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 /* SPDX-License-Identifier: GPL-2.0 */
 #ifndef _LINUX_LIST_H
 #define _LINUX_LIST_H
 
 #include <linux/container_of.h>
 #include <linux/types.h>
 #include <linux/stddef.h>
 #include <linux/poison.h>
 #include <linux/const.h>
 
 #include <asm/barrier.h>
 
 /*
  * Circular doubly linked list implementation.
  *
  * Some of the internal functions ("__xxx") are useful when
  * manipulating whole lists rather than single entries, as
  * sometimes we already know the next/prev entries and we can
  * generate better code by using them directly rather than
  * using the generic single-entry routines.
  */
 
 #define LIST_HEAD_INIT(name) { &(name), &(name) }
 
 #define LIST_HEAD(name) \
     struct list_head name = LIST_HEAD_INIT(name)
 
 /**
  * INIT_LIST_HEAD - Initialize a list_head structure
  * @list: list_head structure to be initialized.
  *
  * Initializes the list_head to point to itself.  If it is a list header,
  * the result is an empty list.
  */
 static inline void INIT_LIST_HEAD(struct list_head *list)
 {
     WRITE_ONCE(list->next, list);
     WRITE_ONCE(list->prev, list);
 }
 
 #ifdef CONFIG_DEBUG_LIST
 extern bool __list_add_valid(struct list_head *new,
                   struct list_head *prev,
                   struct list_head *next);
 extern bool __list_del_entry_valid(struct list_head *entry);
 #else
 static inline bool __list_add_valid(struct list_head *new,
                 struct list_head *prev,
                 struct list_head *next)
 {
     return true;
 }
 static inline bool __list_del_entry_valid(struct list_head *entry)
 {
     return true;
 }
 #endif
 
 /*
  * Insert a new entry between two known consecutive entries.
  *
  * This is only for internal list manipulation where we know
  * the prev/next entries already!
  */
 static inline void __list_add(struct list_head *new,
                   struct list_head *prev,
                   struct list_head *next)
 {
     if (!__list_add_valid(new, prev, next))
         return;
 
     next->prev = new;
     new->next = next;
     new->prev = prev;
     WRITE_ONCE(prev->next, new);
 }
 
 /**
  * list_add - add a new entry
  * @new: new entry to be added
  * @head: list head to add it after
  *
  * Insert a new entry after the specified head.
  * This is good for implementing stacks.
  */
 static inline void list_add(struct list_head *new, struct list_head *head)
 {
     __list_add(new, head, head->next);
 }
 
 
 /**
  * list_add_tail - add a new entry
  * @new: new entry to be added
  * @head: list head to add it before
  *
  * Insert a new entry before the specified head.
  * This is useful for implementing queues.
  */
 static inline void list_add_tail(struct list_head *new, struct list_head *head)
 {
     __list_add(new, head->prev, head);
 }
 
 /*
  * Delete a list entry by making the prev/next entries
  * point to each other.
  *
  * This is only for internal list manipulation where we know
  * the prev/next entries already!
  */
 static inline void __list_del(struct list_head * prev, struct list_head * next)
 {
     next->prev = prev;
     WRITE_ONCE(prev->next, next);
 }
 
 /*
  * Delete a list entry and clear the 'prev' pointer.
  *
  * This is a special-purpose list clearing method used in the networking code
  * for lists allocated as per-cpu, where we don't want to incur the extra
  * WRITE_ONCE() overhead of a regular list_del_init(). The code that uses this
  * needs to check the node 'prev' pointer instead of calling list_empty().
  */
 static inline void __list_del_clearprev(struct list_head *entry)
 {
     __list_del(entry->prev, entry->next);
     entry->prev = NULL;
 }
 
 static inline void __list_del_entry(struct list_head *entry)
 {
     if (!__list_del_entry_valid(entry))
         return;
 
     __list_del(entry->prev, entry->next);
 }
 
 /**
  * list_del - deletes entry from list.
  * @entry: the element to delete from the list.
  * Note: list_empty() on entry does not return true after this, the entry is
  * in an undefined state.
  */
 static inline void list_del(struct list_head *entry)
 {
     __list_del_entry(entry);
     entry->next = LIST_POISON1;
     entry->prev = LIST_POISON2;
 }
 
