Re: [PATCH] printk: Skip console drivers on PREEMPT_RT.

From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To: John Ogness <john.ogness@linutronix.de>
Cc: linux-kernel@vger.kernel.org, Petr Mladek <pmladek@suse.com>,
	Sergey Senozhatsky <senozhatsky@chromium.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH] printk: Skip console drivers on PREEMPT_RT.
Date: Wed, 20 Jul 2022 18:35:35 +0200	[thread overview]
Message-ID: <Ytgu17hATM8iqdGC@linutronix.de> (raw)
In-Reply-To: <87y1wn3g3g.fsf@jogness.linutronix.de>

On 2022-07-20 18:32:43 [+0206], John Ogness wrote:
> Hi Sebastian,
Hi,

> > --- a/kernel/printk/printk.c
> > +++ b/kernel/printk/printk.c
> > @@ -2843,6 +2843,16 @@ void console_unlock(void)
> >  	}
> >  
> >  	/*
> > +	 * On PREEMPT_RT it is not possible to invoke console drivers with
> > +	 * disabled interrupts and or preemption. Therefore all drivers are
> > +	 * skipped and the output can be retrieved from the buffer.
> > +	 */
> > +	if (IS_ENABLED(CONFIG_PREEMPT_RT)) {
> > +		up_console_sem();
> 
> This should be:
> 
>                 __console_unlock();

Why?

> > +		return;
> > +	}
> 
> Note that if @console_may_schedule is 1, then we are in a sleepable
> context and could print. But since that is not very often, it is
> probably better to just have it off all the time as you propose.

No we can't coz printk disables interrupts before invoking the console
drivers.

> John Ogness

Sebastian