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

[Petr Mladek <pmladek@suse.cz>]

On Wed 2015-05-27 16:14:23, Andrew Morton wrote:
> On Mon, 25 May 2015 14:46:27 +0200 Petr Mladek <pmladek@suse.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?

IMHO, irq_work_queue() was primary created for exactly this job.
I mean to move some work from NMI into a more relaxed context.
See the initial commit e360adbe29241a01 ("irq_work: Add generic
hardirq context callbacks").

In each case, the code seems to be well prepared for this. Especially,
there is a big effort to manipulate work->flags lock-less way.

The only exception is irq_work_queue_on() but it seems to be related
to the particular arch_send_call_function_single_ipi(cpu) call
that is not NMI safe.

> arch/arm64/kernel/smp.c uses smp_cross_call() which might use NMI! 

smp_cross_call() is defined by set_smp_cross_call(). I see
only three functions that are assigned this way:

drivers/irqchip/irq-armada-370-xp.c:            set_smp_cross_call(armada_mpic_send_doorbell);
drivers/irqchip/irq-gic-v3.c:   set_smp_cross_call(gic_raise_softirq);
drivers/irqchip/irq-gic.c:              set_smp_cross_call(gic_raise_softirq);
drivers/irqchip/irq-hip04.c:    set_smp_cross_call(hip04_raise_softirq);

They all seems to work with soft IRQ.


> Presumably it'll call directly if the target CPU==this_cpu but I didn't
> run around and audit everything.

IMHO, this is the case of smp_call_function_single() that has the
function as a parameter. But irq_work_queue() always puts the function
into the work list and sends interrupt.

Of course, it is possible that I have missed something. I hope that
Peter or Frederic could confirm my observation.


Best Regards,
Petr