 /**
  * list_replace - replace old entry by new one
  * @old : the element to be replaced
  * @new : the new element to insert
  *
  * If @old was empty, it will be overwritten.
  */
 static inline void list_replace(struct list_head *old,
                 struct list_head *new)
 {
     new->next = old->next;
     new->next->prev = new;
     new->prev = old->prev;
     new->prev->next = new;
 }
 
 /**
  * list_replace_init - replace old entry by new one and initialize the old one
  * @old : the element to be replaced
  * @new : the new element to insert
  *
  * If @old was empty, it will be overwritten.
  */
 static inline void list_replace_init(struct list_head *old,
                      struct list_head *new)
 {
     list_replace(old, new);
     INIT_LIST_HEAD(old);
 }
 
 /**
  * list_swap - replace entry1 with entry2 and re-add entry1 at entry2's position
  * @entry1: the location to place entry2
  * @entry2: the location to place entry1
  */
 static inline void list_swap(struct list_head *entry1,
                  struct list_head *entry2)
 {
     struct list_head *pos = entry2->prev;
 
     list_del(entry2);
     list_replace(entry1, entry2);
     if (pos == entry1)
         pos = entry2;
     list_add(entry1, pos);
 }
 
 /**
  * list_del_init - deletes entry from list and reinitialize it.
  * @entry: the element to delete from the list.
  */
 static inline void list_del_init(struct list_head *entry)
 {
     __list_del_entry(entry);
     INIT_LIST_HEAD(entry);
 }
 
 /**
  * list_move - delete from one list and add as another's head
  * @list: the entry to move
  * @head: the head that will precede our entry
  */
 static inline void list_move(struct list_head *list, struct list_head *head)
 {
     __list_del_entry(list);
     list_add(list, head);
 }
 
 /**
  * list_move_tail - delete from one list and add as another's tail
  * @list: the entry to move
  * @head: the head that will follow our entry
  */
 static inline void list_move_tail(struct list_head *list,
                   struct list_head *head)
 {
     __list_del_entry(list);
     list_add_tail(list, head);
 }
 
 /**
  * list_bulk_move_tail - move a subsection of a list to its tail
  * @head: the head that will follow our entry
  * @first: first entry to move
  * @last: last entry to move, can be the same as first
  *
  * Move all entries between @first and including @last before @head.
  * All three entries must belong to the same linked list.
  */
 static inline void list_bulk_move_tail(struct list_head *head,
                        struct list_head *first,
                        struct list_head *last)
 {
     first->prev->next = last->next;
     last->next->prev = first->prev;
 
     head->prev->next = first;
     first->prev = head->prev;
 
     last->next = head;
     head->prev = last;
 }
 
 /**
  * list_is_first -- tests whether @list is the first entry in list @head
  * @list: the entry to test
  * @head: the head of the list
  */
 static inline int list_is_first(const struct list_head *list, const struct list_head *head)
 {
     return list->prev == head;
 }
 
 /**
  * list_is_last - tests whether @list is the last entry in list @head
  * @list: the entry to test
  * @head: the head of the list
  */
 static inline int list_is_last(const struct list_head *list, const struct list_head *head)
 {
     return list->next == head;
 }
 
 /**
  * list_is_head - tests whether @list is the list @head
  * @list: the entry to test
  * @head: the head of the list
  */
 static inline int list_is_head(const struct list_head *list, const struct list_head *head)
 {
     return list == head;
 }
 
 /**
  * list_empty - tests whether a list is empty
  * @head: the list to test.
  */
 static inline int list_empty(const struct list_head *head)
 {
     return READ_ONCE(head->next) == head;
 }
 
 /**
  * list_del_init_careful - deletes entry from list and reinitialize it.
  * @entry: the element to delete from the list.
  *
  * This is the same as list_del_init(), except designed to be used
  * together with list_empty_careful() in a way to guarantee ordering
  * of other memory operations.
  *
  * Any memory operations done before a list_del_init_careful() are
  * guaranteed to be visible after a list_empty_careful() test.
  */
 static inline void list_del_init_careful(struct list_head *entry)
 {
     __list_del_entry(entry);
     WRITE_ONCE(entry->prev, entry);
     smp_store_release(&entry->next, entry);
 }
 
