Re: [PATCH 02/10] printk: Try harder to get logbuf_lock on NMI

[Andrew Morton <akpm@linux-foundation.org>]

On Fri, 29 May 2015 12:56:07 +0200 Petr Mladek <pmladek@suse.cz> wrote:

> > > +	/* try hard to get the lock but do not wait forever */
> > > +	start_time = cpu_clock(this_cpu);
> > > +	current_time = start_time;
> > > +	while (current_time - start_time < TRY_LOCKBUF_LOCK_MAX_DELAY_NS) {
> > > +		if (raw_spin_trylock(&logbuf_lock))
> > > +			return 1;
> > > +		cpu_relax();
> > > +		current_time = cpu_clock(this_cpu);
> > > +	}
> > > +
> > > +	return 0;
> > > +}
> > 
> > That CPU is now going to spin around for 100us and then time out.
> 
> Yes, there was a deadlock without the patch. So, limited spinning is
> still a win.
> 
> Or would you like to detect the deadlock immediately in all cases?
> I mean to add the proposed wrapper around take/release lock calls
> and set/test some cpu-specific variable there?

Yes.  Pointlessly spinning in NMI for 100us is bad.

> It sounds interesting. Well, the detection will not be 100% correct
> because there is a small race window between taking @logbuf_lock
> and setting @lockbuf_cpu. I wonder if it is worth doing. But I will
> do it if you want.

You might be able to do something with checking logbuf_cpu within the
loop to avoid the worst-case scenarios.