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

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Message-ID: <20200610164140.tgzcn5oip2gzgmze@holly.lan>
Date:   Wed, 10 Jun 2020 17:41:40 +0100
From:   Daniel Thompson <daniel.thompson@...aro.org>
To:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:     Sumit Garg <sumit.garg@...aro.org>, Petr Mladek <pmladek@...e.com>,
        Jason Wessel <jason.wessel@...driver.com>,
        Douglas Anderson <dianders@...omium.org>,
        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 Sat, May 16, 2020 at 01:36:38AM +0900, Sergey Senozhatsky 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 :)

Rather by coincidence (at least I think its a coincidence) the problem
has recently become much more obvious.

0d00449c7a28 ("x86: Replace ist_enter() with nmi_enter()") just brought
this to the surface by treating debug traps as NMIs. This means the CPU
that takes a breakpoint, and where almost all of the kdb printk() calls
take place, will now unconditionally have printk() interception enabled.


Daniel.