 /**
  * list_empty_careful - tests whether a list is empty and not being modified
  * @head: the list to test
  *
  * Description:
  * tests whether a list is empty _and_ checks that no other CPU might be
  * in the process of modifying either member (next or prev)
  *
  * NOTE: using list_empty_careful() without synchronization
  * can only be safe if the only activity that can happen
  * to the list entry is list_del_init(). Eg. it cannot be used
  * if another CPU could re-list_add() it.
  */
 static inline int list_empty_careful(const struct list_head *head)
 {
     struct list_head *next = smp_load_acquire(&head->next);
     return list_is_head(next, head) && (next == READ_ONCE(head->prev));
 }
 
 /**
  * list_rotate_left - rotate the list to the left
  * @head: the head of the list
  */
 static inline void list_rotate_left(struct list_head *head)
 {
     struct list_head *first;
 
     if (!list_empty(head)) {
         first = head->next;
         list_move_tail(first, head);
     }
 }
 
 /**
  * list_rotate_to_front() - Rotate list to specific item.
  * @list: The desired new front of the list.
  * @head: The head of the list.
  *
  * Rotates list so that @list becomes the new front of the list.
  */
 static inline void list_rotate_to_front(struct list_head *list,
                     struct list_head *head)
 {
     /*
      * Deletes the list head from the list denoted by @head and
      * places it as the tail of @list, this effectively rotates the
      * list so that @list is at the front.
      */
     list_move_tail(head, list);
 }
 
 /**
  * list_is_singular - tests whether a list has just one entry.
  * @head: the list to test.
  */
 static inline int list_is_singular(const struct list_head *head)
 {
     return !list_empty(head) && (head->next == head->prev);
 }
 
 static inline void __list_cut_position(struct list_head *list,
         struct list_head *head, struct list_head *entry)
 {
     struct list_head *new_first = entry->next;
     list->next = head->next;
     list->next->prev = list;
     list->prev = entry;
     entry->next = list;
     head->next = new_first;
     new_first->prev = head;
 }
 
 /**
  * list_cut_position - cut a list into two
  * @list: a new list to add all removed entries
  * @head: a list with entries
  * @entry: an entry within head, could be the head itself
  *  and if so we won't cut the list
  *
  * This helper moves the initial part of @head, up to and
  * including @entry, from @head to @list. You should
  * pass on @entry an element you know is on @head. @list
  * should be an empty list or a list you do not care about
  * losing its data.
  *
  */
 static inline void list_cut_position(struct list_head *list,
         struct list_head *head, struct list_head *entry)
 {
     if (list_empty(head))
         return;
     if (list_is_singular(head) && !list_is_head(entry, head) && (entry != head->next))
         return;
     if (list_is_head(entry, head))
         INIT_LIST_HEAD(list);
     else
         __list_cut_position(list, head, entry);
 }
 
 /**
  * list_cut_before - cut a list into two, before given entry
  * @list: a new list to add all removed entries
  * @head: a list with entries
  * @entry: an entry within head, could be the head itself
  *
  * This helper moves the initial part of @head, up to but
  * excluding @entry, from @head to @list.  You should pass
  * in @entry an element you know is on @head.  @list should
  * be an empty list or a list you do not care about losing
  * its data.
  * If @entry == @head, all entries on @head are moved to
  * @list.
  */
 static inline void list_cut_before(struct list_head *list,
                    struct list_head *head,
                    struct list_head *entry)
 {
     if (head->next == entry) {
         INIT_LIST_HEAD(list);
         return;
     }
     list->next = head->next;
     list->next->prev = list;
     list->prev = entry->prev;
     list->prev->next = list;
     head->next = entry;
     entry->prev = head;
 }
 
 static inline void __list_splice(const struct list_head *list,
                  struct list_head *prev,
                  struct list_head *next)
 {
     struct list_head *first = list->next;
     struct list_head *last = list->prev;
 
     first->prev = prev;
     prev->next = first;
 
     last->next = next;
     next->prev = last;
 }
 
 /**
  * list_splice - join two lists, this is designed for stacks
  * @list: the new list to add.
  * @head: the place to add it in the first list.
  */
 static inline void list_splice(const struct list_head *list,
                 struct list_head *head)
 {
     if (!list_empty(list))
         __list_splice(list, head, head->next);
 }
 
