linux-kernel - Re: [RFC patch 08/18] cnt32_to_63 should use smp

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Date:	Fri, 07 Nov 2008 20:32:02 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	David Howells <dhowells@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	akpm@...ux-foundation.org, Ingo Molnar <mingo@...e.hu>,
	linux-kernel@...r.kernel.org, Nicolas Pitre <nico@....org>,
	Ralf Baechle <ralf@...ux-mips.org>, benh@...nel.crashing.org,
	paulus@...ba.org, David Miller <davem@...emloft.net>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-arch@...r.kernel.org
Subject: Re: [RFC patch 08/18] cnt32_to_63 should use smp_rmb()

On Fri, 2008-11-07 at 14:18 -0500, Mathieu Desnoyers wrote:
> * Steven Rostedt (rostedt@...dmis.org) wrote:
> > 
> > On Fri, 7 Nov 2008, Mathieu Desnoyers wrote:
> > > 
> > >   __m_cnt_hi
> > >  is read before
> > >   mmio cnt_lo read
> > > 
> > > for the detailed reasons explained in my previous discussion with
> > > Nicolas here :
> > > http://lkml.org/lkml/2008/10/21/1
> > > 
> > > I use smp_rmb() to do this on SMP systems (hrm, actually, a rmb() could
> > > be required so it works also on UP systems safely wrt interrupts).
> > 
> > smp_rmb turns into a compiler barrier on UP and should prevent the below
> > description.
> > 
> 
> Ah, right, preserving program order on UP should be enough. smp_rmb()
> then.


I'm not quite sure I'm following here. Is this a global hardware clock
you're reading from multiple cpus, if so, are you sure smp_rmb() will
indeed be enough to sync the read?

(In which case the smp_wmb() is provided by the hardware increasing the
clock?)

If these are per-cpu clocks then even in the smp case we'd be good with
a plain barrier() because you'd only ever want to read your own cpu's
clock (and have a separate __m_cnt_hi per cpu).

Or am I totally missing out on something?

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