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

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Date:   Wed, 20 May 2020 13:21:02 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Daniel Thompson <daniel.thompson@...aro.org>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Sumit Garg <sumit.garg@...aro.org>,
        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 (20/05/18 11:21), Petr Mladek wrote:
[..]
> > > Is this guaranteed that we never execute this path from NMI?
> 
> Good question!
> 
> > Absolutely not.
> > 
> > The execution context for kdb is pretty much unique... we are running a
> > debug mode with all CPUs parked in a holding loop with interrupts
> > disabled. One CPU is at an unknown exception state and the others are
> > either handling an IRQ or NMI depending on architecture[1].
> 
> This is similar to the situation in panic() when other CPUs are
> stopped. It is more safe when the CPUs are stopped using IRQ.
> There is higher danger of a deadlock when NMI is used.
> 
> bust_spinlock() is used in panic() to increase the chance to go over
> the deadlock and actually see the messages. It is not enough when
> more locks are used by the console (VT/TTY is good example). And
> it is not guaranteed that the console will still work after
> the hack is disabled by bust_spinlocks(0).

Good point. It's not guaranteed to help, but bust_spinlocks() does
help in general, many serial drivers do check oops_in_progress and
use a deadlock safe approach when locking port lock. I don't see
bust_spinlocks() being used in kdb, so it probably better start
doing so (along with general for_each_console() loop improvements,
like checking if console is enabled/available/etc).

[..]
> > > If so, can this please be added to the commit message? A more
> > > detailed commit message will help a lot.
> 
> What about?
> 
> "KDB has to get messages on consoles even when the system is stopped.
> It uses kdb_printf() internally and calls console drivers on its own.
> 
> It uses a hack to reuse an existing code. It sets "kdb_trap_printk"
> global variable to redirect even the normal printk() into the
> kdb_printf() variant.
> 
> The variable "kdb_trap_printk" is checked in printk_default() and
> it is ignored when printk is redirected to printk_safe in NMI context.
> Solve this by moving the check into printk_func().
> 
> It is obvious that it is not fully safe. But it does not make things
> worse. The console drivers are already called in this context by
> kdb_printf() direct calls."

This looks more informative indeed. Thanks!

	-ss