Re: [PATCH] m68k: nfcon: don't call console_is_registered in nfcon_device

[John Ogness <john.ogness@linutronix.de>]

On 2026-08-20, Petr Mladek <pmladek@suse.com> wrote:
>> > --- a/arch/m68k/emu/nfcon.c
>> > +++ b/arch/m68k/emu/nfcon.c
>> > @@ -49,7 +49,7 @@ static void nfcon_write(struct console *con, const char *str,
>> >  static struct tty_driver *nfcon_device(struct console *con, int *index)
>> >  {
>> >         *index = 0;
>> > -       return console_is_registered(con) ? nfcon_tty_driver : NULL;
>> 
>> Interestingly, that call to console_is_registered() was added in
>> commit de61a1a3a0830710 ("tty: nfcon: use console_is_registered()"),
>> i.e. in the same series that added the extra locking...
>
> Sigh, we clearly did not check the callers of this function properly
> at that time :/
>
>> > +       return nfcon_tty_driver;
>
> Honestly, I do not feel comfortable with removing the check
> completely.
>
> The commit de61a1a3a083071 ("tty: nfcon: use console_is_registered()"
> used console_is_registered() instead of (con->flags & CON_ENABLED).
> And CON_ENABLED is historically cleared when the console is suspended,
> see console_suspend().

Since the NatFeat console driver never implemented suspend/resume
callbacks, I assume it was intended to identify if the console was
registered.

> Is is OK to return a valid struct tty_driver for a non-registered
> or suspended console for all con->device() callers?
>
> I have found only 4 callers:
>
>   $> git grep "\->device("
>   drivers/tty/serial/kgdboc.c:            if (cons->device && cons->device(cons, &idx) == p &&
>   drivers/tty/tty_io.c:           struct tty_driver *drv = cs[i]->device(cs[i], &index);
>   fs/proc/consoles.c:             driver = con->device(con, &index);
>   kernel/printk/printk.c:         driver = c->device(c, index);
>
> These are:
>
>   + configure_kgdboc() in drivers/tty/serial/kgdboc.c
>   + show_cons_active() in drivers/tty/tty_io.c
>   + show_console_dev() in fs/proc/consoles.c
>   + console_device() in kernel/printk/printk.c
>
> , where
>
>   + configure_kgdboc() is used during the system initialization
>     when the console should not be suspended
>
>   + show_cons_active() and show_console_dev() are questionable.
>     Maybe, we should not advertise the device to the userspace
>     when it is suspended.
>
>   + console_device() is used in tty_kopen() -> tty_lookup_driver().
>     Also here we likely should not allow to open a suspended device.

All of these callbacks are done when the console is guaranteed to be
registered (or not yet fully unregistered).

> Let's take a step back:
>
> We should define the locking context for con->device() callback.
> It seems that it is called under console_lock() and
> console_srcu_read_lock() in:
>
>   + configure_kgdboc()
>   + console_device()
>
> and under console_list_lock() and console_lock() in
>
>   + show_console_dev()
>   + show_cons_active()
>
> I would put aside the console_lock(). We are trying to obsolete it.
> So it is down to
>
>    console_srcu_read_lock() vs console_list_lock()
>
> Both of them look reasonable. So, I think about adding a variant
> of console_is_registered_srcu_read_locked() which would work
> with both locking. Something like:
>
> diff --git a/arch/m68k/emu/nfcon.c b/arch/m68k/emu/nfcon.c
> index d41260672e24..d211b6c4da26 100644
> --- a/arch/m68k/emu/nfcon.c
> +++ b/arch/m68k/emu/nfcon.c
> @@ -49,7 +49,7 @@ static void nfcon_write(struct console *con, const char *str,
>  static struct tty_driver *nfcon_device(struct console *con, int *index)
>  {
>  	*index = 0;
> -	return console_is_registered(con) ? nfcon_tty_driver : NULL;
> +	return console_is_registered_srcu_read_locked(con) ? nfcon_tty_driver : NULL;
>  }
>  
>  static struct console nf_console = {
> diff --git a/include/linux/console.h b/include/linux/console.h
> index d624200cfc17..b90527db4eca 100644
> --- a/include/linux/console.h
> +++ b/include/linux/console.h
> @@ -311,7 +311,7 @@ struct nbcon_write_context {
>   * @name:		The name of the console driver
>   * @write:		Legacy write callback to output messages (Optional)
>   * @read:		Read callback for console input (Optional)
> - * @device:		The underlying TTY device driver (Optional)
> + * @device:		The underlying TTY device driver (Optional) [1]
>   * @unblank:		Callback to unblank the console (Optional)
>   * @setup:		Callback for initializing the console (Optional)
>   * @exit:		Callback for teardown of the console (Optional)
> @@ -334,6 +334,9 @@ struct nbcon_write_context {
>   * @kthread:		Printer kthread for this console
>   * @rcuwait:		RCU-safe wait object for @kthread waking
>   * @irq_work:		Defer @kthread waking to IRQ work context
> + *
> + * [1] The @device callback must be called under either console_list_lock()
> + *     or console_srcu_read_lock().
>   */
>  struct console {
>  	char			name[16];
> @@ -473,8 +476,13 @@ struct console {
>  };
>  
>  #ifdef CONFIG_LOCKDEP
> +extern bool console_list_lock_is_held(void);
>  extern void lockdep_assert_console_list_lock_held(void);
>  #else
> +static inline bool console_list_lock_is_held(void)
> +{
> +	return 1;
> +}
>  static inline void lockdep_assert_console_list_lock_held(void)
>  {
>  }
> @@ -555,6 +563,16 @@ static inline bool console_is_registered_locked(const struct console *con)
>  	return !hlist_unhashed(&con->node);
>  }
>  
> +/*
> + * Variant of console_is_registered() when either the console_srcu_read_lock
> + * or console_list_lock is held.
> +*/
> +static inline bool console_is_registered_srcu_read_locked(const struct console *con)
> +{
> +	lockdep_assert(console_srcu_read_lock_is_held() || console_list_lock_is_held());
> +	return !hlist_unhashed(&con->node);
> +}
> +
>  /*
>   * console_is_registered - Check if the console is registered
>   * @con:	struct console pointer of console to check
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index 2fe9a963c823..9b632b91f096 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -109,6 +109,12 @@ static struct lockdep_map console_lock_dep_map = {
>  	.name = "console_lock"
>  };
>  
> +bool console_list_lock_is_held(void)
> +{
> +	return lockdep_is_held(&console_mutex);
> +}
> +EXPORT_SYMBOL(console_list_lock_is_held);
> +
>  void lockdep_assert_console_list_lock_held(void)
>  {
>  	lockdep_assert_held(&console_mutex);
>
>
> Alternative solution would be to require the srcu locking.
> IMHO, it should be perfectly fine to use the srcu_read_lock in
> both show_console_dev() and show_cons_active().
>
> What do you think, please?
> John?

I do not think any of that is necessary. I think it is enough to allow
the ->device() callback to assume it is registered. I see no reason why
the _callback_ should ever need to take the console_list_lock or the
console_srcu_read_lock.

John