 /**
  * list_splice_tail - join two lists, each list being a queue
  * @list: the new list to add.
  * @head: the place to add it in the first list.
  */
 static inline void list_splice_tail(struct list_head *list,
                 struct list_head *head)
 {
     if (!list_empty(list))
         __list_splice(list, head->prev, head);
 }
 
 /**
  * list_splice_init - join two lists and reinitialise the emptied list.
  * @list: the new list to add.
  * @head: the place to add it in the first list.
  *
  * The list at @list is reinitialised
  */
 static inline void list_splice_init(struct list_head *list,
                     struct list_head *head)
 {
     if (!list_empty(list)) {
         __list_splice(list, head, head->next);
         INIT_LIST_HEAD(list);
     }
 }
 
 /**
  * list_splice_tail_init - join two lists and reinitialise the emptied list
  * @list: the new list to add.
  * @head: the place to add it in the first list.
  *
  * Each of the lists is a queue.
  * The list at @list is reinitialised
  */
 static inline void list_splice_tail_init(struct list_head *list,
                      struct list_head *head)
 {
     if (!list_empty(list)) {
         __list_splice(list, head->prev, head);
         INIT_LIST_HEAD(list);
     }
 }
 
 /**
  * list_entry - get the struct for this entry
  * @ptr:    the &struct list_head pointer.
  * @type:   the type of the struct this is embedded in.
  * @member: the name of the list_head within the struct.
  */
 #define list_entry(ptr, type, member) \
     container_of(ptr, type, member)
 
 /**
  * list_first_entry - get the first element from a list
  * @ptr:    the list head to take the element from.
  * @type:   the type of the struct this is embedded in.
  * @member: the name of the list_head within the struct.
  *
  * Note, that list is expected to be not empty.
  */
 #define list_first_entry(ptr, type, member) \
     list_entry((ptr)->next, type, member)
 
 /**
  * list_last_entry - get the last element from a list
  * @ptr:    the list head to take the element from.
  * @type:   the type of the struct this is embedded in.
  * @member: the name of the list_head within the struct.
  *
  * Note, that list is expected to be not empty.
  */
 #define list_last_entry(ptr, type, member) \
     list_entry((ptr)->prev, type, member)
 
 /**
  * list_first_entry_or_null - get the first element from a list
  * @ptr:    the list head to take the element from.
  * @type:   the type of the struct this is embedded in.
  * @member: the name of the list_head within the struct.
  *
  * Note that if the list is empty, it returns NULL.
  */
 #define list_first_entry_or_null(ptr, type, member) ({ \
     struct list_head *head__ = (ptr); \
     struct list_head *pos__ = READ_ONCE(head__->next); \
     pos__ != head__ ? list_entry(pos__, type, member) : NULL; \
 })
 
 /**
  * list_next_entry - get the next element in list
  * @pos:    the type * to cursor
  * @member: the name of the list_head within the struct.
  */
 #define list_next_entry(pos, member) \
     list_entry((pos)->member.next, typeof(*(pos)), member)
 
 /**
  * list_next_entry_circular - get the next element in list
  * @pos:    the type * to cursor.
  * @head:   the list head to take the element from.
  * @member: the name of the list_head within the struct.
  *
  * Wraparound if pos is the last element (return the first element).
  * Note, that list is expected to be not empty.
  */
 #define list_next_entry_circular(pos, head, member) \
     (list_is_last(&(pos)->member, head) ? \
     list_first_entry(head, typeof(*(pos)), member) : list_next_entry(pos, member))
 
 /**
  * list_prev_entry - get the prev element in list
  * @pos:    the type * to cursor
  * @member: the name of the list_head within the struct.
  */
 #define list_prev_entry(pos, member) \
     list_entry((pos)->member.prev, typeof(*(pos)), member)
 
