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:	Wed, 18 Feb 2009 14:52:12 +0100
From:	Nick Piggin <npiggin@...e.de>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	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>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	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 Tue, Feb 17, 2009 at 02:39:10PM -0800, Paul E. McKenney wrote:
> On Tue, Feb 17, 2009 at 10:45:18PM +0100, Oleg Nesterov wrote:
> > > If all of the above is executed by the same task, tripping the BUG_ON()
> > > means either a compiler or CPU bug.
> > 
> > I think you misunderstood...
> > 
> > smp_send_xxx() sends the ipi to another CPU, and smp_xxx_interrupt() is
> > the handler.
> 
> You are right, I did miss that completely.  :-/
> 
> I have seen hardware in which the IPI could beat the cache invalidation
> from the sending CPU to the interrupted CPU, and in which one or both of
> the CPUs would have to execute special cache-flush/invalidation/whatever
> instructions for the interrupted CPU to have a consistent view of the
> data (in your example, "COND").
> 
> But we had a little chat with the hardware designers, and in subsequent
> hardware, the IPI interacted with the cache-coherence protocol so as to
> prevent the above bug from firing.  However, this was x86-based hardware,
> which orders writes.  Weakly ordered systems would likely need a memory
> barrier somewhere, whether as shown above or buried in the smp_send_xxx()
> primitive.

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.
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