x86: Select CONFIG_HAVE_IRQ_EXIT_ON_IRQ_STACK

Now that all invocations of irq_exit_rcu() happen on the irq stack, turn on
CONFIG_HAVE_IRQ_EXIT_ON_IRQ_STACK which causes the core code to invoke
__do_softirq() directly without going through do_softirq_own_stack().

That means do_softirq_own_stack() is only invoked from task context which
means it can't be on the irq stack. Remove the conditional from
run_softirq_on_irqstack_cond() and rename the function accordingly.
Signed-off-by: NThomas Gleixner <tglx@linutronix.de>
Reviewed-by: NKees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20210210002513.068033456@linutronix.de

arch/x86/Kconfig

@@ -187,6 +187,7 @@ config X86
 	select HAVE_HW_BREAKPOINT
 	select HAVE_IDE
 	select HAVE_IOREMAP_PROT
+	select HAVE_IRQ_EXIT_ON_IRQ_STACK	if X86_64
 	select HAVE_IRQ_TIME_ACCOUNTING
 	select HAVE_KERNEL_BZIP2
 	select HAVE_KERNEL_GZIP

arch/x86/include/asm/irq_stack.h

@@ -189,19 +189,16 @@
 	"call %P[__func]				\n"
 
 /*
- * Macro to invoke __do_softirq on the irq stack. Contrary to the above
- * the only check which is necessary is whether the interrupt stack is
- * in use already.
+ * Macro to invoke __do_softirq on the irq stack. This is only called from
+ * task context when bottom halfs are about to be reenabled and soft
+ * interrupts are pending to be processed. The interrupt stack cannot be in
+ * use here.
  */
-#define run_softirq_on_irqstack_cond()					\
+#define run_softirq_on_irqstack()					\
 {									\
-	if (__this_cpu_read(hardirq_stack_inuse)) {			\
-		__do_softirq();						\
-	} else {							\
-		__this_cpu_write(hardirq_stack_inuse, true);		\
-		call_on_irqstack(__do_softirq, ASM_CALL_SOFTIRQ);	\
-		__this_cpu_write(hardirq_stack_inuse, false);		\
-	}								\
+	__this_cpu_write(hardirq_stack_inuse, true);			\
+	call_on_irqstack(__do_softirq, ASM_CALL_SOFTIRQ);		\
+	__this_cpu_write(hardirq_stack_inuse, false);			\
 }
 
 #else /* CONFIG_X86_64 */

arch/x86/kernel/irq_64.c

@@ -76,5 +76,5 @@ int irq_init_percpu_irqstack(unsigned int cpu)
 
 void do_softirq_own_stack(void)
 {
-	run_softirq_on_irqstack_cond();
+	run_softirq_on_irqstack();
 }