linux-kernel - Re: [PATCH RT] kernel/printk: Don't try to print from IRQ/NMI region

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Date:	Fri, 27 May 2016 16:56:01 +0200
From:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	linux-rt-users@...r.kernel.org, linux-kernel@...r.kernel.org,
	tglx@...utronix.de
Subject: Re: [PATCH RT] kernel/printk: Don't try to print from IRQ/NMI region

* Steven Rostedt | 2016-05-27 10:12:51 [-0400]:

>On Fri, 27 May 2016 15:58:12 +0200
>Sebastian Andrzej Siewior <bigeasy@...utronix.de> wrote:
>
>> On -RT we try to acquire sleeping locks which might lead to warnings
>> from lockdep or a warn_on() from spin_try_lock() (which is a rtmutex on
>> RT).
>> We don't print in general from a IRQ off region so we should not try
>> this via console_unblank() / bust_spinlocks() as well.
>> 
>> 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
>> @@ -1502,6 +1502,11 @@ static void call_console_drivers(int lev
>>  	if (!console_drivers)
>>  		return;
>>  
>> +	if (IS_ENABLED(CONFIG_PREEMPT_RT_BASE)) {
>> +		if (in_irq() || in_nmi())
>> +			return;
>> +	}
>> +
>
>We use to have a patch where a console could flag itself as atomic.
>That is, that it doesn't call any sleeping locks. What happened to that.
>
>IIRC, the video console was one such console. Otherwise, we lose out on
>backtraces in irq context.

video, like VT?
The call_console_drivers() does not check such a flag and I don't
remember about removing something like that. I was actually thinking
about adding such a flag… I remember you had something in your tree to
print from IRQ off regions via UART.

We don't print from a context with interrupts disabled or even a preempt
disabled region. In such cases we just wake_up_klogd() and print it
later.
The reason for the patch was the HW/SW lockup detector which comes with
interrupts disabled or NMI context and gets to the console(s). Usually
we defer everything until later except in this case. And I got busted
later in the VT code (and the problem was that one sleeping while atomic
warning which led to more stuff to be printed).

Sebastian