Re: [PATCH 02/19] printk: Introduce console_is_nbcon

Petr Mladek
2026-01-13T17:38:26Z

On Sat 2025-12-27 09:16:09, Marcos Paulo de Souza wrote:
> Besides checking if the current console is NBCON or not, console->flags
> is also being read in order to serve as argument of the console_is_usable
> function.
>
> But CON_NBCON flag is unique: it's set just once in the console
> registration and never cleared. In this case it can be possible to read
> the flag when console_srcu_lock is held (which is the case when using
> for_each_console).
>
> This change makes possible to remove the flags argument from
> console_is_usable in the next patches.
>
> Signed-off-by: Petr Mladek <pmladek@suse.com>
> Signed-off-by: Marcos Paulo de Souza <mpdesouza@suse.com>
> ---
> include/linux/console.h | 27 +++++++++++++++++++++++++++
> kernel/debug/kdb/kdb_io.c | 2 +-
> kernel/printk/nbcon.c | 2 +-
> kernel/printk/printk.c | 15 ++++++---------
> 4 files changed, 35 insertions(+), 11 deletions(-)
>
> diff --git a/include/linux/console.h b/include/linux/console.h
> index 35c03fc4ed51..dd4ec7a5bff9 100644
> --- a/include/linux/console.h
> +++ b/include/linux/console.h
> @@ -561,6 +561,33 @@ static inline void console_srcu_write_flags(struct console *con, short flags)
> WRITE_ONCE(con->flags, flags);
> }
>
> +/**
> + * console_srcu_is_nbcon - Locklessly check whether the console is nbcon

There is _srcu in the function name, see below.

> + * @con: struct console pointer of console to check
> + *
> + * Requires console_srcu_read_lock to be held, which implies that @con might
> + * be a registered console. The purpose of holding console_srcu_read_lock is
> + * to guarantee that no exit/cleanup routines will run if the console
> + * is currently undergoing unregistration.
> + *
> + * If the caller is holding the console_list_lock or it is _certain_ that
> + * @con is not and will not become registered, the caller may read
> + * @con->flags directly instead.
> + *
> + * Context: Any context.
> + * Return: True when CON_NBCON flag is set.
> + */
> +static inline bool console_is_nbcon(const struct console *con)

And here it is without _srcu.

I would prefer the variant with _srcu to make it clear that it
can be called only under _srcu. Similar to console_srcu_read_flags(con).

> +{
> + WARN_ON_ONCE(!console_srcu_read_lock_is_held());
> +
> + /*
> + * The CON_NBCON flag is statically initialized and is never
> + * set or cleared at runtime.
> + */
> + return data_race(con->flags & CON_NBCON);
> +}
> +
> /* Variant of console_is_registered() when the console_list_lock is held. */
> static inline bool console_is_registered_locked(const struct console *con)
> {

Otherwise, it looks good to me.

With a consistent name, feel free to use:

Reviewed-by: Petr Mladek <pmladek@suse.com>

Best Regards,
Petr