[PATCH 1/4] arm64: topology: Implement basic CPU topology support

[will.deacon@arm.com (Will Deacon)]

On Tue, Feb 11, 2014 at 08:15:19AM +0000, Vincent Guittot wrote:
> On 10 February 2014 17:46, Mark Brown <broonie@kernel.org> wrote:
> > On Mon, Feb 10, 2014 at 04:22:31PM +0000, Catalin Marinas wrote:
> >> On Mon, Feb 10, 2014 at 01:02:01PM +0000, Mark Brown wrote:
> >
> >> > +           if (cpu != cpuid)
> >> > +                   cpumask_set_cpu(cpu, &cpuid_topo->thread_sibling);
> >> > +   }
> >> > +   smp_wmb();
> >
> >> I now noticed there are a couple of smp_wmb() calls in this patch. What
> >> are they for?
> >
> > To be honest I mostly cargo culted them from the ARM implementation; I
> > did look a bit but didn't fully dig into it - it seemed they were
> > required to ensure that the updates for the new CPU are visible over all
> > CPUs.  Vincent?
> 
> Yes that's it. we must ensure that updates are made visible to other CPUs

In relation to what? The smp_* barriers ensure ordering of observability
between a number of independent accesses, so you must be ensuring
ordering against something else. Also, you need to guarantee ordering on the
read-side too -- how is this achieved? I can't see any smp_rmb calls from a
quick grep, so I assume you're making use of address dependencies?

/confused

Will