timer: Further simplify the SMP and HOTPLUG logic

Remove one CONFIG_HOTPLUG_CPU #ifdef in trade for introducing one
CONFIG_SMP #ifdef.

The CONFIG_SMP ifdef avoids declaring the per-CPU __tvec_bases storage
on UP systems since they already have boot_tvec_bases.

Also (re)add a runtime check on the base alignment -- for the paranoid
amongst us :-)

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/fdd2d35e169bdc554ffa3fe77f77716298c75ada.1427814611.git.viresh.kumar@linaro.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/time/timer.c

@@ -101,7 +101,6 @@ struct tvec_base {
  */
 struct tvec_base boot_tvec_bases;
 EXPORT_SYMBOL(boot_tvec_bases);
-static DEFINE_PER_CPU(struct tvec_base, __tvec_bases);
 
 static DEFINE_PER_CPU(struct tvec_base *, tvec_bases) = &boot_tvec_bases;
 
@@ -1038,6 +1037,8 @@ int try_to_del_timer_sync(struct timer_list *timer)
 EXPORT_SYMBOL(try_to_del_timer_sync);
 
 #ifdef CONFIG_SMP
+static DEFINE_PER_CPU(struct tvec_base, __tvec_bases);
+
 /**
  * del_timer_sync - deactivate a timer and wait for the handler to finish.
  * @timer: the timer to be deactivated
@@ -1591,12 +1592,10 @@ static void migrate_timers(int cpu)
 	spin_unlock_irq(&new_base->lock);
 	put_cpu_var(tvec_bases);
 }
-#endif /* CONFIG_HOTPLUG_CPU */
 
 static int timer_cpu_notify(struct notifier_block *self,
 				unsigned long action, void *hcpu)
 {
-#ifdef CONFIG_HOTPLUG_CPU
 	switch (action) {
 	case CPU_DEAD:
 	case CPU_DEAD_FROZEN:
@@ -1605,18 +1604,24 @@ static int timer_cpu_notify(struct notifier_block *self,
 	default:
 		break;
 	}
-#endif
+
 	return NOTIFY_OK;
 }
 
-static struct notifier_block timers_nb = {
-	.notifier_call	= timer_cpu_notify,
-};
+static inline void timer_register_cpu_notifier(void)
+{
+	cpu_notifier(timer_cpu_notify, 0);
+}
+#else
+static inline void timer_register_cpu_notifier(void) { }
+#endif /* CONFIG_HOTPLUG_CPU */
 
 static void __init init_timer_cpu(struct tvec_base *base, int cpu)
 {
 	int j;
 
+	BUG_ON(base != tbase_get_base(base));
+
 	base->cpu = cpu;
 	per_cpu(tvec_bases, cpu) = base;
 	spin_lock_init(&base->lock);
@@ -1643,8 +1648,10 @@ static void __init init_timer_cpus(void)
 	for_each_possible_cpu(cpu) {
 		if (cpu == local_cpu)
 			base = &boot_tvec_bases;
+#ifdef CONFIG_SMP
 		else
 			base = per_cpu_ptr(&__tvec_bases, cpu);
+#endif
 
 		init_timer_cpu(base, cpu);
 	}
@@ -1657,7 +1664,7 @@ void __init init_timers(void)
 
 	init_timer_cpus();
 	init_timer_stats();
-	register_cpu_notifier(&timers_nb);
+	timer_register_cpu_notifier();
 	open_softirq(TIMER_SOFTIRQ, run_timer_softirq);
 }