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

* Linus Torvalds <torvalds@linux-foundation.org> wrote:

> On Wed, 18 Feb 2009, Ingo Molnar wrote:
> >
> > But ... WRMSR should already be serializing - it is documented
> > as a serializing instruction.
>
> Hmm. I was thinking about this some more, and I think I've
> come up with an explanation.
>
> "wrmsr" probably serializes _after_ doing the write. After
> all, it's historically used for changing internal CPU state,
> so you want to do the write, and then wait until the effects
> of the write are "stable" in the core.
>
> That would explain how x2apic can use both a serializing
> instruction (wrmsr) and still effectively cause the IPI to
> happen out of sequence: the IPI can reach the destination CPU
> before the source CPU has flushed its store buffers, because
> the IPI is actually sent before serializing the core.
>
> But I would very strongly put this in the "x2apic code bug"
> column. If this is a true issue (and your TLB patch does imply
> it is), then we should just make sure that the x2apic IPI
> calls always do a 'sfence' before they happen - regardless of
> whether they are for TLB flushes or for generic kernel
> cross-calls, or for anything else.

Yeah, that makes perfect sense. IPIs are an out of band
signalling mechanism that do not listen to the normal cache
coherency rules.

Moving the smp_mb() to the x2apic specific code will also speed
up the normal mmio-mapped IPI sequence a bit. It should be an
smp_wmb() i suspect - which turns it into an sfence.

Ingo


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