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

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Date:   Mon, 18 May 2020 11:49:55 +0530
From:   Sumit Garg <sumit.garg@...aro.org>
To:     Doug Anderson <dianders@...omium.org>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:     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

On Fri, 15 May 2020 at 22:22, Doug Anderson <dianders@...omium.org> wrote:
>
> 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 :)

It's been noticed now since I started playing with NMIs support on
arm64 for kgdb. And that's been only possible with the advent of
pseudo NMIs on arm64 since Linux 5.3 release.

>
> 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.

There's a case for the primary CPU to enter kgdb in NMI context too.
Consider hard-lockup detection based kernel panic. I guess that's
already been working on x86 and should be able to work on arm64 after
this patch [1].

[1] http://lists.infradead.org/pipermail/linux-arm-kernel/2020-May/732227.html

-Sumit

>  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