Re: [PATCH 2/9] perf_counter: fix update_userpage()

[Paul Mackerras <paulus@samba.org>]

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.