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:	Thu, 19 Feb 2009 01:12:20 +0100
From:	Nick Piggin <npiggin@...e.de>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	"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>,
	Ingo Molnar <mingo@...e.hu>,
	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 Wed, Feb 18, 2009 at 08:09:21AM -0800, Linus Torvalds wrote:
> 
> 
> On Wed, 18 Feb 2009, Nick Piggin wrote:
> > 
> > I agree with you both that we *should* make arch interrupt code
> > do the ordering, but given the subtle lockups on some architectures
> > in this new code, I didn't want to make it significantly weaker...
> > 
> > Though perhaps it appears that I have, if I have removed an smp_mb
> > that x86 was relying on to emit an mfence to serialise the apic.
> 
> The thing is, if the architecture doesn't order IPI wrt cache coherency, 
> then the "smp_mb()" doesn't really do so _either_. 

Oh yes agreed three, which is why I'm saying it is just a hack
and should be removed at some point.

 
> It might hide some architecture-specific implementation issue, of course, 
> so random amounts of "smp_mb()"s sprinkled around might well make some 
> architecture "work", but it's in no way guaranteed. A smp_mb() does not 
> guarantee that some separate IPI network is ordered - that may well take 
> some random machine-specific IO cycle.

Yes, but I didn't want to pull out that smp_mb() at least until
arch maintainers can ack it. Just because there might indeed be
some random issue hidden by it.

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