linux-kernel - Re: [patch] Let smp_call_function

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Date:	Mon, 14 May 2007 12:18:18 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Heiko Carstens <heiko.carstens@...ibm.com>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	randy.dunlap@...cle.com, paulus@...ba.org,
	benh@...nel.crashing.org, David Miller <davem@...emloft.net>,
	Andi Kleen <ak@...e.de>
Subject: Re: [patch] Let smp_call_function_single return -EBUSY.

On Mon, 14 May 2007 12:11:37 -0700
Andrew Morton <akpm@...ux-foundation.org> wrote:

> > This of course raises another question: it is not clear in which context
> > the smp_call_function* functions are supposed to be called. Should it be
> > with preemption disabled or is preemption enabled allowed as well?
> > If calling with preemption enabled is allowed then the powerpc implementation
> > is broken, since smp_processor_id() as well as num_online_cpus() may change
> > while they are accessed.
> 
> These are all excellent questions.  And important ones.

erk, I see your point.  If a caller is calling this with preemption enabled
then the current thread might at any time migrate onto the target CPU,
causing the smp_call_function_single() attempt to fail.  So the effects of
that call are basically a random crapshoot.

Often but not always, any code which is hanging onto a variable called
"cpu" while preemption enabled is buggy.

So yes, I'd say that from a sanity-of-implementation POV and for general
defensiveness, we should require that the called of
smp_call_function_single() has disabled preemption.

What a crock.
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