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:	Fri, 20 Feb 2009 08:49:25 +1100
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Nick Piggin <npiggin@...e.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"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>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Steven Rostedt <rostedt@...dmis.org>,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org
Subject: Re: Q: smp.c && barriers (Was: [PATCH 1/4] generic-smp: remove
 single ipi fallback for smp_call_function_many())

On Thu, 2009-02-19 at 16:06 +0100, Ingo Molnar wrote:
> > So your arch_send_call_function_single_ipi etc need to ensure 
> > this, right?  Generic code obviously has no idea about how to 
> > do it.
> 
> The thing is, the most widespread way to send IPIs (x86 
> non-x2apic local APIC) does not need any barriers in the generic 
> code or elsewhere, because the local APIC is mapped uncached so 
> it's implicitly ordered.
> 
> So the right solution is to add barriers to those IPI 
> implementations that need it. This means that the generic code 
> should not have a barrier for IPI sending.

I agree. In fact, our current code should be fine in any case because
our writel() which will be used to generate the IPI has a sync in it
anyway for other reasons.

Cheers,
Ben.


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