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

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Date:   Wed, 20 Jul 2022 19:56:01 +0206
From:   John Ogness <john.ogness@...utronix.de>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     linux-kernel@...r.kernel.org, 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.

On 2022-07-20, Sebastian Andrzej Siewior <bigeasy@...utronix.de> wrote:
>> > --- 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?

Because that is the official function that performs the console
unlocking. With upcoming patches, that function does more. But even
without those patches, by directly calling up_console_sem() you are
leaving @console_locked set. The @console_locked variable is used by the
vt code to validate correct console locking.

I realize you are just copy/pasting from the condition above (checking
@console_suspended), but that condition is special. When consoles are
suspended, we still allow @console_sem to be locked/unlocked, but
without changing the locked status of the console. See suspend_console()
and resume_console() if you really want to see the gory details.

John