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

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Message-ID: <20140423081525.0e9b863f@gandalf.local.home>
Date:	Wed, 23 Apr 2014 08:15:25 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:	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>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: BUG: spinlock trylock failure on UP, i.MX28 3.12.15-rt25

On Wed, 23 Apr 2014 09:14:33 +0200
Sebastian Andrzej Siewior <bigeasy@...utronix.de> 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.

Although, as Peter said FULL_NO_HZ is pretty pointless on UP, but I've
been thinking this from a non PREEMPT_RT viewpoint. In non PREEMPT_RT,
spin_locks() are nops, where this is an issue, but in PREEMPT_RT, they
are mutexes, BUT!

The rt_mutex use wait_lock to protect its internal state, which is a raw
spinlock, and in UP it's a nop. That means there's nothing preventing
an interrupt here from corrupting the rtmutex's internal state. I see
this is still an issue, and the warning is still valid.

-- Steve
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