asm-generic: Conditionally enable do_softirq_own_stack() via Kconfig.

Remove the CONFIG_PREEMPT_RT symbol from the ifdef around
do_softirq_own_stack() and move it to Kconfig instead.

Enable softirq stacks based on SOFTIRQ_ON_OWN_STACK which depends on
HAVE_SOFTIRQ_ON_OWN_STACK and its default value is set to !PREEMPT_RT.
This ensures that softirq stacks are not used on PREEMPT_RT and avoids
a 'select' statement on an option which has a 'depends' statement.

Link: https://lore.kernel.org/[e-mail]
Signed-off-by: Sebastian Andrzej Siewior <[e-mail]>
Signed-off-by: Arnd Bergmann <[e-mail]>

arch/Kconfig

···
 	  Architecture provides a function to run __do_softirq() on a
 	  separate stack.
 
+config SOFTIRQ_ON_OWN_STACK
+	def_bool HAVE_SOFTIRQ_ON_OWN_STACK && !PREEMPT_RT
+
 config ALTERNATE_USER_ADDRESS_SPACE
 	bool
 	help

arch/arm/kernel/irq.c

···
 	}
 }
 
-#ifndef CONFIG_PREEMPT_RT
+#ifdef CONFIG_SOFTIRQ_ON_OWN_STACK
 static void ____do_softirq(void *arg)
 {
 	__do_softirq();

arch/parisc/kernel/irq.c

···
 	*irq_stack_in_use = 1;
 }
 
-#ifndef CONFIG_PREEMPT_RT
+#ifdef CONFIG_SOFTIRQ_ON_OWN_STACK
 void do_softirq_own_stack(void)
 {
 	execute_on_irq_stack(__do_softirq, 0);

arch/powerpc/kernel/irq.c

···
 	}
 }
 
-#ifndef CONFIG_PREEMPT_RT
+#ifdef CONFIG_SOFTIRQ_ON_OWN_STACK
 static __always_inline void call_do_softirq(const void *sp)
 {
 	/* Temporarily switch r1 to sp, call __do_softirq() then restore r1. */
···
 void *softirq_ctx[NR_CPUS] __read_mostly;
 void *hardirq_ctx[NR_CPUS] __read_mostly;
 
-#ifndef CONFIG_PREEMPT_RT
+#ifdef CONFIG_SOFTIRQ_ON_OWN_STACK
 void do_softirq_own_stack(void)
 {
 	call_do_softirq(softirq_ctx[smp_processor_id()]);

arch/s390/include/asm/softirq_stack.h

···
 #include <asm/lowcore.h>
 #include <asm/stacktrace.h>
 
-#ifndef CONFIG_PREEMPT_RT
+#ifdef CONFIG_SOFTIRQ_ON_OWN_STACK
 static inline void do_softirq_own_stack(void)
 {
 	call_on_stack(0, S390_lowcore.async_stack, void, __do_softirq);

arch/sh/kernel/irq.c

···
 	hardirq_ctx[cpu] = NULL;
 }
 
-#ifndef CONFIG_PREEMPT_RT
+#ifdef CONFIG_SOFTIRQ_ON_OWN_STACK
 void do_softirq_own_stack(void)
 {
 	struct thread_info *curctx;

arch/sparc/kernel/irq_64.c

···
 	set_irq_regs(old_regs);
 }
 
-#ifndef CONFIG_PREEMPT_RT
+#ifdef CONFIG_SOFTIRQ_ON_OWN_STACK
 void do_softirq_own_stack(void)
 {
 	void *orig_sp, *sp = softirq_stack[smp_processor_id()];

arch/x86/include/asm/irq_stack.h

···
 			      IRQ_CONSTRAINTS, regs, vector);		\
 }
 
-#ifndef CONFIG_PREEMPT_RT
+#ifdef CONFIG_SOFTIRQ_ON_OWN_STACK
 /*
  * Macro to invoke __do_softirq on the irq stack. This is only called from
  * task context when bottom halves are about to be reenabled and soft

arch/x86/kernel/irq_32.c

···
 	return 0;
 }
 
-#ifndef CONFIG_PREEMPT_RT
+#ifdef CONFIG_SOFTIRQ_ON_OWN_STACK
 void do_softirq_own_stack(void)
 {
 	struct irq_stack *irqstk;

include/asm-generic/softirq_stack.h

···
 #ifndef __ASM_GENERIC_SOFTIRQ_STACK_H
 #define __ASM_GENERIC_SOFTIRQ_STACK_H
 
-#if defined(CONFIG_HAVE_SOFTIRQ_ON_OWN_STACK) && !defined(CONFIG_PREEMPT_RT)
+#ifdef CONFIG_SOFTIRQ_ON_OWN_STACK
 void do_softirq_own_stack(void);
 #else
 static inline void do_softirq_own_stack(void)