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

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Date:	Fri, 2 May 2014 20:38:24 +0200
From:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:	Steven Rostedt <rostedt@...dmis.org>
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

* Steven Rostedt | 2014-04-22 14:16:50 [-0400]:

>On Tue, 22 Apr 2014 13:48:02 -0400
>Steven Rostedt <rostedt@...dmis.org> wrote:
>
>> I need to take a deeper look into the actual code. But as trylocks on
>> UP are nops (always succeed), and if it expects to be able to do
>> something in a critical section that is protected by spinlocks (again
>> nops on UP), this would be broken for UP.
>
>Reading the code, I see it's broken. We should add something like this:
>
>Signed-off-by: Steven Rostedt <rostedt@...dmis.org>
>---
>diff --git a/kernel/timer.c b/kernel/timer.c
>index cc34e42..a03164a 100644
>--- 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
>+
> /*
>  * 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.

Sebastian
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