linux-kernel - Re: [PATCH] printk: nbcon: Check for device

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Message-ID: <aTbTAfJtVeOgtHAr@pathway>
Date: Mon, 8 Dec 2025 14:30:41 +0100
From: Petr Mladek <pmladek@...e.com>
To: John Ogness <john.ogness@...utronix.de>
Cc: Marcos Paulo de Souza <mpdesouza@...e.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Sergey Senozhatsky <senozhatsky@...omium.org>,
	linux-kernel@...r.kernel.org, marcos@...esouza.com
Subject: Re: [PATCH] printk: nbcon: Check for device_{lock,unlock} callbacks

On Mon 2025-12-08 10:21:09, John Ogness wrote:
> On 2025-12-05, Marcos Paulo de Souza <mpdesouza@...e.com> wrote:
> > These callbacks are necessary to synchronize ->write_thread callback
> > against other operations using the same device.
> >
> > Signed-off-by: Marcos Paulo de Souza <mpdesouza@...e.com>
> > ---
> > I found this issue while creating a custom kernel module that implements
> > the nbcon interfaces.

Great catch!

> > --- a/kernel/printk/nbcon.c
> > +++ b/kernel/printk/nbcon.c
> > @@ -1764,6 +1764,10 @@ bool nbcon_alloc(struct console *con)
> >  	if (WARN_ON(!con->write_thread))
> >  		return false;
> >  
> > +	/* The device_lock() and device_unlock() callbacks are mandatory. */
> > +	if (WARN_ON(!con->device_lock || !con->device_unlock))
> > +		return false;
> > +
> 
> The code itself is fine and this is an important contribution. But I
> would prefer to combine it with the previous mandatory check. So it
> looks more like this:
> 
> diff --git a/kernel/printk/nbcon.c b/kernel/printk/nbcon.c
> index 3fa403f9831f..f096282b0625 100644
> --- a/kernel/printk/nbcon.c
> +++ b/kernel/printk/nbcon.c
> @@ -1760,9 +1758,12 @@ bool nbcon_alloc(struct console *con)
>  	/* Synchronize the kthread start. */
>  	lockdep_assert_console_list_lock_held();
>  
> -	/* The write_thread() callback is mandatory. */
> -	if (WARN_ON(!con->write_thread))
> +	/* Check for mandatory nbcon callbacks. */
> +	if (WARN_ON(!con->write_thread ||
> +		    !con->device_lock ||
> +		    !con->device_unlock)) {
>  		return false;
> +	}

I agree that this looks better. ;-)

>  
>  	rcuwait_init(&con->rcuwait);
>  	init_irq_work(&con->irq_work, nbcon_irq_work);

Best Regards,
Petr