linux-kernel - Re: [PATCH printk v1 02/18] printk: Add NMI check to down_trylock_console

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Date:   Thu, 13 Apr 2023 15:42:08 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH printk v1 02/18] printk: Add NMI check to
 down_trylock_console_sem()

On Fri 2023-03-17 12:43:56, John Ogness wrote:
> On 2023-03-07, Petr Mladek <pmladek@...e.com> wrote:
> > So that this change would cause a non-paired console_unlock().
> > And console_unlock might still deadlock on the console_sem->lock.
> 
> Yes, but at least it would have flushed beforehand.
> 
> > One solution would be to call console_flush_all() directly in
> > console_flush_on_panic() without taking console_lock().
> >
> > It should not be worse than the current code which ignores
> > the console_trylock() return value.
> 
> I think your suggestion is acceptable.
> 
> > Note that it mostly works because console_flush_on_panic() is called
> > when other CPUs are supposed to be stopped.
> >
> > We only would need to prevent other CPUs from flushing messages
> > as well if they were still running by chance. But we actually already
> > do this, see abandon_console_lock_in_panic(). Well, we should
> > make sure that the abandon_console_lock_in_panic() check is
> > done before flushing the first message.
> >
> > All these changes together would prevent deadlock on
> > console_sem->lock.  But the synchronization "guarantees" should stay
> > the same.
> 
> We could also update console_trylock() and console_lock() to fail and
> infinitely sleep, respectively, when abandon_console_lock_in_panic() is
> true. That would prevent CPUs from newly acquiring the console lock and
> interfering with the panic CPU.

Interesting idea. It should be safe after panic() tries to
stop the CPUs. But I am slightly worried to do this earlier.

I wonder if it might block, for example, trigger_all_cpu_backtrace()
that is called when (panic_print & PANIC_PRINT_ALL_CPU_BT) bit is set.

Best Regards.
Petr