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

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Message-ID: <20140423084937.GH11096@twins.programming.kicks-ass.net>
Date:	Wed, 23 Apr 2014 10:49:37 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Stanislav Meduna <stano@...una.org>,
	"linux-rt-users@...r.kernel.org" <linux-rt-users@...r.kernel.org>,
	Linux ARM Kernel <linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: BUG: spinlock trylock failure on UP, i.MX28 3.12.15-rt25

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.
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