x86/entry: Clarify irq_{enter,exit}_rcu()

Because:

  irq_enter_rcu() includes lockdep_hardirq_enter()
  irq_exit_rcu() does *NOT* include lockdep_hardirq_exit()

Which resulted in two 'stray' lockdep_hardirq_exit() calls in
idtentry.h, and me spending a long time trying to find the matching
enter calls.
Signed-off-by: NPeter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: NThomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20200529213321.359433429@infradead.org

arch/x86/include/asm/idtentry.h

@@ -206,7 +206,6 @@ __visible noinstr void func(struct pt_regs *regs,			\
 	kvm_set_cpu_l1tf_flush_l1d();					\
 	__##func (regs, (u8)error_code);				\
 	irq_exit_rcu();							\
-	lockdep_hardirq_exit();						\
 	instrumentation_end();						\
 	idtentry_exit_cond_rcu(regs, rcu_exit);				\
 }									\
@@ -249,7 +248,6 @@ __visible noinstr void func(struct pt_regs *regs)			\
 	kvm_set_cpu_l1tf_flush_l1d();					\
 	run_on_irqstack_cond(__##func, regs, regs);			\
 	irq_exit_rcu();							\
-	lockdep_hardirq_exit();						\
 	instrumentation_end();						\
 	idtentry_exit_cond_rcu(regs, rcu_exit);				\
 }									\

kernel/softirq.c

@@ -404,12 +404,7 @@ static inline void tick_irq_exit(void)
 #endif
 }
 
-/**
- * irq_exit_rcu() - Exit an interrupt context without updating RCU
- *
- * Also processes softirqs if needed and possible.
- */
-void irq_exit_rcu(void)
+static inline void __irq_exit_rcu(void)
 {
 #ifndef __ARCH_IRQ_EXIT_IRQS_DISABLED
 	local_irq_disable();
@@ -424,6 +419,18 @@ void irq_exit_rcu(void)
 	tick_irq_exit();
 }
 
+/**
+ * irq_exit_rcu() - Exit an interrupt context without updating RCU
+ *
+ * Also processes softirqs if needed and possible.
+ */
+void irq_exit_rcu(void)
+{
+	__irq_exit_rcu();
+	 /* must be last! */
+	lockdep_hardirq_exit();
+}
+
 /**
  * irq_exit - Exit an interrupt context, update RCU and lockdep
  *
@@ -431,7 +438,7 @@ void irq_exit_rcu(void)
  */
 void irq_exit(void)
 {
-	irq_exit_rcu();
+	__irq_exit_rcu();
 	rcu_irq_exit();
 	 /* must be last! */
 	lockdep_hardirq_exit();