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

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Message-ID: <20090220194054.GA2444@elte.hu>
Date:	Fri, 20 Feb 2009 20:40:54 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Suresh Siddha <suresh.b.siddha@...el.com>
Cc:	Nick Piggin <npiggin@...e.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Oleg Nesterov <oleg@...hat.com>,
	Jens Axboe <jens.axboe@...cle.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.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>,
	steiner@....com
Subject: Re: smp.c && barriers (Was: [PATCH 1/4] generic-smp: remove single
	ipi fallback for smp_call_function_many())


* Suresh Siddha <suresh.b.siddha@...el.com> wrote:

> > So the question is, is an SFENCE there enough to serialize 
> > the WRMSR with previous memory-writes? It's not specified in 
> > the x2apic docs as far as i could see.
> 
> No. sfence is not enough (wrmsr to x2apic regs was still 
> passing ahead). We have done a small experiment to demonstrate 
> the issue and adding mfence fixes the issue but not sfence. We 
> need a serializing instruction or mfence. I will try to get 
> the SDM updated.

Fair enough!

	Ingo
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