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Subject: Re: [PATCH][2.5] fix preempt-issues with smp_call_function()
From: Alan Cox <alan@lxorguk.ukuu.org.uk>
To: Andrew Morton <akpm@digeo.com>
Cc: Dave Jones <davej@codemonkey.org.uk>, schlicht@uni-mannheim.de,
       Linus Torvalds <torvalds@transmeta.com>, hugh@veritas.com,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Wed, 2003-02-26 at 10:28, Andrew Morton wrote:
> Dave Jones <davej@codemonkey.org.uk> wrote:
> >
> > btw, (unrelated) shouldn't smp_call_function be doing magick checks
> > with cpu_online() ?
> 
> Looks OK?  It sprays the IPI out to all the other CPUs in cpu_online_map,
> and waits for num_online_cpus()-1 CPUs to answer.

You cannot do that by counting without a lot of care. IPI messages do not have
guaranteed "once only" semantics. On an error a resend can and has been observed
to cause a reissue of an IPI on PII/PIII setups

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