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Mon, 08 Dec 2025 05:30:43 -0800 (PST) Date: Mon, 8 Dec 2025 14:30:41 +0100 From: Petr Mladek To: John Ogness Cc: Marcos Paulo de Souza , Steven Rostedt , Sergey Senozhatsky , linux-kernel@vger.kernel.org, marcos@mpdesouza.com Subject: Re: [PATCH] printk: nbcon: Check for device_{lock,unlock} callbacks Message-ID: References: <20251205-nbcon-device-cb-fix-v1-1-906bdff5760b@suse.com> <87ecp5wd0i.fsf@jogness.linutronix.de> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87ecp5wd0i.fsf@jogness.linutronix.de> On Mon 2025-12-08 10:21:09, John Ogness wrote: > On 2025-12-05, Marcos Paulo de Souza wrote: > > These callbacks are necessary to synchronize ->write_thread callback > > against other operations using the same device. > > > > Signed-off-by: Marcos Paulo de Souza > > --- > > 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