Re: [PATCH v2] printk/console: Enable console kthreads only when there is no boot console left

[Sergey Senozhatsky <senozhatsky@chromium.org>]

On (22/06/21 11:09), Petr Mladek wrote:
> Threaded console printing does not take into consideration that boot
> consoles may be accessing the same hardware as normal consoles and thus
> must not be called in parallel.
> 
> Since it is currently not possible to identify which consoles are
> accessing the same hardware, delay threaded console printing activation
> until it is known that there are no boot consoles registered.
> 
> Link: https://lore.kernel.org/r/20220619204949.50d9154d@thinkpad
> Link: https://lore.kernel.org/r/2a82eae7-a256-f70c-fd82-4e510750906e@samsung.com
> Link: https://lore.kernel.org/r/20220619204949.50d9154d@thinkpad
> Reported-by: Marek Behún <kabel@kernel.org>
> [john.ogness@linutronix.de: Better description of the problem.]
> Signed-off-by: Petr Mladek <pmladek@suse.com>
> Tested-by: Marek Behún <kabel@kernel.org>

Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>

[..]
> +static int __init printk_activate_kthreads(void)
> +{
> +	struct console *con;
> +
> +	console_lock();
> +	printk_kthreads_available = true;
> +	for_each_console(con)
> +		printk_start_kthread(con);
> +	console_unlock();
> +
> +	return 0;
> +}
> +
>  /*
>   * Some boot consoles access data that is in the init section and which will
>   * be discarded after the initcalls have been run. To make sure that no code
> @@ -3567,6 +3580,7 @@ void __init console_init(void)
>   */
>  static int __init printk_late_init(void)
>  {
> +	bool no_bootcon = true;
>  	struct console *con;
>  	int ret;
>  
> @@ -3588,7 +3602,10 @@ static int __init printk_late_init(void)
>  			pr_warn("bootconsole [%s%d] uses init memory and must be disabled even before the real one is ready\n",
>  				con->name, con->index);
>  			unregister_console(con);
> +			continue;
>  		}
> +
> +		no_bootcon = false;
>  	}
>  	ret = cpuhp_setup_state_nocalls(CPUHP_PRINTK_DEAD, "printk:dead", NULL,
>  					console_cpu_notify);
> @@ -3597,23 +3614,19 @@ static int __init printk_late_init(void)
>  					console_cpu_notify, NULL);
>  	WARN_ON(ret < 0);
>  	printk_sysctl_init();
> -	return 0;
> -}
> -late_initcall(printk_late_init);
> -
> -static int __init printk_activate_kthreads(void)
> -{
> -	struct console *con;
>  
> -	console_lock();
> -	printk_kthreads_available = true;
> -	for_each_console(con)
> -		printk_start_kthread(con);
> -	console_unlock();
> +	/*
> +	 * Boot consoles may be accessing the same hardware as normal
> +	 * consoles and thus must not be called in parallel. Therefore
> +	 * only activate threaded console printing if it is known that
> +	 * there are no boot consoles registered.
> +	 */
> +	if (no_bootcon)
> +		printk_activate_kthreads();

A quick question. Here we still can have bootcon which can unregistered
later, right? Do you think it'll make sense to check if printing kthreads
can be safely started and start them if so (if no CON_BOOT found and kthreads
are not already created) at the end of unregister_console()?