Re: [RFC][PATCH] x86: make text_poke() atomic

Mathieu Desnoyers
2009-03-03T00:00:47Z

* Ingo Molnar (mingo@elte.hu) wrote:
>
> * Masami Hiramatsu <mhiramat@redhat.com> wrote:
>
> > Ingo Molnar wrote:
> > >>> So perhaps another approach to (re-)consider would be to go back
> > >>> to atomic fixmaps here. It spends 3 slots but that's no big
> > >>> deal.
> > >> Oh, it's a good idea! fixmaps must make it simpler.
> > >>
> > >>> In exchange it will be conceptually simpler, and will also scale
> > >>> much better than a global spinlock. What do you think?
> > >> I think even if I use fixmaps, we have to use a spinlock to protect
> > >> the fixmap area from other threads...
> > >
> > > that's why i suggested to use an atomic-kmap, not a fixmap.
> >
> > Even if the mapping is atomic, text_poke() has to protect pte
> > from other text_poke()s while changing code.
> > AFAIK, atomic-kmap itself doesn't ensure that, does it?
>
> Well, but text_poke() is not a serializing API to begin with.
> It's normally used in code patching sequences when we 'know'
> that there cannot be similar parallel activities. The kprobes
> usage of text_poke() looks unsafe - and that needs to be fixed.
>
> So indeed a new global lock is needed there.
>
> It's fixable and we'll fixit, but text_poke() is really more
> complex than i'd like it to be.
>
> stop_machine_run() is essentially instantaneous in practice and
> obviously serializing so it warrants a second look at least.
> Have you tried to use it in kprobes?
>
> Ingo

This is why I prepared

text-edit-lock-architecture-independent-code.patch
text-edit-lock-kprobes-architecture-independent-support.patch

A while ago. I'll post them right away.

Mathieu

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Mathieu Desnoyers
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