Re: [patch] i386/x86_64: smp_call_function locking inconsistency

From: Avi Kivity
Date: Sun Jun 10 2007 - 03:38:58 EST

Satyam Sharma wrote:
On 6/7/07, Avi Kivity <avi@xxxxxxxxxxxx> 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.

We don't catch incorrect cpu values in smp (because we can't); there's no reason to do so in UP IMO.

error compiling committee.c: too many arguments to function

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