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

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Message-ID: <875x8j3xre.fsf@jogness.linutronix.de>
Date: Fri, 30 Jan 2026 16:56:45 +0106
From: John Ogness <john.ogness@...utronix.de>
To: Marcos Paulo de Souza <mpdesouza@...e.com>, Richard Weinberger
 <richard@....at>, Anton Ivanov <anton.ivanov@...bridgegreys.com>, Johannes
 Berg <johannes@...solutions.net>, Greg Kroah-Hartman
 <gregkh@...uxfoundation.org>, Jason Wessel <jason.wessel@...driver.com>,
 Daniel Thompson <danielt@...nel.org>, Douglas Anderson
 <dianders@...omium.org>, Petr Mladek <pmladek@...e.com>, Steven Rostedt
 <rostedt@...dmis.org>, Sergey Senozhatsky <senozhatsky@...omium.org>, Jiri
 Slaby <jirislaby@...nel.org>, Breno Leitao <leitao@...ian.org>, Andrew
 Lunn <andrew+netdev@...n.ch>, "David S. Miller" <davem@...emloft.net>,
 Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
 Paolo Abeni <pabeni@...hat.com>, Geert Uytterhoeven
 <geert@...ux-m68k.org>, Kees Cook <kees@...nel.org>, Tony Luck
 <tony.luck@...el.com>, "Guilherme G. Piccoli" <gpiccoli@...lia.com>,
 Madhavan Srinivasan <maddy@...ux.ibm.com>, Michael Ellerman
 <mpe@...erman.id.au>, Nicholas Piggin <npiggin@...il.com>, Christophe
 Leroy <christophe.leroy@...roup.eu>, Andreas Larsson
 <andreas@...sler.com>, Alexander Shishkin
 <alexander.shishkin@...ux.intel.com>, Maxime Coquelin
 <mcoquelin.stm32@...il.com>, Alexandre Torgue
 <alexandre.torgue@...s.st.com>, Jacky Huang <ychuang3@...oton.com>,
 Shan-Chun Hung <schung@...oton.com>, Laurentiu Tudor
 <laurentiu.tudor@....com>
Cc: linux-um@...ts.infradead.org, linux-kernel@...r.kernel.org,
 kgdb-bugreport@...ts.sourceforge.net, linux-serial@...r.kernel.org,
 netdev@...r.kernel.org, linux-m68k@...ts.linux-m68k.org,
 linux-hardening@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
 sparclinux@...r.kernel.org, linux-stm32@...md-mailman.stormreply.com,
 linux-arm-kernel@...ts.infradead.org, linux-fsdevel@...r.kernel.org,
 Marcos Paulo de Souza <mpdesouza@...e.com>
Subject: Re: [PATCH 02/19] printk: Introduce console_is_nbcon

On 2025-12-27, Marcos Paulo de Souza <mpdesouza@...e.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