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

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Date:	Thu, 2 Apr 2009 20:15:46 +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:

> > That means that we don't need any CPU memory barriers on either side.
> > All the kernel needs to do is to increment `lock' when it updates
> > things, and the user side can be:
> > 
> > 	do {
> > 		seq = pc->lock;
> > 		index = pc->index;
> > 		offset = pc->offset;
> > 		barrier();
> > 	} while (pc->lock != seq);
> > 
> > and all that's needed is a compiler barrier to stop the compiler from
> > optimizing too much.
> 
> Can this work at all?
> 
> I mean, user-space could get preempted/rescheduled after we read the
> mmap() data using that seqlock and before we actually did the read-pmc
> bit.
> 
> In that case, the counter can have changed underneath us and we're
> reading rubbish.

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;

No?

Paul.
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