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Date: Thu, 19 Feb 2009 01:12:20 +0100
From: Nick Piggin <npiggin@suse.de>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
       Oleg Nesterov <oleg@redhat.com>,
       Peter Zijlstra <a.p.zijlstra@chello.nl>,
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       Steven Rostedt <rostedt@goodmis.org>, linux-kernel@vger.kernel.org,
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Subject: Re: Q: smp.c && barriers (Was: [PATCH 1/4] generic-smp: remove single ipi fallback for smp_call_function_many())
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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.