x86/softirq: 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 safe some memory and
ensure that do_softirq_own_stack() is not used which is not expected.

 [ bigeasy: commit description. ]

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20210924161245.2357247-1-bigeasy@linutronix.de

arch/x86/include/asm/irq_stack.h

@@ -185,6 +185,7 @@
 			      IRQ_CONSTRAINTS, regs, vector);		\
 }
 
+#ifndef CONFIG_PREEMPT_RT
 #define ASM_CALL_SOFTIRQ						\
 	"call %P[__func]				\n"
 
@@ -201,6 +202,8 @@
 	__this_cpu_write(hardirq_stack_inuse, false);			\
 }
 
+#endif
+
 #else /* CONFIG_X86_64 */
 /* System vector handlers always run on the stack they interrupted. */
 #define run_sysvec_on_irqstack_cond(func, regs)				\

arch/x86/kernel/irq_32.c

@@ -132,6 +132,7 @@ int irq_init_percpu_irqstack(unsigned int cpu)
 	return 0;
 }
 
+#ifndef CONFIG_PREEMPT_RT
 void do_softirq_own_stack(void)
 {
 	struct irq_stack *irqstk;
@@ -148,6 +149,7 @@ void do_softirq_own_stack(void)
 
 	call_on_stack(__do_softirq, isp);
 }
+#endif
 
 void __handle_irq(struct irq_desc *desc, struct pt_regs *regs)
 {