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

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Message-ID: <9302.1226100479@redhat.com>
Date:	Fri, 07 Nov 2008 23:27:59 +0000
From:	David Howells <dhowells@...hat.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	dhowells@...hat.com, Andrew Morton <akpm@...ux-foundation.org>,
	Nicolas Pitre <nico@....org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	linux-kernel@...r.kernel.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>,
	Steven Rostedt <rostedt@...dmis.org>,
	linux-arch@...r.kernel.org
Subject: Re: [RFC patch 08/18] cnt32_to_63 should use smp_rmb()

Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca> wrote:

> Yes. Do you think the synchronization of the cycles counters is
> _perfect_ across CPUs so that there is no possible way whatsoever that
> two cycle counter values appear to go backward between CPUs ? (also
> taking in account delays in __m_cnt_hi write-back...)

Given there's currently only one CPU allowed, yes, I think it's perfect:-)

It's something to re-evaluate should Panasonic decide to do SMP.

> If we expect the only correct use-case to be with readl(), I don't see
> the problem with added synchronization.

It might be expensive if you don't actually want to call readl().  But that's
on a par with using funky instructions to read the TSC, I guess.

David
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