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

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Date:	Fri, 2 May 2014 21:36:15 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
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>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: BUG: spinlock trylock failure on UP, i.MX28 3.12.15-rt25

On Fri, 2 May 2014, Thomas Gleixner wrote:
> On Fri, 2 May 2014, Sebastian Andrzej Siewior wrote:
> > * Steven Rostedt | 2014-04-22 14:16:50 [-0400]:
> > > /*
> > >  * Called by the local, per-CPU timer interrupt on SMP.
> > >  */
> > >@@ -1467,7 +1473,7 @@ void run_local_timers(void)
> > > 		return;
> > > 	}
> > > 
> > >-	if (!spin_do_trylock(&base->lock)) {
> > >+	if (timer_should_raise_softirq(&base->lock)) {
> > > 		raise_softirq(TIMER_SOFTIRQ);
> > > 		return;
> > > 	}
> > 
> > Okay. So Peter said that it is okay to apply this since FULL_NO_HZ users
> > wouldn't complain on UP. I still wouldn't say it is broken but that is a
> > different story.
> > We have two users of this trylock. run_local_timers() which pops up
> > quite often (and you patched here) and the other is
> > get_next_timer_interrupt(). What do you suggest we do here? It is
> > basically the same thing.
> 
> It's different as it CANNOT fail on UP. That's called from the idle
> code and there is no way that anything holds that lock on UP when idle
> runs.

So yeah, you are right, that it's called from irq_exit() so it needs
an annotation at least. Maybe it's really cleaner to make it #if SMP
as well just for clarity raisins.

Thanks,

	tglx
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