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 _______________________________________________ linux-pm mailing list linux-pm@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/linux-pm
