linux-kernel - Re: [linux-pm] [uclinux-dist-devel] freezer: should barriers be smp?

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Date:	Thu, 14 Apr 2011 00:34:41 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Mike Frysinger <vapier@...too.org>,
	uclinux-dist-devel@...ckfin.uclinux.org,
	linux-pm@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [linux-pm] [uclinux-dist-devel] freezer: should barriers be smp?

On Thursday, April 14, 2011, Alan Stern wrote:
> On Wed, 13 Apr 2011, Rafael J. Wysocki wrote:
> 
> > The above means that smp_*mb() are defined as *mb() if CONFIG_SMP is set,
> > which basically means that *mb() are more restrictive than the corresponding
> > smp_*mb().  More precisely, they also cover the cases in which the CPU
> > reorders instructions on uniprocessor, which we definitely want to cover.
> > 
> > IOW, your patch would break things on uniprocessor where the CPU reorders
> > instructions.
> 
> How could anything break on a UP system?  CPUs don't reorder 
> instructions that drastically.  For example, no CPU will ever violate
> this assertion:
> 
> 	x = 0;
> 	y = x;
> 	x = 1;
> 	assert(y == 0);
> 
> even if it does reorder the second and third statements internally.  
> This is guaranteed by the C language specification.

Well, you conveniently removed the patch from your reply. :-)

For example, there's no reason why the CPU cannot reorder things so that
the "if (frozen(p))" is (speculatively) done before the "if (!freezing(p))"
if there's only a compiler barrier between them.

> > > Documentation/memory-barriers.txt:
> > > SMP memory barriers are reduced to compiler barriers on uniprocessor compiled
> > > systems because it is assumed that a CPU will appear to be self-consistent,
> > > and will order overlapping accesses correctly with respect to itself.
> > 
> > Exactly, which is not guaranteed in general (e.g. on Alpha).  That is, some
> > CPUs can reorder instructions in such a way that a compiler barrier is not
> > sufficient to prevent breakage.
> 
> I don't think this is right.  You _can_ assume that Alphas appear to be
> self-consistent.  If they didn't, you wouldn't be able to use them at
> all.

I'm quite convinced that the statement "some CPUs can reorder instructions in
such a way that a compiler barrier is not sufficient to prevent breakage" is
correct.

Thanks,
Rafael
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