linux-kernel - Re: [PATCH] printk: Skip console drivers on PREEMPT

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Message-ID: <87y1wn3g3g.fsf@jogness.linutronix.de>
Date:   Wed, 20 Jul 2022 18:32:43 +0206
From:   John Ogness <john.ogness@...utronix.de>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        linux-kernel@...r.kernel.org
Cc:     Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] printk: Skip console drivers on PREEMPT_RT.

Hi Sebastian,

On 2022-07-20, Sebastian Andrzej Siewior <bigeasy@...utronix.de> wrote:
> printk might be invoked in a context with disabled interrupts and or
> preemption and additionally disables interrupts before it invokes the
> console drivers. This is behaviour is not compatible with PREEMPT_RT.
>
> Disable console printing until the return of atomic consoles and the
> printing thread. This allows to retrieve the log buffer from user space
> which is not possible by disable printk.
>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> ---
>  kernel/printk/printk.c |   10 ++++++++++
>  1 file changed, 10 insertions(+)
>
> --- 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();

> +		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.

John Ogness