#ifndef _LINUX_RCULIST_NULLS_H
  #define _LINUX_RCULIST_NULLS_H
  
  #ifdef __KERNEL__
  
  /*
   * RCU-protected list version
   */
  #include <linux/list_nulls.h>
  #include <linux/rcupdate.h>
  
  /**
   * hlist_nulls_del_init_rcu - deletes entry from hash list with re-initialization
   * @n: the element to delete from the hash list.
   *
   * Note: hlist_nulls_unhashed() on the node return true after this. It is
   * useful for RCU based read lockfree traversal if the writer side
   * must know if the list entry is still hashed or already unhashed.
   *
   * In particular, it means that we can not poison the forward pointers
   * that may still be used for walking the hash list and we can only
   * zero the pprev pointer so list_unhashed() will return true after
   * this.
   *
   * The caller must take whatever precautions are necessary (such as
   * holding appropriate locks) to avoid racing with another
   * list-mutation primitive, such as hlist_nulls_add_head_rcu() or
   * hlist_nulls_del_rcu(), running on this same list.  However, it is
   * perfectly legal to run concurrently with the _rcu list-traversal
   * primitives, such as hlist_nulls_for_each_entry_rcu().
   */
  static inline void hlist_nulls_del_init_rcu(struct hlist_nulls_node *n)
  {
  	if (!hlist_nulls_unhashed(n)) {
  		__hlist_nulls_del(n);
  		n->pprev = NULL;
  	}
  }

  #define hlist_nulls_first_rcu(head) \
  	(*((struct hlist_nulls_node __rcu __force **)&(head)->first))
  
  #define hlist_nulls_next_rcu(node) \
  	(*((struct hlist_nulls_node __rcu __force **)&(node)->next))

  /**
   * hlist_nulls_del_rcu - deletes entry from hash list without re-initialization
   * @n: the element to delete from the hash list.
   *
   * Note: hlist_nulls_unhashed() on entry does not return true after this,
   * the entry is in an undefined state. It is useful for RCU based
   * lockfree traversal.
   *
   * In particular, it means that we can not poison the forward
   * pointers that may still be used for walking the hash list.
   *
   * The caller must take whatever precautions are necessary
   * (such as holding appropriate locks) to avoid racing
   * with another list-mutation primitive, such as hlist_nulls_add_head_rcu()
   * or hlist_nulls_del_rcu(), running on this same list.
   * However, it is perfectly legal to run concurrently with
   * the _rcu list-traversal primitives, such as
   * hlist_nulls_for_each_entry().
   */
  static inline void hlist_nulls_del_rcu(struct hlist_nulls_node *n)
  {
  	__hlist_nulls_del(n);
  	n->pprev = LIST_POISON2;
  }
  
  /**
   * hlist_nulls_add_head_rcu
   * @n: the element to add to the hash list.
   * @h: the list to add to.
   *
   * Description:
   * Adds the specified element to the specified hlist_nulls,
   * while permitting racing traversals.
   *
   * The caller must take whatever precautions are necessary
   * (such as holding appropriate locks) to avoid racing
   * with another list-mutation primitive, such as hlist_nulls_add_head_rcu()
   * or hlist_nulls_del_rcu(), running on this same list.
   * However, it is perfectly legal to run concurrently with
   * the _rcu list-traversal primitives, such as
   * hlist_nulls_for_each_entry_rcu(), used to prevent memory-consistency
   * problems on Alpha CPUs.  Regardless of the type of CPU, the
   * list-traversal primitive must be guarded by rcu_read_lock().
   */
  static inline void hlist_nulls_add_head_rcu(struct hlist_nulls_node *n,
  					struct hlist_nulls_head *h)
  {
  	struct hlist_nulls_node *first = h->first;
  
  	n->next = first;
  	n->pprev = &h->first;

  	rcu_assign_pointer(hlist_nulls_first_rcu(h), n);

  	if (!is_a_nulls(first))
  		first->pprev = &n->next;
  }
  /**
   * hlist_nulls_for_each_entry_rcu - iterate over rcu list of given type
   * @tpos:	the type * to use as a loop cursor.
   * @pos:	the &struct hlist_nulls_node to use as a loop cursor.
   * @head:	the head for your list.
   * @member:	the name of the hlist_nulls_node within the struct.
   *

   * The barrier() is needed to make sure compiler doesn't cache first element [1],
   * as this loop can be restarted [2]
   * [1] Documentation/atomic_ops.txt around line 114
   * [2] Documentation/RCU/rculist_nulls.txt around line 146

   */

  #define hlist_nulls_for_each_entry_rcu(tpos, pos, head, member)			\

  	for (({barrier();}),							\
  	     pos = rcu_dereference_raw(hlist_nulls_first_rcu(head));		\

  		(!is_a_nulls(pos)) &&						\

  		({ tpos = hlist_nulls_entry(pos, typeof(*tpos), member); 1; }); \

  		pos = rcu_dereference_raw(hlist_nulls_next_rcu(pos)))

  
  #endif
  #endif