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

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Date:   Fri, 17 Mar 2023 12:43:56 +0106
From:   John Ogness <john.ogness@...utronix.de>
To:     Petr Mladek <pmladek@...e.com>
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 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.

John