Subject: x86/softirq: Disable softirq stacks on PREEMPT_RT
From: Thomas Gleixner <[email-masked]>
Date: Fri, 24 Sep 2021 18:12:45 +0200

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 <[email-masked]>
Signed-off-by: Sebastian Andrzej Siewior <[email-masked]>
Link: https://lore.kernel.org/r/[email-masked]
---
 arch/x86/include/asm/irq_stack.h |    3 +++
 arch/x86/kernel/irq_32.c         |    2 ++
 2 files changed, 5 insertions(+)

--- a/arch/x86/include/asm/irq_stack.h
+++ b/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)				\
--- a/arch/x86/kernel/irq_32.c
+++ b/arch/x86/kernel/irq_32.c
@@ -132,6 +132,7 @@ int irq_init_percpu_irqstack(unsigned in
 	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)
 {