linux-kernel - Re: smp_call_function

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Message-ID: <20150220201123.GD3603@gmail.com>
Date:	Fri, 20 Feb 2015 21:11:23 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Linus Torvalds <torvalds@...ux-foundation.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


* Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> 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,

Ah, ok! Then the patch looks good to me.

( Originally we didn't wait for the ICR bit either, but 
  then it was added due to later erratas and was eventually 
  made an architectural requirement. )

> 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".

Yeah.

Thanks,

	Ingo
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