[PATCH RT 2/4] timer: do not spin_trylock() on UP

[Steven Rostedt <rostedt@goodmis.org>]

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3.12.22-rt35-rc1 stable review patch.
If anyone has any objections, please let me know.

------------------

From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

This will void a warning comming from the spin-lock debugging code. The
lock avoiding idea is from Steven Rostedt.

Cc: stable-rt@vger.kernel.org
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
---
 kernel/timer.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/kernel/timer.c b/kernel/timer.c
index 611a5973d9e1..3b79da2f9c67 100644
--- a/kernel/timer.c
+++ b/kernel/timer.c
@@ -1461,6 +1461,19 @@ void run_local_timers(void)
 	 * the timer softirq.
 	 */
 #ifdef CONFIG_PREEMPT_RT_FULL
+
+#ifndef CONFIG_SMP
+	/*
+	 * The spin_do_trylock() later may fail as the lock may be hold before
+	 * the interrupt arrived. The spin-lock debugging code will raise a
+	 * warning if the try_lock fails on UP. Since this is only an
+	 * optimization for the FULL_NO_HZ case (not to run the timer softirq on
+	 * an nohz_full CPU) we don't really care and shedule the softirq.
+	 */
+	raise_softirq(TIMER_SOFTIRQ);
+	return;
+#endif
+
 	/* On RT, irq work runs from softirq */
 	if (irq_work_needs_cpu()) {
 		raise_softirq(TIMER_SOFTIRQ);
-- 
2.0.0