#include <linux/user-return-notifier.h>
  #include <linux/percpu.h>
  #include <linux/sched.h>

  #include <linux/export.h>

  
  static DEFINE_PER_CPU(struct hlist_head, return_notifier_list);

  /*
   * Request a notification when the current cpu returns to userspace.  Must be
   * called in atomic context.  The notifier will also be called in atomic
   * context.
   */
  void user_return_notifier_register(struct user_return_notifier *urn)
  {
  	set_tsk_thread_flag(current, TIF_USER_RETURN_NOTIFY);

  	hlist_add_head(&urn->link, &__get_cpu_var(return_notifier_list));

  }
  EXPORT_SYMBOL_GPL(user_return_notifier_register);
  
  /*
   * Removes a registered user return notifier.  Must be called from atomic

   * context, and from the same cpu registration occurred in.

   */
  void user_return_notifier_unregister(struct user_return_notifier *urn)
  {
  	hlist_del(&urn->link);

  	if (hlist_empty(&__get_cpu_var(return_notifier_list)))

  		clear_tsk_thread_flag(current, TIF_USER_RETURN_NOTIFY);
  }
  EXPORT_SYMBOL_GPL(user_return_notifier_unregister);
  
  /* Calls registered user return notifiers */
  void fire_user_return_notifiers(void)
  {
  	struct user_return_notifier *urn;
  	struct hlist_node *tmp1, *tmp2;
  	struct hlist_head *head;
  
  	head = &get_cpu_var(return_notifier_list);
  	hlist_for_each_entry_safe(urn, tmp1, tmp2, head, link)
  		urn->on_user_return(urn);

  	put_cpu_var(return_notifier_list);

  }