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

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Date:	Fri, 7 Nov 2008 19:55:25 -0500 (EST)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	David Howells <dhowells@...hat.com>
cc:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	akpm@...ux-foundation.org, Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	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, 7 Nov 2008, David Howells wrote:

> Steven Rostedt <rostedt@...dmis.org> wrote:
> 
> > > 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.
> 
> Note that that does not guarantee that the two reads will be done in the order
> you want.  The compiler barrier _only_ affects the compiler.  It does not stop
> the CPU from doing the reads in any order it wants.  You need something
> stronger than smp_rmb() if you need the reads to be so ordered.

For reading hardware devices that can indeed be correct. But for normal 
memory access on a uniprocessor, if the CPU were to reorder the reads that 
would effect the actual algorithm then that CPU is broken.

 read a
    <---  interrupt  - should see read a here before read b is done.
 read b

Now the fact that one of the reads is a hardware clock, then this 
statement might not be too strong. But the fact that it is a clock, and 
not some memory mapped device register, I still think smp_rmb is 
sufficient.

-- Steve

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