Messages in this thread | | | Date | Tue, 3 Mar 2009 00:49:10 +0100 | From | Ingo Molnar <> | Subject | Re: [RFC][PATCH] x86: make text_poke() atomic |
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* 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
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