These callbacks are necessary to synchronize ->write_thread callback
against other operations using the same device.

Signed-off-by: Marcos Paulo de Souza <mpdesouza@suse.com>
---
I found this issue while creating a custom kernel module that implements
the nbcon interfaces.
---
 kernel/printk/nbcon.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/kernel/printk/nbcon.c b/kernel/printk/nbcon.c
index 3fa403f9831f..02809c44d7ea 100644
--- 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;
+
 	rcuwait_init(&con->rcuwait);
 	init_irq_work(&con->irq_work, nbcon_irq_work);
 	atomic_long_set(&ACCESS_PRIVATE(con, nbcon_prev_seq), -1UL);

---
base-commit: 322530a9c2ab1da67e5b0988fa29aca64d96ec8f
change-id: 20251205-nbcon-device-cb-fix-7ce745a21ba8

Best regards,
--  
Marcos Paulo de Souza <mpdesouza@suse.com>

On 2025-12-05, Marcos Paulo de Souza <mpdesouza@suse.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@suse.com>
> ---
> I found this issue while creating a custom kernel module that implements
> the nbcon interfaces.
> ---
>  kernel/printk/nbcon.c | 4 ++++
>  1 file changed, 4 insertions(+)
>
> diff --git a/kernel/printk/nbcon.c b/kernel/printk/nbcon.c
> index 3fa403f9831f..02809c44d7ea 100644
> --- 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;
+	}
 
 	rcuwait_init(&con->rcuwait);
 	init_irq_work(&con->irq_work, nbcon_irq_work);

John Ogness

On Mon 2025-12-08 10:21:09, John Ogness wrote:
> On 2025-12-05, Marcos Paulo de Souza <mpdesouza@suse.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@suse.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