linux-kernel - Re: Q: smp.c && barriers (Was: [PATCH 1/4] generic-smp: remove single ipi fallback for smp_call_function

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Date:	Wed, 18 Feb 2009 10:33:46 -0800
From:	Suresh Siddha <suresh.b.siddha@...el.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Ingo Molnar <mingo@...e.hu>,
	"Pallipadi, Venkatesh" <venkatesh.pallipadi@...el.com>,
	Yinghai Lu <yinghai@...nel.org>, Nick Piggin <npiggin@...e.de>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Jens Axboe <jens.axboe@...cle.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Steven Rostedt <rostedt@...dmis.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	asit.k.mallick@...el.com
Subject: Re: Q: smp.c && barriers (Was: [PATCH 1/4] generic-smp: remove
	single ipi fallback for smp_call_function_many())

On Wed, 2009-02-18 at 09:14 -0800, Linus Torvalds wrote:
> 
> On Wed, 18 Feb 2009, Ingo Molnar wrote:
> >
> > an off-list comment pointed out this piece of information as 
> > well:
> > 
> >   http://www.sandpile.org/ia32/coherent.htm
> > 
> >   A WRMSR to one of the x2APIC MSRs (0000_0800h...0000_0BFFh) is 
> >   not guaranteed to be serializing.
> > 
> > So i suspect we should just enclose it in smp_mb() pairs to make 
> > sure it's a full barrier in both directions?
> 
> Why would we care about "both directions"?

on x86 we don't need in both directions.

> 
> I think putting an sfence _before_ the wrmsr (and not even all of them - 
> just put it in front of the "send IPI" sequence) should be fine. Any other 
> ordering sounds like just unnecessary overhead to me.

For x2apic ipi's, we should use a serializing instruction or a "mfence"
instruction. "sfence" will not help in this scenario.

thanks,
suresh

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