[PATCH v2] printk: nbcon: Check for device_{lock,unlock} callbacks

[Marcos Paulo de Souza <mpdesouza@suse.com>]

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.
---
Changes in v2:
- Merged the checks device locks with write_thread check, suggested by John.
- Link to v1: https://patch.msgid.link/20251205-nbcon-device-cb-fix-v1-1-906bdff5760b@suse.com
---
 kernel/printk/nbcon.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/kernel/printk/nbcon.c b/kernel/printk/nbcon.c
index 3fa403f9831f..1dfc4a6a44bf 100644
--- a/kernel/printk/nbcon.c
+++ b/kernel/printk/nbcon.c
@@ -1760,9 +1760,12 @@ 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);

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

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