linux-kernel - Re: [PATCH] kdb: Make kdb_printf robust to run in NMI context

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Message-ID: <CAFA6WYPVY2Jz6YF_tpc80b2LqEnzMa60NaVjQEmBwQEOY3o+2g@mail.gmail.com>
Date:   Fri, 22 May 2020 16:22:30 +0530
From:   Sumit Garg <sumit.garg@...aro.org>
To:     Daniel Thompson <daniel.thompson@...aro.org>
Cc:     kgdb-bugreport@...ts.sourceforge.net,
        Jason Wessel <jason.wessel@...driver.com>,
        Douglas Anderson <dianders@...omium.org>,
        Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] kdb: Make kdb_printf robust to run in NMI context

On Fri, 22 May 2020 at 16:08, Daniel Thompson
<daniel.thompson@...aro.org> wrote:
>
> On Fri, May 22, 2020 at 03:32:26PM +0530, Sumit Garg wrote:
> > While rounding up CPUs via NMIs, its possible that 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, invoke bust_spinlocks() prior to invocation
> > of console handlers.
> >
> > Also, add a check for console port to be enabled prior to invocation of
> > corresponding handler.
> >
> > Suggested-by: Petr Mladek <pmladek@...e.com>
> > Suggested-by: Sergey Senozhatsky <sergey.senozhatsky@...il.com>
> > Signed-off-by: Sumit Garg <sumit.garg@...aro.org>
> > ---
> >  kernel/debug/kdb/kdb_io.c | 8 ++++++++
> >  1 file changed, 8 insertions(+)
> >
> > diff --git a/kernel/debug/kdb/kdb_io.c b/kernel/debug/kdb/kdb_io.c
> > index 924bc92..e32ece6 100644
> > --- a/kernel/debug/kdb/kdb_io.c
> > +++ b/kernel/debug/kdb/kdb_io.c
> > @@ -699,7 +699,11 @@ int vkdb_printf(enum kdb_msgsrc src, const char *fmt, va_list ap)
> >                       }
> >               }
> >               for_each_console(c) {
> > +                     if (!(c->flags & CON_ENABLED))
> > +                             continue;
> > +                     bust_spinlocks(1);
> >                       c->write(c, cp, retlen - (cp - kdb_buffer));
> > +                     bust_spinlocks(0);
>
> I think it might actually be better to directly manipulate oops_in_progress
> here.
>

Okay will directly manipulate oops_in_progress here.

> bust_spinlocks(0) will call unblank_screen() and console_unblank() and
> I don't think calling these is worth the risk (and whilst kicking klogd
> is safe I think its also not required).

Makes sense.

-Sumit

>
>
> Daniel.
>
>
>
> >                       touch_nmi_watchdog();
> >               }
> >       }
> > @@ -761,7 +765,11 @@ int vkdb_printf(enum kdb_msgsrc src, const char *fmt, va_list ap)
> >                       }
> >               }
> >               for_each_console(c) {
> > +                     if (!(c->flags & CON_ENABLED))
> > +                             continue;
> > +                     bust_spinlocks(1);
> >                       c->write(c, moreprompt, strlen(moreprompt));
> > +                     bust_spinlocks(0);
> >                       touch_nmi_watchdog();
> >               }
> >
> > --
> > 2.7.4
> >