linux-kernel - Re: [PATCH] printk/kdb: Redirect printk messages into kdb in any context

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Date:   Fri, 15 May 2020 09:52:16 -0700
From:   Doug Anderson <dianders@...omium.org>
To:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:     Sumit Garg <sumit.garg@...aro.org>, Petr Mladek <pmladek@...e.com>,
        Daniel Thompson <daniel.thompson@...aro.org>,
        Jason Wessel <jason.wessel@...driver.com>,
        kgdb-bugreport@...ts.sourceforge.net,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        John Ogness <john.ogness@...utronix.de>
Subject: Re: [PATCH] printk/kdb: Redirect printk messages into kdb in any context

Hi,

On Fri, May 15, 2020 at 9:36 AM Sergey Senozhatsky
<sergey.senozhatsky@...il.com> wrote:
>
> On (20/05/15 17:32), Sumit Garg wrote:
> > > Can I please have some context what problem does this solve?
> >
> > You can find the problem description here [1] which leads to this fix.
>
> [..]
>
> > [1] https://lkml.org/lkml/2020/5/12/213
>
> Thanks for the link. I'm slightly surprised it took so many years
> to notice the addition of printk_nmi/printk_safe :)

I haven't looked at all the details, but IIUC we don't normally enter
kgdb on the primary CPU through a NMI context, but the secondary ones
(on x86) always do.  Most things are run on the primary CPU and I
think it's relatively unlikely for people to change the primary CPU
(though it is possible).

Probably things got worse when I changed the way "btc" worked to make
it common between all architectures.  See commit 9ef50a686b53
("UPSTREAM: kdb: Fix stack crawling on 'running' CPUs that aren't the
master").  Though theoretically someone could have changed masters and
reproduced the problem with a simple "bt" before my patch, now a
relatively normal command "btc" would tickle the problem.  I didn't
notice it because I work almost totally on arm/arm64 machines and they
don't have NMI (yet).

In general I've always wondered about why (historically) kgdb bugs
have sometimes gone unnoticed for a period of time.  That does seem to
be changing, though, and I've seen a few longstanding bugs getting
fixed recently.  :-)

-Doug