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From:   Doug Anderson <dianders@chromium.org>
Date:   Fri, 15 May 2020 09:52:16 -0700
Message-ID: <CAD=FV=XohG8ErL=_fyuA+MXEmfp55aW+SxSunb6YPRYj77bOxQ@mail.gmail.com>
Subject: Re: [PATCH] printk/kdb: Redirect printk messages into kdb in any context
To:     Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Cc:     Sumit Garg <sumit.garg@linaro.org>, Petr Mladek <pmladek@suse.com>,
        Daniel Thompson <daniel.thompson@linaro.org>,
        Jason Wessel <jason.wessel@windriver.com>,
        kgdb-bugreport@lists.sourceforge.net,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Arnd Bergmann <arnd@arndb.de>,
        Andrew Morton <akpm@linux-foundation.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        John Ogness <john.ogness@linutronix.de>
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Hi,

On Fri, May 15, 2020 at 9:36 AM Sergey Senozhatsky
<sergey.senozhatsky@gmail.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