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From: "Satyam Sharma" <satyam.sharma@gmail.com>
To: "Avi Kivity" <avi@qumranet.com>
Cc: "Heiko Carstens" <heiko.carstens@de.ibm.com>,
	"Jan Glauber" <jan.glauber@de.ibm.com>,
	"David Miller" <davem@davemloft.net>,
	akpm@osdl.org, mingo@elte.hu, ak@suse.de, schwidefsky@de.ibm.com,
	linux-kernel@vger.kernel.org, "Alan Cox" <alan@redhat.com>
Subject: Re: [patch] i386/x86_64: smp_call_function locking inconsistency
Date: Thu, 7 Jun 2007 23:03:08 +0530	[thread overview]
Message-ID: <a781481a0706071033y4bd86301v51bb0ebeb0e6f8b9@mail.gmail.com> (raw)
In-Reply-To: <46683EC4.4030604@qumranet.com>

On 6/7/07, Avi Kivity <avi@qumranet.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.

  reply	other threads:[~2007-06-07 17:33 UTC|newest]

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2007-02-08 20:32 Heiko Carstens
2007-02-08 20:43 ` David Miller
2007-02-09  8:42   ` Heiko Carstens
2007-02-09 12:57     ` Jan Glauber
2007-06-07 14:07       ` Satyam Sharma
2007-06-07 16:27         ` Heiko Carstens
2007-06-07 16:54           ` Satyam Sharma
2007-06-07 17:18             ` Satyam Sharma
2007-06-07 17:22               ` Avi Kivity
2007-06-07 17:33                 ` Satyam Sharma [this message]
2007-06-10  7:38                   ` Avi Kivity
2007-06-08 19:43             ` Andi Kleen
2007-06-08 19:42         ` Andi Kleen
2007-02-09  7:40 ` Andi Kleen

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