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

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Message-ID: <Pine.LNX.4.44L0.1104131805090.2005-100000@iolanthe.rowland.org>
Date:	Wed, 13 Apr 2011 18:11:48 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
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 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.

> > 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.

Alan Stern

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