Re: [GIT PULL] printk for 5.20

From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Petr Mladek <pmladek@suse.com>,
	Sergey Senozhatsky <senozhatsky@chromium.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	John Ogness <john.ogness@linutronix.de>,
	Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
	Rasmus Villemoes <linux@rasmusvillemoes.dk>,
	Thomas Gleixner <tglx@linutronix.de>, Jan Kara <jack@suse.cz>,
	Peter Zijlstra <peterz@infradead.org>,
	linux-kernel@vger.kernel.org
Subject: Re: [GIT PULL] printk for 5.20
Date: Wed, 3 Aug 2022 18:34:30 +0200	[thread overview]
Message-ID: <YuqjloJDmXl7YQkI@linutronix.de> (raw)
In-Reply-To: <CAHk-=wj0umHOtL1TB_7Tvinoqhxz6BnL-jEC0eQfVihHOmbWjw@mail.gmail.com>

On 2022-08-03 09:08:07 [-0700], Linus Torvalds wrote:
> But thinking some more about it, even the "serious RT" people almost
> certainly don't really want some kind of static "disable it all". Not
> even if it was a separate Kconfig question like I suggested.
> 
> You'd most likely want it to be dynamic, because things like "log to
> console" is different at bootup when the system hasn't started yet -
> you can't really have realtime response when your hardware hasn't even
> initialized yet - and when things are actually running.
> 
> So I think even then you really just want a "turn off console logging"
> dynamic flag, not a Kconfig option.

Having such a flag would mean we have to set it starting with
SYSTEM_SCHEDULING because otherwise we risk a deadlock once the console
driver tries to acquire its lock. The printk code invokes the console
drivers with disabled interrupts and the console driver use spinlock_t
locking which become sleeping locks on PREEMPT_RT and must not be
acquired with disabled interrupts.
The long term solution is to have a printing thread for the console
driver and the atomic console support to print directly in an emergency
case.

> Which I think we already have, in the form of log levels. No?
> 
>           Linus

Sebastian