linux-kernel - Re: smp_call_function

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Date:	Fri, 20 Feb 2015 12:03:03 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Rafael David Tinoco <inaddy@...ntu.com>,
	Peter Anvin <hpa@...or.com>,
	Jiang Liu <jiang.liu@...ux.intel.com>,
	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Jens Axboe <axboe@...nel.dk>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Gema Gomez <gema.gomez-solano@...onical.com>,
	Christopher Arges <chris.j.arges@...onical.com>,
	"the arch/x86 maintainers" <x86@...nel.org>
Subject: Re: smp_call_function_single lockups

On Fri, Feb 20, 2015 at 11:41 AM, Ingo Molnar <mingo@...nel.org> wrote:
>
> I'm not so sure about that aspect: I think disabling IRQs
> might be necessary with some APICs (if lower levels don't
> disable IRQs), to make sure the 'local APIC busy' bit isn't
> set:

Right. But afaik not for the x2apic case, which this is.  The x2apic
doesn't even have a busy bit, and sending the ipi is a single write,

I agree that when doing other apic implementations, we may need to
guarantee atomicity for things like "wait for apic idle, then send the
ipi".

                        Linus
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