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

Nick Piggin
2009-02-25T03:32:47Z

On Fri, Feb 20, 2009 at 10:56:54AM -0800, Suresh B wrote:
> On Fri, 2009-02-20 at 02:56 -0800, Ingo Molnar wrote:
> > * Suresh Siddha <suresh.b.siddha@intel.com> wrote:
> >
> > > On Thu, 2009-02-19 at 04:20 -0800, Ingo Molnar wrote:
> > > > Could you please refresh this patch to latest tip:master? The
> > > > APIC drivers moved to arch/x86/kernel/apic/.
> > >
> > > Appended the refreshed patch. Thanks.
> >
> > thanks. Two details i noticed:
> >
> > Firstly:
> >
> > > +++ b/arch/x86/kernel/apic/x2apic_cluster.c
> > > +++ b/arch/x86/kernel/apic/x2apic_phys.c
> >
> > how about x2apic_uv.c? It uses uv_write_global_mmr64() in its
> > IPI sending method, which uses:
> >
> > static inline void uv_write_global_mmr64(int pnode, unsigned long offset,
> > unsigned long val)
> > {
> > *uv_global_mmr64_address(pnode, offset) = val;
> > }
> >
> > which uses ->mmr_base, which is mapped via:
> >
> > init_extra_mapping_uc(UV_LOCAL_MMR_BASE, UV_LOCAL_MMR_SIZE);
> >
> > so it should be fine because uncached - but at minimum we should
> > put a comment into x2apic_uv.c that the generic IPI code relies
> > on the lowlevel code serializing - i.e. relies on the UC PAT
> > attribute.
>
> Oops. I forgot to mention that and copy Jack to confirm our
> understanding. I can send a followup patch adding the comments for UV.
> Jack, are you ok?
>
> >
> > Secondly, you added smp_mb(), which will translate to an MFENCE.
> >
> > But in theory it should be enough to have a wmb() here. [Note,
> > not an smp_wmb() that i suggested before.] That will translate
> > to an SFENCE - which will serialize writes but still allows
> > reads/prefetches to pass.
> >
> > So the question is, is an SFENCE there enough to serialize the
> > WRMSR with previous memory-writes? It's not specified in the
> > x2apic docs as far as i could see.
>
> No. sfence is not enough (wrmsr to x2apic regs was still passing ahead).
> We have done a small experiment to demonstrate the issue and adding
> mfence fixes the issue but not sfence. We need a serializing instruction
> or mfence. I will try to get the SDM updated.

Just if I may add something -- I would probably slightly prefer if
you explicitly used an sfence or other serializing instruction rather
than smp_mb(). Maybe call it wrmsr_fence() or something. Apart from
being self documenting, and less confusing (wrmsr is not part of
normal ordering), I assume you technically also need it on UP
systems?