linux-kernel - Re: [PATCHv2 3/4] printk: factor out register

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Message-ID: <20190523065144.GA18333@jagdpanzerIV>
Date:   Thu, 23 May 2019 15:51:44 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: [PATCHv2 3/4] printk: factor out register_console() code

Hello,

On (05/15/19 16:36), Petr Mladek wrote:
[..]
> >  
> >  	console_unlock();
> >  	console_sysfs_notify();
> > +	console_lock();
> 
> I have got an idea how to get rid of this weirdness:
>
> 1. The check for bcon seems to be just an optimization. There is not need
>    to remove boot consoles when there are none.
> 
> 2. The condition (newcon->flags & (CON_CONSDEV|CON_BOOT)) == CON_CONSDEV)
>    is valid only when the preferred console was really added.
> 
> Therefore we could move the code to a separate function, e.g.
> 
> void unregister_boot_consoles(void)
> {
> 	struct console *bcon;
> 
> 	console_lock();
> 	for_each_console(bcon)
> 		if (bcon->flags & CON_BOOT)
> 			__unregister_console(bcon);
> 	}
> 	console_unlock();
> 	console_sysfs_notify();
> }
> 
> Then we could do something like:
> 
> void register_console(struct console *newcon)
> {
> 	bool newcon_is_preferred = false;
> 
> 	console_lock();
> 	__register_console(newcon);
> 	if ((newcon->flags & (CON_CONSDEV|CON_BOOT)) == CON_CONSDEV)
> 		newcon_is_preferred = true;
> 	console_unlock();
> 	console_sysfs_notify();
> 
> 	/*
> 	 * By unregistering the bootconsoles after we enable the real console
> 	 * we get the "console xxx enabled" message on all the consoles -
> 	 * boot consoles, real consoles, etc - this is to ensure that end
> 	 * users know there might be something in the kernel's log buffer that
> 	 * went to the bootconsole (that they do not see on the real console)
> 	 */
> 	if (newcon_is_preferred && !keep_bootcon)
> 		unregister_boot_consoles();
> }
> 
> How does that sound?

Hmm, may be I'm missing something. I think that the 'weirdness'
is still needed. This

	console_lock();
	__unregister_console(bcon);  // pr_info("%sconsole disabled\n")
	console_unlock();

is going to change the visible behaviour - we need to show
pr_info("%sconsole [%s%d] disabled\n") on all consoles, especially
on the console which we are disabling. Who knows, maybe that's the
last remaining properly working console. Doing __unregister_console()
under console_sem will end up in a lost/missing message on bcon (or
on any other console we are unregistering).

	-ss