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

From: Sebastian Andrzej Siewior
Date: Fri May 27 2016 - 10:56:10 EST

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

>On Fri, 27 May 2016 15:58:12 +0200
>Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx>
>> ---
>> 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 (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 flagâ 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
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).