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

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Date:	Fri, 7 Nov 2008 16:34:36 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Mathieu Desnoyers <compudj@...stal.dyndns.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	David Howells <dhowells@...hat.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, Nov 07, 2008 at 04:04:48PM -0500, Steven Rostedt wrote:
> 
> On Fri, 7 Nov 2008, Paul E. McKenney wrote:
> > > > Would that make more sense ?
> > > > 
> > > 
> > > Oh, actually, I got things reversed in this email : the readl(io_addr)
> > > must be done _after_ the __m_cnt_hi read.
> > > 
> > > Therefore, two consecutive executions would look like :
> > > 
> > > barrier();  /* Make sure the compiler does not reorder __m_cnt_hi and
> > >                previous mmio read. */
> > > read __m_cnt_hi
> > > smp_rmb();  /* Waits for every cached memory reads to complete */
> > 
> > If these are MMIO reads, then you need rmb() rather than smp_rmb(),
> > at least on architectures that can reorder writes (Power, Itanium,
> > and I believe also ARM, ...).
> 
> The read is from a clock source. The only writes that are happening is 
> by the clock itself.
> 
> On a UP system, is a rmb still needed? That is, can you have two reads on 
> the same CPU from the clock source that will produce a backwards clock? 
> That to me sounds like the clock interface is broken.

I do not believe that all CPUs are guaranteed to execute a sequence
of MMIO reads in order.

							Thanx, Paul
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