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

Ingo Molnar
2009-03-02T23:50:41Z

* 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