,
	"Alan Cox" <alan@...hat.com>
Subject: Re: [patch] i386/x86_64: smp_call_function locking inconsistency

On 6/7/07, Avi Kivity <avi@...ranet.com> wrote:
> Satyam Sharma wrote:
> >
> > Oh wait, the on_one_cpu() patch proposes on UP:
> >
> > +static inline int on_one_cpu(int cpu, void (*func)(void *info), void
> > *info,
> > +                 int retry, int wait)
> > +{
> >
> > /* this needs a if (cpu == 0) check here, IMO */
> >
> > +    local_irq_disable();
> > +    func(info);
> > +    local_irq_enable();
> > +    return 0;
> >
> > /* else WARN and return -EINVAL; */
> >
> > +}
> >
> > which is broken without the suggested additions, IMHO
> > (this is what got me into this in the first place). There
> > _is_ a difference between on_each_cpu() and the
> > smp_call_function* semantics (as discussed on the other
> > thread -- gargh! my mistake for opening this discussion up
> > on so many threads), and in its current form on_one_cpu()
> > has quite confused semantics, trying to mix the two. I guess
> > on_one_cpu() would be better off simply being just an
> > atomic wrapper over smp_processor_id() and
> > smp_call_function_single() (which is the *real* issue that
> > needs solving in the first place), and do it well.
> >
>
> This is on UP, so (cpu == 0) is trivially true.

Yes, the caller code might derive the value for the cpu arg in
such a manner to always only ever yield 0 on UP. OTOH,
WARN_ON(!...)'s are often added for such assumptions that are
understood to be trivially true. Note that a warning for cpu != 0
would be perfectly justified, we'd clearly want to flag such
(errant) users.

Anyway, I guess another problem being tackled here is avoding
#ifdef CONFIG_SMP's to mask calls to smp_call_function* (and
thus on_cpu()) in kernel code(?) Avoiding putting WARN_ON() in
the UP cases could be useful to minimize noise, in that case. It
(and smp_call_function*) could still return -EINVAL for the invalid
cases, though.
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