linux-kernel - Re: [PATCH printk v2 2/5] printk: Add NMI safety to console_flush_on_panic() and console

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Message-ID: <20230711160758.GG12154@google.com>
Date:   Wed, 12 Jul 2023 01:07:58 +0900
From:   Sergey Senozhatsky <senozhatsky@...omium.org>
To:     Petr Mladek <pmladek@...e.com>
Cc:     John Ogness <john.ogness@...utronix.de>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH printk v2 2/5] printk: Add NMI safety to
 console_flush_on_panic() and console_unblank()

On (23/07/11 17:43), Petr Mladek wrote:
> On Mon 2023-07-10 15:51:21, John Ogness wrote:
> > The printk path is NMI safe because it only adds content to the
> > buffer and then triggers the delayed output via irq_work. If the
> > console is flushed or unblanked on panic (from NMI context) then it
> > can deadlock in down_trylock_console_sem() because the semaphore is
> > not NMI safe.
> 
> <thinking loudly>
> 
> Just to be sure. The semaphore is not NMI safe because even the
> trylock takes an internal spin lock. Am I right, please?
> 
> Alternative solution would be to make down_trylock() NMI safe
> by using raw_spin_trylock_irqsave() for the internal lock.
> 
> But this actually would not solve the whole problem. If the NMI safe
> down_trylock() succeeded then up() would need to be called
> in NMI as well. And up() really needs to take the spin lock
> which might get blocked in the meantime.

I guess another problem with up() is that it also may call
try_to_wake_up(), that may attempt to acquire a bunch of other
spin_lock-s.