[PATCH 02/19] printk: Introduce console_is_nbcon

[John Ogness]

On 2025-12-27, Marcos Paulo de Souza <mpdesouza at suse.com> 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.

Note that console_is_usable() now also checks for the flag
CON_NBCON_ATOMIC_UNSAFE as well.

> 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
> + * @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)
> +{
> +	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);

If this flag is statically initialized and is never set or cleared at
runtime, why is the console_srcu_read_lock required? Why not just:

static inline bool console_is_nbcon(const struct console *con)
{
        /*
	 * The CON_NBCON flag is statically initialized and is never
	 * set or cleared at runtime.
	 */
	return data_race(con->flags & CON_NBCON);
}

And even if you do need the console_srcu_read_lock, why copy/paste the
implementation and comments of console_srcu_read_flags()? Just do:

static inline bool console_is_nbcon(const struct console *con)
{
	return console_srcu_read_flags(con) & CON_NBCON;
}

John Ogness