genirq: Force interrupt threading on RT

Switch force_irqthreads from a boot time modifiable variable to a compile
time constant when CONFIG_PREEMPT_RT is enabled.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20190816160923.12855-1-bigeasy@linutronix.de

include/linux/interrupt.h

@@ -472,7 +472,11 @@ extern int irq_set_irqchip_state(unsigned int irq, enum irqchip_irq_state which,
 				 bool state);
 
 #ifdef CONFIG_IRQ_FORCED_THREADING
+# ifdef CONFIG_PREEMPT_RT
+#  define force_irqthreads	(true)
+# else
 extern bool force_irqthreads;
+# endif
 #else
 #define force_irqthreads	(0)
 #endif

kernel/irq/manage.c

@@ -23,7 +23,7 @@
 
 #include "internals.h"
 
-#ifdef CONFIG_IRQ_FORCED_THREADING
+#if defined(CONFIG_IRQ_FORCED_THREADING) && !defined(CONFIG_PREEMPT_RT)
 __read_mostly bool force_irqthreads;
 EXPORT_SYMBOL_GPL(force_irqthreads);