linux-kernel - Re: [PATCH printk 06/18] printk: Protect [un]register

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Message-ID: <YzvgtQLLTKQEXlnD@alley>
Date:   Tue, 4 Oct 2022 09:28:53 +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,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH printk 06/18] printk: Protect [un]register_console() with
 a mutex

On Mon 2022-10-03 21:41:22, John Ogness wrote:
> On 2022-10-03, Petr Mladek <pmladek@...e.com> wrote:
> > What is exactly wrong with console_lock, please?
> 
> It is ambiguously performing multiple tasks:
> 
> - protecting the console list
> - protecting individual console fields
> - serializing console printing
> - stopping all console printing
> 
> And the answer to that is: A BKL is preventing us from optimizing the
> kernel by decoupling unrelated activities.
>
> > The above proposal suggests that it might be something like:
> >
> > register_console()
> > {
> > 	console_list_lock();
> >
> > 	if (!need_console())
> > 		goto out;
> >
> > 	if (!try_enable_console())
> > 		goto out;
> >
> > 	if (!(con->flags & CON_NOBLK))
> > 		console_lock()
> 
> Why are you taking the console_lock here? The console_list_lock needs to
> replace this responsibility. I realize the RFC and this v1 series does
> not do this. For v2, it will be clear.

This is the important information that I missed. It is a great idea.
I agree that console_list_lock() would be a step forward if this worked.

As you say, in the RFC and this v1, console_lock() was still used
to synchronize the list and the metadata manipulation. It means that
console_lock() was as complex as before. In fact, it was even
more complex because console_list_lock() appeared in its lock
dependency chains. And it was not clear that v2 would be
any different in this regard.

Best Regards,
Petr