 /**
  * list_prev_entry_circular - get the prev element in list
  * @pos:    the type * to cursor.
  * @head:   the list head to take the element from.
  * @member: the name of the list_head within the struct.
  *
  * Wraparound if pos is the first element (return the last element).
  * Note, that list is expected to be not empty.
  */
 #define list_prev_entry_circular(pos, head, member) \
     (list_is_first(&(pos)->member, head) ? \
     list_last_entry(head, typeof(*(pos)), member) : list_prev_entry(pos, member))
 
 /**
  * list_for_each    -   iterate over a list
  * @pos:    the &struct list_head to use as a loop cursor.
  * @head:   the head for your list.
  */
 #define list_for_each(pos, head) \
     for (pos = (head)->next; !list_is_head(pos, (head)); pos = pos->next)
 
 /**
  * list_for_each_rcu - Iterate over a list in an RCU-safe fashion
  * @pos:    the &struct list_head to use as a loop cursor.
  * @head:   the head for your list.
  */
 #define list_for_each_rcu(pos, head)          \
     for (pos = rcu_dereference((head)->next); \
          !list_is_head(pos, (head)); \
          pos = rcu_dereference(pos->next))
 
 /**
  * list_for_each_continue - continue iteration over a list
  * @pos:    the &struct list_head to use as a loop cursor.
  * @head:   the head for your list.
  *
  * Continue to iterate over a list, continuing after the current position.
  */
 #define list_for_each_continue(pos, head) \
     for (pos = pos->next; !list_is_head(pos, (head)); pos = pos->next)
 
 /**
  * list_for_each_prev   -   iterate over a list backwards
  * @pos:    the &struct list_head to use as a loop cursor.
  * @head:   the head for your list.
  */
 #define list_for_each_prev(pos, head) \
     for (pos = (head)->prev; !list_is_head(pos, (head)); pos = pos->prev)
 
 /**
  * list_for_each_safe - iterate over a list safe against removal of list entry
  * @pos:    the &struct list_head to use as a loop cursor.
  * @n:      another &struct list_head to use as temporary storage
  * @head:   the head for your list.
  */
 #define list_for_each_safe(pos, n, head) \
     for (pos = (head)->next, n = pos->next; \
          !list_is_head(pos, (head)); \
          pos = n, n = pos->next)
 
 /**
  * list_for_each_prev_safe - iterate over a list backwards safe against removal of list entry
  * @pos:    the &struct list_head to use as a loop cursor.
  * @n:      another &struct list_head to use as temporary storage
  * @head:   the head for your list.
  */
 #define list_for_each_prev_safe(pos, n, head) \
     for (pos = (head)->prev, n = pos->prev; \
          !list_is_head(pos, (head)); \
          pos = n, n = pos->prev)
 
 /**
  * list_entry_is_head - test if the entry points to the head of the list
  * @pos:    the type * to cursor
  * @head:   the head for your list.
  * @member: the name of the list_head within the struct.
  */
 #define list_entry_is_head(pos, head, member)               \
     (&pos->member == (head))
 
 /**
  * list_for_each_entry  -   iterate over list of given type
  * @pos:    the type * to use as a loop cursor.
  * @head:   the head for your list.
  * @member: the name of the list_head within the struct.
  */
 #define list_for_each_entry(pos, head, member)              \
     for (pos = list_first_entry(head, typeof(*pos), member);    \
          !list_entry_is_head(pos, head, member);            \
          pos = list_next_entry(pos, member))
 
 /**
  * list_for_each_entry_reverse - iterate backwards over list of given type.
  * @pos:    the type * to use as a loop cursor.
  * @head:   the head for your list.
  * @member: the name of the list_head within the struct.
  */
 #define list_for_each_entry_reverse(pos, head, member)          \
     for (pos = list_last_entry(head, typeof(*pos), member);     \
          !list_entry_is_head(pos, head, member);            \
          pos = list_prev_entry(pos, member))
 
 /**
  * list_prepare_entry - prepare a pos entry for use in list_for_each_entry_continue()
  * @pos:    the type * to use as a start point
  * @head:   the head of the list
  * @member: the name of the list_head within the struct.
  *
  * Prepares a pos entry for use as a start point in list_for_each_entry_continue().
  */
 #define list_prepare_entry(pos, head, member) \
     ((pos) ? : list_entry(head, typeof(*pos), member))
 
