linux-kernel - Re: [PATCH 2/9] perf_counter: fix update

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Message-ID: <18900.35934.799877.893556@cargo.ozlabs.ibm.com>
Date:	Thu, 2 Apr 2009 20:58:54 +1100
From:	Paul Mackerras <paulus@...ba.org>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	Mike Galbraith <efault@....de>,
	Arjan van de Ven <arjan@...radead.org>,
	Wu Fengguang <fengguang.wu@...el.com>
Subject: Re: [PATCH 2/9] perf_counter: fix update_userpage()

Peter Zijlstra writes:

> > Good point.  This should work, though:
> > 
> > 	do {
> > 		seq = pc->lock;
> > 		barrier();
> > 		value = read_pmc(pc->index) + pc->offset;
> > 		barrier();
> > 	} while (pc->lock != seq);
> > 	return value;
> 
> I don't think you need the first barrier(), all you need to avoid is it
> reusing the first pc->lock read, so one should suffice.

I need it to make sure that the compiler doesn't put the load of
pc->index or pc->offset before the first load of pc->lock.  The second
barrier is needed to make sure the compiler puts the second load of
pc->lock after the loads of pc->index and pc->offset.  So I think I do
need to barrier()s (but only compiler barriers, not cpu memory
barriers).

> Also, you need to handle the !pc->index case.

Hmmm, yeah.  I claim that read_pmc(0) always returns 0. :)

Paul.
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