Re: [PATCH v4 1/5] printk/nmi: generic solution for safe printk in NMI

[Petr Mladek <pmladek@suse.com>]

On Mon 2016-04-04 13:49:28, Sergey Senozhatsky wrote:
> Hello,
> 
> On (03/30/16 17:53), Petr Mladek wrote:
> [..]
> > @@ -67,10 +67,12 @@ extern void irq_exit(void);
> >  		preempt_count_add(NMI_OFFSET + HARDIRQ_OFFSET);	\
> >  		rcu_nmi_enter();				\
> >  		trace_hardirq_enter();				\
> > +		printk_nmi_enter();				\
> >  	} while (0)
> >  
> >  #define nmi_exit()						\
> >  	do {							\
> > +		printk_nmi_exit();				\
> >  		trace_hardirq_exit();				\
> >  		rcu_nmi_exit();					\
> >  		BUG_ON(!in_nmi());				\
> 
> isn't it a bit too early to printk_nmi_exit()? rcu_nmi_exit() can
> WARN_ON_ONCE() in 3 places.
> 
> the same goes for printk_nmi_enter(). rcu_nmi_enter() can WARN_ON_ONCE().
> 
> seems that in both cases we can endup having WARN_ON_ONCE() from nmi,
> but with default printk function.

Great catch! You are right.


> > +/*
> > + * Flush data from the associated per_CPU buffer. The function
> > + * can be called either via IRQ work or independently.
> > + */
> > +static void __printk_nmi_flush(struct irq_work *work)
> > +{
> > +	static raw_spinlock_t read_lock =
> > +		__RAW_SPIN_LOCK_INITIALIZER(read_lock);
> > +	struct nmi_seq_buf *s = container_of(work, struct nmi_seq_buf, work);
> > +	unsigned long flags;
> > +	size_t len, size;
> > +	int i, last_i;
> > +
> > +	/*
> > +	 * The lock has two functions. First, one reader has to flush all
> > +	 * available message to make the lockless synchronization with
> > +	 * writers easier. Second, we do not want to mix messages from
> > +	 * different CPUs. This is especially important when printing
> > +	 * a backtrace.
> > +	 */
> > +	raw_spin_lock_irqsave(&read_lock, flags);
> > +
> 
> hm... so here we have
> 	for (; i < size; i++)
> 		printk()
> 
> under the spinlock. the thing is that one of printk() can end up
> in console_unlock()->call_console_drivers() loop, iterating there
> long enough to spinlock lockup other CPUs that might want to flush
> NMI buffers (if any), assuming that there are enough printk() (or
> may be a slow serial console) happening concurrently on other CPUs
> to keep the current ->read_lock busy. async printk can help here,
> but user can request sync version of printk.

I think that printk() is called on many other locations under
a spinlock and they all are waiting for the async printk.


> how about using deferred printk for nmi flush?
> print_nmi_seq_line()->printk_deferred() ?

But this is great idea. It will help to avoid the ugly macro
deferred_console_in_nmi() as you mentioned in the other mail.

Heh, I remember that I thought about this but I did not want
to override the original log level of the messages. Now, I see
that LOGLEVEL_SCHED is added on top and the original level
is preserved.

Thanks a lot for review. I am going to wait with respin a bit
and give others some time for feedback.


Best Regards,
Petr