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    SubjectRe: [RFC 0/5] kernel: backtrace unwind support
    On Sat, Feb 11, 2012 at 03:38:09PM +0100, Ingo Molnar wrote:
    >
    > * Jiri Olsa <jolsa@redhat.com> wrote:
    >
    > > > I had a quick peek and I don't think it's constructed in a
    > > > resilent enough form right now. For example there's no clear
    > > > separation and checking of what comes from GCC and what not.
    > >
    > > yes, there's nothing like this in now, I'll see what can be
    > > done about that..
    >
    > Another resilience feature of lockdep is the 'one strike and you
    > are out!' aspect: the first error or unexpected condition we
    > detect results in the very quick shutting down of all things
    > lockdep. It prints exactly one error message, then it
    > deactivates and never ever runs again.
    >
    > The equivalent of this in the scope of your dwarf unwind kernel
    > feature would be to fall back to the regular guess and
    > framepointer based stack backtrace method the moment any error
    > is detected.
    >
    > Maybe print a single line that indicates that the fallback has
    > been activated, and after that the dwarf code should never run
    > again. Make sure nobody comes away a "oh, no, the dwarf unwind
    > messed up things!' impression, even if it *does* run into some
    > trouble (such as unexpected debuginfo generated by GCC - or
    > debuginfo *corrupted* by a kernel bug [a very real
    > possibility]).

    right, such fallback seems necessary

    >
    > What is totally unacceptable is for the dwarf code to *cause*
    > crashes, or to destroy stack trace information.
    >
    > > yep, looks interesting.. not sure about the mathematical proof
    > > though ;)
    >
    > In the physical sense even mathematics is always and unavoidably
    > probability based (or brain and all our senses are
    > probabilistic), so you can probably replace 'mathematical proof'
    > with 'very robust design and a very, very good track record',
    > before bothering Linus with it next time around ;-)

    I wasn't aware of such kernel unwind history ;) was just curious,
    if anyone is interested, before spending more time on that..

    >
    > And we might as well conclude "it's simply not worth it", at
    > some point down he road. I *do* think that it's worth it though,
    > and I do think it can be designed and implemented robustly, so
    > I'd be willing to try out these patches in -tip for a kernel
    > release or two, without pushing it to Linus.

    thanks a lot for your ideas, I'll start working on that

    jirka


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