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

    * 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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