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

Ingo Molnar
2009-02-18T19:18:55Z

* Suresh Siddha <suresh.b.siddha@intel.com> wrote:

> On Wed, 2009-02-18 at 05:59 -0800, Nick Piggin wrote:
> > You're saying the problem is in generic_exec_single because I've
> > removed the smp_mb that inadvertently also serialises memory with
> > the x2apic on x86?
>
> yes.
>
> >
> > Indeed that could cause problems on some architectures which I
> > had hoped to avoid. So the patch is probably better off to first
> > add the smp_mb() to arch_send_call_function_xxx arch code, unless
> > it is immediately obvious or confirmed by arch maintainer that
> > such barrier is not required.
>
> For x2apic specific operations we should add the smp_mb() sequence. But
> we need to make sure that we don't end up doing it twice (once in
> generic code and another in arch code) for all the ipi paths.

right now we do have an smp_mb() due to your fix in November.

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