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

From: Alan Stern
Date: Wed Apr 13 2011 - 18:11:54 EST

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

Alan Stern

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