linux-kernel - Re: [PATCH v3 3/4] printk: nbcon: Allow KDB to acquire the NBCON context

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Message-ID: <aL7y1AvuAAsOSJ4q@pathway.suse.cz>
Date: Mon, 8 Sep 2025 17:14:28 +0200
From: Petr Mladek <pmladek@...e.com>
To: Marcos Paulo de Souza <mpdesouza@...e.com>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	John Ogness <john.ogness@...utronix.de>,
	Sergey Senozhatsky <senozhatsky@...omium.org>,
	Jason Wessel <jason.wessel@...driver.com>,
	Daniel Thompson <danielt@...nel.org>,
	Douglas Anderson <dianders@...omium.org>,
	linux-kernel@...r.kernel.org, kgdb-bugreport@...ts.sourceforge.net
Subject: Re: [PATCH v3 3/4] printk: nbcon: Allow KDB to acquire the NBCON
 context

On Tue 2025-09-02 15:33:54, Marcos Paulo de Souza wrote:
> KDB can interrupt any console to execute the "mirrored printing" at any
> time, so add an exception to nbcon_context_try_acquire_direct to allow
> to get the context if the current CPU is the same as kdb_printf_cpu.
> 
> This change will be necessary for the next patch, which fixes
> kdb_msg_write to work with NBCON consoles by calling ->write_atomic on
> such consoles. But to print it first needs to acquire the ownership of
> the console, so nbcon_context_try_acquire_direct is fixed here.
> 
> --- a/kernel/printk/nbcon.c
> +++ b/kernel/printk/nbcon.c
> @@ -247,6 +248,8 @@ static int nbcon_context_try_acquire_direct(struct nbcon_context *ctxt,
>  		 * Panic does not imply that the console is owned. However,
>  		 * since all non-panic CPUs are stopped during panic(), it
>  		 * is safer to have them avoid gaining console ownership.
> +		 * The only exception is if kdb is active, which may print
> +		 * from multiple CPUs during a panic.

Strictly speaking this is not the only exception. The reacquire is
another one. I would put this into a separate paragraph and write:

		 * One exception is when kdb is active, which may print
		 * from multiple CPUs during a panic.

>  		 * If this acquire is a reacquire (and an unsafe takeover

And here start the paragrah with

		 * Second exception is a reacquire (and an usafe ...

>  		 * has not previously occurred) then it is allowed to attempt

Best Regards,
Petr