netdev - Re: [PATCH 02/19] printk: Introduce console_is

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Message-ID: <aWaCzZ8_UuyAa6xp@pathway.suse.cz>
Date: Tue, 13 Jan 2026 18:37:17 +0100
From: Petr Mladek <pmladek@...e.com>
To: Marcos Paulo de Souza <mpdesouza@...e.com>
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Subject: Re: [PATCH 02/19] printk: Introduce console_is_nbcon

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@...e.com>
> Signed-off-by: Marcos Paulo de Souza <mpdesouza@...e.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@...e.com>

Best Regards,
Petr