linux-kernel - Re: [RFC][PATCH 1/2] printk: lock console_sem before we unregister boot consoles

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Date:   Wed, 24 Apr 2019 16:49:24 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: [RFC][PATCH 1/2] printk: lock console_sem before we unregister
 boot consoles

On Tue 2019-04-23 15:25:10, Sergey Senozhatsky wrote:
> The following pattern is not completely safe:
> 
>      for_each_console(bcon)
>          if (bcon->flags & CON_BOOT)
>              unregister_console(bcon);
> 
> Because, in theory, console drivers list and console drivers
> can be modified concurrently from another CPU. Take console_sem
> lock, which protects console drivers list and console drivers,
> before we start iterating console drivers list.
> 
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index 17102fd4c136..b0e361ca1bea 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -2777,62 +2819,34 @@ void register_console(struct console *newcon)
>  	pr_info("%sconsole [%s%d] enabled\n",
>  		(newcon->flags & CON_BOOT) ? "boot" : "" ,
>  		newcon->name, newcon->index);
> -	if (bcon &&
> -	    ((newcon->flags & (CON_CONSDEV | CON_BOOT)) == CON_CONSDEV) &&
> -	    !keep_bootcon) {
> -		/* We need to iterate through all boot consoles, to make
> +
> +	if (keep_bootcon)
> +		return;
> +
> +	if (bcon && (newcon->flags & (CON_CONSDEV|CON_BOOT)) == CON_CONSDEV) {
> +		console_lock();
> +		/*
> +		 * We need to iterate through all boot consoles, to make
>  		 * sure we print everything out, before we unregister them.
>  		 */

I wondered if moving the console locking could break the above
statement.

It seems that the comment has been invalid since the commit
8259cf4342029aad37660e ("printk: Ensure that "console
enabled" messages are printed on the console").

Could we remove it in this patch? It touches it indirectly anyway.

Otherwise, the patch looks fine to me:

Reviewed-by: Petr Mladek <pmladek@...e.com>

>  		for_each_console(bcon)
>  			if (bcon->flags & CON_BOOT)
> -				unregister_console(bcon);
> +				__unregister_console(bcon);
> +		console_unlock();
> +		console_sysfs_notify();
>  	}
>  }
>  EXPORT_SYMBOL(register_console);

Best Regards,
Petr