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

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Date:	Tue, 22 Apr 2014 14:16:50 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	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 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;
 	}
--
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