arch/*: Disable softirq stacks on PREEMPT_RT.

PREEMPT_RT preempts softirqs and the current implementation avoids
do_softirq_own_stack() and only uses __do_softirq().

Disable the unused softirqs stacks on PREEMPT_RT to save some memory and
ensure that do_softirq_own_stack() is not used bwcause it is not expected.
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>

arch/arm/kernel/irq.c

@@ -70,6 +70,7 @@ static void __init init_irq_stacks(void)
 	}
 }
 
+#ifndef CONFIG_PREEMPT_RT
 static void ____do_softirq(void *arg)
 {
 	__do_softirq();
@@ -80,7 +81,7 @@ void do_softirq_own_stack(void)
 	call_with_stack(____do_softirq, NULL,
 			__this_cpu_read(irq_stack_ptr));
 }
-
+#endif
 #endif
 
 int arch_show_interrupts(struct seq_file *p, int prec)

arch/parisc/kernel/irq.c

@@ -480,10 +480,12 @@ static void execute_on_irq_stack(void *func, unsigned long param1)
 	*irq_stack_in_use = 1;
 }
 
+#ifndef CONFIG_PREEMPT_RT
 void do_softirq_own_stack(void)
 {
 	execute_on_irq_stack(__do_softirq, 0);
 }
+#endif
 #endif /* CONFIG_IRQSTACKS */
 
 /* ONLY called from entry.S:intr_extint() */

arch/powerpc/kernel/irq.c

@@ -611,6 +611,7 @@ static inline void check_stack_overflow(void)
 	}
 }
 
+#ifndef CONFIG_PREEMPT_RT
 static __always_inline void call_do_softirq(const void *sp)
 {
 	/* Temporarily switch r1 to sp, call __do_softirq() then restore r1. */
@@ -629,6 +630,7 @@ static __always_inline void call_do_softirq(const void *sp)
 		   "r11", "r12"
 	);
 }
+#endif
 
 static __always_inline void call_do_irq(struct pt_regs *regs, void *sp)
 {
@@ -747,10 +749,12 @@ void *mcheckirq_ctx[NR_CPUS] __read_mostly;
 void *softirq_ctx[NR_CPUS] __read_mostly;
 void *hardirq_ctx[NR_CPUS] __read_mostly;
 
+#ifndef CONFIG_PREEMPT_RT
 void do_softirq_own_stack(void)
 {
 	call_do_softirq(softirq_ctx[smp_processor_id()]);
 }
+#endif
 
 irq_hw_number_t virq_to_hw(unsigned int virq)
 {

arch/s390/include/asm/softirq_stack.h

@@ -5,9 +5,10 @@
 #include <asm/lowcore.h>
 #include <asm/stacktrace.h>
 
+#ifndef CONFIG_PREEMPT_RT
 static inline void do_softirq_own_stack(void)
 {
 	call_on_stack(0, S390_lowcore.async_stack, void, __do_softirq);
 }
-
+#endif
 #endif /* __ASM_S390_SOFTIRQ_STACK_H */

arch/sh/kernel/irq.c

@@ -149,6 +149,7 @@ void irq_ctx_exit(int cpu)
 	hardirq_ctx[cpu] = NULL;
 }
 
+#ifndef CONFIG_PREEMPT_RT
 void do_softirq_own_stack(void)
 {
 	struct thread_info *curctx;
@@ -176,6 +177,7 @@ void do_softirq_own_stack(void)
 		  "r5", "r6", "r7", "r8", "r9", "r15", "t", "pr"
 	);
 }
+#endif
 #else
 static inline void handle_one_irq(unsigned int irq)
 {

arch/sparc/kernel/irq_64.c

@@ -855,6 +855,7 @@ void __irq_entry handler_irq(int pil, struct pt_regs *regs)
 	set_irq_regs(old_regs);
 }
 
+#ifndef CONFIG_PREEMPT_RT
 void do_softirq_own_stack(void)
 {
 	void *orig_sp, *sp = softirq_stack[smp_processor_id()];
@@ -869,6 +870,7 @@ void do_softirq_own_stack(void)
 	__asm__ __volatile__("mov %0, %%sp"
 			     : : "r" (orig_sp));
 }
+#endif
 
 #ifdef CONFIG_HOTPLUG_CPU
 void fixup_irqs(void)

include/asm-generic/softirq_stack.h

@@ -2,7 +2,7 @@
 #ifndef __ASM_GENERIC_SOFTIRQ_STACK_H
 #define __ASM_GENERIC_SOFTIRQ_STACK_H
 
-#ifdef CONFIG_HAVE_SOFTIRQ_ON_OWN_STACK
+#if defined(CONFIG_HAVE_SOFTIRQ_ON_OWN_STACK) && !defined(CONFIG_PREEMPT_RT)
 void do_softirq_own_stack(void);
 #else
 static inline void do_softirq_own_stack(void)