softirq: Consolidate common code in __tasklet_[hi]_schedule()

__tasklet_schedule() and __tasklet_hi_schedule() are almost identical.
Move the common code from both function into __tasklet_schedule_common()
and let both functions invoke it with different arguments.

[ bigeasy: Splitted out from RT's "tasklet: Prevent tasklets from going
  	   into infinite spin in RT" and added commit message. Use
  	   this_cpu_ptr(headp) in __tasklet_schedule_common() as suggested
  	   by Julia Cartwright ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Julia Cartwright <juliac@eso.teric.us>
Link: https://lkml.kernel.org/r/20180227164808.10093-2-bigeasy@linutronix.de

kernel/softirq.c

@@ -460,29 +460,33 @@ struct tasklet_head {
 static DEFINE_PER_CPU(struct tasklet_head, tasklet_vec);
 static DEFINE_PER_CPU(struct tasklet_head, tasklet_hi_vec);
 
-void __tasklet_schedule(struct tasklet_struct *t)
+static void __tasklet_schedule_common(struct tasklet_struct *t,
+				      struct tasklet_head __percpu *headp,
+				      unsigned int softirq_nr)
 {
+	struct tasklet_head *head;
 	unsigned long flags;
 
 	local_irq_save(flags);
+	head = this_cpu_ptr(headp);
 	t->next = NULL;
-	*__this_cpu_read(tasklet_vec.tail) = t;
-	__this_cpu_write(tasklet_vec.tail, &(t->next));
-	raise_softirq_irqoff(TASKLET_SOFTIRQ);
+	*head->tail = t;
+	head->tail = &(t->next);
+	raise_softirq_irqoff(softirq_nr);
 	local_irq_restore(flags);
 }
+
+void __tasklet_schedule(struct tasklet_struct *t)
+{
+	__tasklet_schedule_common(t, &tasklet_vec,
+				  TASKLET_SOFTIRQ);
+}
 EXPORT_SYMBOL(__tasklet_schedule);
 
 void __tasklet_hi_schedule(struct tasklet_struct *t)
 {
-	unsigned long flags;
-
-	local_irq_save(flags);
-	t->next = NULL;
-	*__this_cpu_read(tasklet_hi_vec.tail) = t;
-	__this_cpu_write(tasklet_hi_vec.tail,  &(t->next));
-	raise_softirq_irqoff(HI_SOFTIRQ);
-	local_irq_restore(flags);
+	__tasklet_schedule_common(t, &tasklet_hi_vec,
+				  HI_SOFTIRQ);
 }
 EXPORT_SYMBOL(__tasklet_hi_schedule);