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

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Date:	Fri, 26 Feb 2016 15:57:22 +0100
From:	Petr Mladek <pmladek@...e.com>
To:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:	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

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.

I think about a compromise. We should try to get the messages
out only when kdump is not enabled. If the kdump is enabled
we should avoid the risk a the deadlock here and let people
to find the messages in the dump.

I am going to play with this a bit.

Best Regards,
Petr

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.