Re: [PATCH v3 3/4] kdb: Make kdb_printf robust to run in NMI context

[Sumit Garg <sumit.garg@linaro.org>]

On Wed, 27 May 2020 at 19:56, Daniel Thompson
<daniel.thompson@linaro.org> wrote:
>
> On Wed, May 27, 2020 at 11:55:58AM +0530, Sumit Garg wrote:
> > While rounding up CPUs via NMIs, its possible that a rounded up CPU
>
> This problem does not just impact NMI roundup (breakpoints,

I guess here via breakpoints you meant if we add a compiled breakpoint
or runtime breakpoint in console handler code while its holding the
spin lock could lead to a deadlock, correct?

> including
> implicit breakpoint-on-oops can have the same effect).
>

Isn't the breakpoint-on-oops case already handled via bust_spinlocks()
usage in panic handler here [1]?

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/kernel/panic.c#n207

>
> > maybe holding a console port lock leading to kgdb master CPU stuck in
> > a deadlock during invocation of console write operations. So in order
> > to avoid such a deadlock, enable oops_in_progress prior to invocation
> > of console handlers.
> >
> > Suggested-by: Petr Mladek <pmladek@suse.com>
> > Signed-off-by: Sumit Garg <sumit.garg@linaro.org>
> > ---
> >  kernel/debug/kdb/kdb_io.c | 10 ++++++++++
> >  1 file changed, 10 insertions(+)
> >
> > diff --git a/kernel/debug/kdb/kdb_io.c b/kernel/debug/kdb/kdb_io.c
> > index 349dfcc..f848482 100644
> > --- a/kernel/debug/kdb/kdb_io.c
> > +++ b/kernel/debug/kdb/kdb_io.c
> > @@ -566,7 +566,17 @@ static void kdb_msg_write(char *msg, int msg_len)
> >       for_each_console(c) {
> >               if (!(c->flags & CON_ENABLED))
> >                       continue;
> > +             /*
> > +              * While rounding up CPUs via NMIs, its possible that
>
> Ditto.
>
> > +              * a rounded up CPU maybe holding a console port lock
> > +              * leading to kgdb master CPU stuck in a deadlock during
> > +              * invocation of console write operations. So in order
> > +              * to avoid such a deadlock, enable oops_in_progress
> > +              * prior to invocation of console handlers.
>
> Actually looking at this comment as a whole I think it spends to many
> words on what and not enough on why (e.g. what the tradeoffs are and
> why we are not using bust_spinlocks() which would be a more obvious
> approach).
>
>   Set oops_in_progress to encourage the console drivers to disregard
>   their internal spin locks: in the current calling context
>   the risk of deadlock is a bigger problem than risks due to
>   re-entering the console driver. We operate directly on
>   oops_in_progress rather than using bust_spinlocks() because
>   the calls bust_spinlocks() makes on exit are not appropriate
>   for this calling context.

Sounds reasonable, will use it instead.

-Sumit

>
>
> Daniel.
>
>
> > +              */
> > +             ++oops_in_progress;
> >               c->write(c, msg, msg_len);
> > +             --oops_in_progress;
> >               touch_nmi_watchdog();
> >       }
> >  }
> > --
> > 2.7.4
> >