linux-kernel - Re: [PATCH] printk/nmi: restore printk_func in nmi

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Date:	Sat, 27 Feb 2016 11:19:44 +0900
From:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:	Petr Mladek <pmladek@...e.com>
Cc:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jan Kara <jack@...e.cz>, Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: [PATCH] printk/nmi: restore printk_func in nmi_panic

Hello Petr,

On (02/26/16 15:57), Petr Mladek wrote:
> On Fri 2016-02-26 12:37:20, Sergey Senozhatsky wrote:
> > When watchdog detects a hardlockup and calls nmi_panic() `printk_func'
> > must be restored via printk_nmi_exit() call, so panic() will be able
> > to flush nmi buf and show backtrace and panic message. We also better
> > explicitly ask nmi to printk_nmi_flush() in console_flush_on_panic(),
> > because it may be too late to rely on irq work.
> > 
> > Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@...il.com>
> > ---
> >  include/linux/kernel.h | 6 ++++--
> >  kernel/printk/printk.c | 1 +
> >  2 files changed, 5 insertions(+), 2 deletions(-)
> > 
> > diff --git a/include/linux/kernel.h b/include/linux/kernel.h
> > index f4fa2b2..3ee33d5 100644
> > --- a/include/linux/kernel.h
> > +++ b/include/linux/kernel.h
> > @@ -469,10 +469,12 @@ do {									\
> >  	cpu = raw_smp_processor_id();					\
> >  	old_cpu = atomic_cmpxchg(&panic_cpu, PANIC_CPU_INVALID, cpu);	\
> >  									\
> > -	if (old_cpu == PANIC_CPU_INVALID)				\
> > +	if (old_cpu == PANIC_CPU_INVALID) {				\
> > +		printk_nmi_exit();					\
> 
> This might end up in a deadlock that printk_nmi() wanted to avoid.

aha, I see.

> I think about a compromise. We should try to get the messages
> out only when kdump is not enabled.

can we zap_locks() if we are on nmi_panic()->panic()->console_flush_on_panic() path?
console_flush_on_panic() is happening after we send out smp_send_stop().


> PS: I am sorry for not responding much about the printk problems
> this week. I have attended a training and did not have much time
> for a real work.

no prob.

	-ss