linux-kernel - Re: [PATCH 04/10] printk: Merge and flush NMI buffer predictably via IRQ work

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Date:	Wed, 27 May 2015 16:14:23 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Petr Mladek <pmladek@...e.cz>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Dave Anderson <anderson@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Kay Sievers <kay@...y.org>, Jiri Kosina <jkosina@...e.cz>,
	Michal Hocko <mhocko@...e.cz>, Jan Kara <jack@...e.cz>,
	linux-kernel@...r.kernel.org, Wang Long <long.wanglong@...wei.com>,
	peifeiyue@...wei.com, dzickus@...hat.com, morgan.wang@...wei.com,
	sasha.levin@...cle.com
Subject: Re: [PATCH 04/10] printk: Merge and flush NMI buffer predictably
 via IRQ work

On Mon, 25 May 2015 14:46:27 +0200 Petr Mladek <pmladek@...e.cz> wrote:

> It might take ages until users see messages from NMI context. They cannot
> be flushed to the console because the operation involves taking and
> releasing a bunch of locks. Everything gets fixed by the followup printk
> in normal context but it is not predictable.
> 
> The same problem has printk_sched() and this patch reuses the existing
> solution.
> 
> There is no special printk() variant for NMI context. Hence the IRQ work
> need to get queued from vprintk_emit().
> 
> ...
>
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -1554,9 +1554,6 @@ int printk_deferred(const char *fmt, ...)
>  	va_start(args, fmt);
>  	r = vprintk_emit(0, LOGLEVEL_SCHED, NULL, 0, fmt, args);
>  	va_end(args);
> -
> -	__this_cpu_or(printk_pending, PRINTK_PENDING_OUTPUT);
> -	irq_work_queue(this_cpu_ptr(&wake_up_klogd_work));
>  	preempt_enable();
>  
>  	return r;
> @@ -1880,7 +1877,10 @@ asmlinkage int vprintk_emit(int facility, int level,
>  	 * If called from the scheduler or NMI context, we can not get console
>  	 * without a possible deadlock.
>  	 */
> -	if (!in_sched && !in_nmi()) {
> +	if (in_sched || in_nmi()) {
> +		__this_cpu_or(printk_pending, PRINTK_PENDING_OUTPUT);
> +		irq_work_queue(this_cpu_ptr(&wake_up_klogd_work));
> +	} else {

This looks hairy.  Why irq_work_queue() OK to call from NMI? 
arch/arm64/kernel/smp.c uses smp_cross_call() which might use NMI! 
Presumably it'll call directly if the target CPU==this_cpu but I didn't
run around and audit everything.

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