Re: smp.c && barriers (Was: [PATCH 1/4] generic-smp: remove single ipi fallback for smp_call_function_many())

Nick Piggin
2009-02-19T12:30:04Z

On Thu, Feb 19, 2009 at 01:20:31PM +0100, Ingo Molnar wrote:
>
> * Suresh Siddha <suresh.b.siddha@intel.com> wrote:
>
> > On Wed, 2009-02-18 at 11:17 -0800, Ingo Molnar wrote:
> > > * Suresh Siddha <suresh.b.siddha@intel.com> wrote:
> > > So what should happen is to move that smp_mb() from the x86
> > > generic IPI path to the x86 x2apic IPI path. (and turn it into
> > > an smp_wmb() - that should be enough - we dont care about future
> > > reads being done sooner than this point.)
> >
> > Ingo, smp_wmb() won't help. x2apic register writes can still
> > go ahead of the sfence. According to the SDM, we need a
> > serializing instruction or mfence. Our internal experiments
> > also proved this.
>
> ah, yes - i got confused about how an x2apic write can pass a
> _store_ fence.

And about how smp_wmb() doesn't emit a store fence ;)