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

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Message-ID: <466BAA87.8000400@qumranet.com>
Date:	Sun, 10 Jun 2007 10:38:47 +0300
From:	Avi Kivity <avi@...ranet.com>
To:	Satyam Sharma <satyam.sharma@...il.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

Satyam Sharma wrote:
> 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.

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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