linux-kernel - Re: [PATCH printk v2 06/18] printk: nbcon: Add context to console_is

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Date: Thu, 13 Jun 2024 15:22:21 +0200
From: Petr Mladek <pmladek@...e.com>
To: John Ogness <john.ogness@...utronix.de>
Cc: Sergey Senozhatsky <senozhatsky@...omium.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH printk v2 06/18] printk: nbcon: Add context to
 console_is_usable()

On Tue 2024-06-04 01:30:41, John Ogness wrote:
> The nbcon consoles have two callbacks to be used for different
> contexts. In order to determine if an nbcon console is usable,
> console_is_usable() needs to know if it is a context that will
> use the write_atomic() callback or the write_thread() callback.
> 
> Add an extra parameter @use_atomic to specify this.
> 
> Signed-off-by: John Ogness <john.ogness@...utronix.de>
> ---
>  kernel/printk/internal.h | 16 ++++++++++------
>  kernel/printk/nbcon.c    |  8 ++++----
>  kernel/printk/printk.c   |  6 ++++--
>  3 files changed, 18 insertions(+), 12 deletions(-)
> 
> diff --git a/kernel/printk/internal.h b/kernel/printk/internal.h
> index 38680c6b2b39..243d3d3bc889 100644
> --- a/kernel/printk/internal.h
> +++ b/kernel/printk/internal.h
> @@ -100,7 +100,7 @@ void nbcon_kthread_create(struct console *con);
>   * which can also play a role in deciding if @con can be used to print
>   * records.
>   */
> -static inline bool console_is_usable(struct console *con, short flags)
> +static inline bool console_is_usable(struct console *con, short flags, bool use_atomic)
>  {
>  	if (!(flags & CON_ENABLED))
>  		return false;
> @@ -109,10 +109,13 @@ static inline bool console_is_usable(struct console *con, short flags)
>  		return false;
>  
>  	if (flags & CON_NBCON) {
> -		if (!con->write_atomic)
> -			return false;
> -		if (!con->write_thread)
> -			return false;
> +		if (use_atomic) {
> +			if (!con->write_atomic)
> +				return false;
> +		} else {
> +			if (!con->write_thread)
> +				return false;
> +		}
>  	} else {
>  		if (!con->write)
>  			return false;
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -4018,8 +4018,10 @@ static bool __pr_flush(struct console *con, int timeout_ms, bool reset_on_progre
>  			 * that they make forward progress, so only increment
>  			 * @diff for usable consoles.
>  			 */
> -			if (!console_is_usable(c, flags))
> +			if (!console_is_usable(c, flags, true) &&
> +			    !console_is_usable(c, flags, false)) {
>  				continue;
> +			}
>  
>  			if (flags & CON_NBCON) {
>  				printk_seq = nbcon_seq_read(c);

I wonder if it is worth it. Do we seriously want to support nbcon
without con->write_kthread() or con->write_atomic() callbacks?

For example, I see in kernel/printk/printk.c:

void register_console(struct console *newcon)
{
	bool use_device_lock = (newcon->flags & CON_NBCON) && newcon->write_atomic;


We would need to extend this check if we wanted to allow nbcon
consoles without con->nbcon_atomic()...

Best Regards,
Petr