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    SubjectRe: [patch] i386/x86_64: smp_call_function locking inconsistency
    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.
    -
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