linux-kernel - Re: 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 20:17:57 +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>
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:

> On Wed, 2009-02-18 at 05:59 -0800, Nick Piggin wrote:
> > You're saying the problem is in generic_exec_single because I've
> > removed the smp_mb that inadvertently also serialises memory with
> > the x2apic on x86?
> 
> yes.
> 
> > 
> > Indeed that could cause problems on some architectures which I
> > had hoped to avoid. So the patch is probably better off to first
> > add the smp_mb() to arch_send_call_function_xxx arch code, unless
> > it is immediately obvious or confirmed by arch maintainer that
> > such barrier is not required.
> 
> For x2apic specific operations we should add the smp_mb() sequence. But
> we need to make sure that we don't end up doing it twice (once in
> generic code and another in arch code) for all the ipi paths.

right now we do have an smp_mb() due to your fix in November.

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
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