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    SubjectRe: lockdep and debug objects together are broken?

    * Nick Piggin <npiggin@suse.de> wrote:

    > On Wed, Jan 21, 2009 at 12:42:29PM +0100, Ingo Molnar wrote:
    > >
    > > * Nick Piggin <npiggin@suse.de> wrote:
    > >
    > > > On Tue, Jan 20, 2009 at 10:11:47PM +0100, Vegard Nossum wrote:
    > > > > On Tue, Jan 20, 2009 at 9:55 AM, Nick Piggin <npiggin@suse.de> wrote:
    > > > > > Hi,
    > > > > >
    > > > > > I've had a problem frustrating my testing because lockdep was silently turning
    > > > > > itself off... I patched out the code to disable lockdep after the first error,
    > > > > > and it started showing up weird errors. kernel/fork.c:990 seemed to be the
    > > > > > first to trigger (hard irqs disabled) from a call_usermodehelper call. Later,
    > > > > > migration thread was reported to try to unlock rq->lock although it was
    > > > > > holding no locks. Then init was reported to return to userspace without
    > > > > > releasing an objectdebug hash lock.
    > > > > >
    > > > > > All that went away and everything seemed to work properly with debug objects
    > > > > > configured out.
    > > > > >
    > > > > > I didn't get too far in trying to debug the problem. But it should be easy
    > > > > > enough to reproduce (if not, I can post traces or test patches).
    > > > >
    > > > > I just built a kernel with lockdep and debugobjects enabled, and
    > > > > everything seemed fine. I think you should post your kernel version,
    > > > > config, and the lockdep patch (if needed -- it didn't seem to turn
    > > > > itself off here).
    > > >
    > > > Are you sure? Ie. sysrq+D a still works properly? In that case, you
    > > > wouldn't need the lockdep patch because it just prevents the feature from being
    > > > switched off.
    > > >
    > > > I'll have to dig a bit further, then. The annoying thing is that
    > > > lockdep turns itself off at the drop of a hat (and this particular
    > > > problem seems to happen without any backtraces), so it invalidates
    > > > all your lockdep testing if you don't realise it has turned itself
    > > > off.
    > > >
    > > > Is there a way to re-arm lockdep? That would be neat.
    > >
    > > Not at the moment, and it looks somewhat complicated. All lock state
    > > freezes the moment lockdep disarms itself. That's very much a key design
    > > element: rarely will you see any real lockdep-inflicted crash - even if it
    > > has a bug it is self-disabling itself and running for the door very
    > > efficiently.
    >
    > Lockdep isn't exactly for production systems though, is it? If you want
    > to debug some problem but you have other code (that you don't have
    > knowledge to debug) is switching it off...
    >
    > Also, I'd guess that most bugs in lockdep would probably fall pretty
    > neatly into either the "pretty harmless" or "completely take down the
    > system" categories ;)

    i think lockdep could be expanded into production use via code patching
    techniques.

    So in that sense the rearm bit could be useful - it would give us a
    lockdep variant that would run for the first 5 minutes of uptime (where
    90% of all lockdep reports trigger: lockdep maps the dependencies very
    quickly) - and could turn itself off after that, and patch out / disable
    its callbacks.

    The memory footprint would still remain, but that is not nearly as much of
    a problem for production systems as runtime overhead.

    Ingo


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