BUG: spinlock trylock failure on UP, i.MX28 3.12.15-rt25

[Peter Zijlstra]

On Wed, Apr 23, 2014 at 09:14:33AM +0200, Sebastian Andrzej Siewior wrote:
> On 04/22/2014 08:16 PM, Steven Rostedt wrote:
> > --- a/kernel/timer.c
> > +++ b/kernel/timer.c
> > @@ -1447,6 +1447,12 @@ static void run_timer_softirq(struct softirq_action *h)
> >  		__run_timers(base);
> >  }
> >  
> > +#ifdef CONFIG_SMP
> > +#define timer_should_raise_softirq(lock)	!spin_do_trylock(lock)
> > +#else
> > +#define timer_should_raise_softirq(lock)	1
> > +#endif
> > +
> 
> No. The lock may be taken but it also may be available no matter if UP
> or not. With this patch applied the lockdep splat will go away but the
> FULL_NO_HZ people will come back because the timer softirq is scheduled
> even if no timer has expired.

You can do FULL_NO_HZ on UP ? That seems rather pointless, as there
isn't a 'spare' CPU to do all the timekeeping on.