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

["Satyam Sharma" <satyam.sharma@gmail.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.