linux-kernel - Re: PREEMPT_RT (2.6.33-rt17) disabled printk-to-console after console

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Date:	Thu, 20 May 2010 13:23:07 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	"Nikita V. Youshchenko" <yoush@...msu.su>
cc:	Sujit K M <sjt.kar@...il.com>, linux-rt-users@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: PREEMPT_RT (2.6.33-rt17) disabled printk-to-console after
 console_init

On Thu, 20 May 2010, Nikita V. Youshchenko wrote:

> > > Still questions:
> > >
> > > 1) why does that prevent klogd from working?
> >
> > Patch below.
> > ...
> > @@ -1084,18 +1084,8 @@ void release_console_mutex(void)
> >  #endif
> >  	}
> >  	console_locked = 0;
> > -	raw_spin_unlock_irqrestore(&logbuf_lock, flags);
> >  	mutex_unlock(&console_mutex);
> 
> Hmm... that lock is taken inside loop body, then control goes out of loop 
> at 'break' statement, and then, if this line is deteled, lock is still 
> held at function return. Looks wrong.

Stupid me. Yes. Drop that line.
 
> > > 3) I believe that 8250 serial driver is aware of preempt-rt.
> > > Could you please comment on my "2.6.33.2-rt13: RFC: fix serial
> > > console" post to linux-rt-users list
> > > (http://eeek.borgchat.net/lists/linux-rt-users/msg05569.html)
> >
> > While that can work due to the trylock, it can introduce massive
> > latencies just in case some driver reports a status change or what
> > ever.
> 
> Isn't it better just manually disable console (with 'quiet' kernel command 
> line option) when running in production? This may be recommended in proper 
> guidelines.

No, anything which needs extra parameters for normal operation is
wrong as it affects users. 

Adding a "I'm debugging, I don't care" flag for developers is the
right thing to do.

Thanks,

	tglx

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