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: <20090219124551.GB22044@elte.hu>
Date:	Thu, 19 Feb 2009 13:45:51 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Nick Piggin <npiggin@...e.de>
Cc:	Suresh Siddha <suresh.b.siddha@...el.com>,
	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>
Subject: Re: smp.c && barriers (Was: [PATCH 1/4] generic-smp: remove single
	ipi fallback for smp_call_function_many())


* Nick Piggin <npiggin@...e.de> wrote:

> On Thu, Feb 19, 2009 at 01:20:31PM +0100, Ingo Molnar wrote:
> > 
> > * Suresh Siddha <suresh.b.siddha@...el.com> wrote:
> > 
> > > On Wed, 2009-02-18 at 11:17 -0800, Ingo Molnar wrote: 
> > > > * Suresh Siddha <suresh.b.siddha@...el.com> wrote:
> > > > So what should happen is to move that smp_mb() from the x86 
> > > > generic IPI path to the x86 x2apic IPI path. (and turn it into 
> > > > an smp_wmb() - that should be enough - we dont care about future 
> > > > reads being done sooner than this point.)
> > > 
> > > Ingo, smp_wmb() won't help. x2apic register writes can still 
> > > go ahead of the sfence. According to the SDM, we need a 
> > > serializing instruction or mfence. Our internal experiments 
> > > also proved this.
> > 
> > ah, yes - i got confused about how an x2apic write can pass a 
> > _store_ fence.
> 
> And about how smp_wmb() doesn't emit a store fence ;)

yeah ;-) Only wmb() emits a SFENCE all the time. Writes are 
normally ordered so we map smp_wmb() to barrier().

	Ingo
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