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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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