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Date: Fri, 20 Feb 2009 20:40:54 +0100
From: Ingo Molnar <mingo@elte.hu>
To: Suresh Siddha <suresh.b.siddha@intel.com>
Cc: Nick Piggin <npiggin@suse.de>, Peter Zijlstra <a.p.zijlstra@chello.nl>,
       Oleg Nesterov <oleg@redhat.com>, Jens Axboe <jens.axboe@oracle.com>,
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       "linux-arch@vger.kernel.org" <linux-arch@vger.kernel.org>,
       steiner@sgi.com
Subject: Re: smp.c && barriers (Was: [PATCH 1/4] generic-smp: remove single
	ipi fallback for smp_call_function_many())
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* Suresh Siddha <suresh.b.siddha@intel.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