 /**
  * list_for_each_entry_continue - continue iteration over list of given type
  * @pos:    the type * to use as a loop cursor.
  * @head:   the head for your list.
  * @member: the name of the list_head within the struct.
  *
  * Continue to iterate over list of given type, continuing after
  * the current position.
  */
 #define list_for_each_entry_continue(pos, head, member)         \
     for (pos = list_next_entry(pos, member);            \
          !list_entry_is_head(pos, head, member);            \
          pos = list_next_entry(pos, member))
 
 /**
  * list_for_each_entry_continue_reverse - iterate backwards from the given point
  * @pos:    the type * to use as a loop cursor.
  * @head:   the head for your list.
  * @member: the name of the list_head within the struct.
  *
  * Start to iterate over list of given type backwards, continuing after
  * the current position.
  */
 #define list_for_each_entry_continue_reverse(pos, head, member)     \
     for (pos = list_prev_entry(pos, member);            \
          !list_entry_is_head(pos, head, member);            \
          pos = list_prev_entry(pos, member))
 
 /**
  * list_for_each_entry_from - iterate over list of given type from the current point
  * @pos:    the type * to use as a loop cursor.
  * @head:   the head for your list.
  * @member: the name of the list_head within the struct.
  *
  * Iterate over list of given type, continuing from current position.
  */
 #define list_for_each_entry_from(pos, head, member)             \
     for (; !list_entry_is_head(pos, head, member);          \
          pos = list_next_entry(pos, member))
 
 /**
  * list_for_each_entry_from_reverse - iterate backwards over list of given type
  *                                    from the current point
  * @pos:    the type * to use as a loop cursor.
  * @head:   the head for your list.
  * @member: the name of the list_head within the struct.
  *
  * Iterate backwards over list of given type, continuing from current position.
  */
 #define list_for_each_entry_from_reverse(pos, head, member)     \
     for (; !list_entry_is_head(pos, head, member);          \
          pos = list_prev_entry(pos, member))
 
 /**
  * list_for_each_entry_safe - iterate over list of given type safe against removal of list entry
  * @pos:    the type * to use as a loop cursor.
  * @n:      another type * to use as temporary storage
  * @head:   the head for your list.
  * @member: the name of the list_head within the struct.
  */
 #define list_for_each_entry_safe(pos, n, head, member)          \
     for (pos = list_first_entry(head, typeof(*pos), member),    \
         n = list_next_entry(pos, member);           \
          !list_entry_is_head(pos, head, member);            \
          pos = n, n = list_next_entry(n, member))
 
 /**
  * list_for_each_entry_safe_continue - continue list iteration safe against removal
  * @pos:    the type * to use as a loop cursor.
  * @n:      another type * to use as temporary storage
  * @head:   the head for your list.
  * @member: the name of the list_head within the struct.
  *
  * Iterate over list of given type, continuing after current point,
  * safe against removal of list entry.
  */
 #define list_for_each_entry_safe_continue(pos, n, head, member)         \
     for (pos = list_next_entry(pos, member),                \
         n = list_next_entry(pos, member);               \
          !list_entry_is_head(pos, head, member);                \
          pos = n, n = list_next_entry(n, member))
 
 /**
  * list_for_each_entry_safe_from - iterate over list from current point safe against removal
  * @pos:    the type * to use as a loop cursor.
  * @n:      another type * to use as temporary storage
  * @head:   the head for your list.
  * @member: the name of the list_head within the struct.
  *
  * Iterate over list of given type from current point, safe against
  * removal of list entry.
  */
 #define list_for_each_entry_safe_from(pos, n, head, member)             \
     for (n = list_next_entry(pos, member);                  \
          !list_entry_is_head(pos, head, member);                \
          pos = n, n = list_next_entry(n, member))
 
 /**
  * list_for_each_entry_safe_reverse - iterate backwards over list safe against removal
  * @pos:    the type * to use as a loop cursor.
  * @n:      another type * to use as temporary storage
  * @head:   the head for your list.
  * @member: the name of the list_head within the struct.
  *
  * Iterate backwards over list of given type, safe against removal
  * of list entry.
  */
 #define list_for_each_entry_safe_reverse(pos, n, head, member)      \
     for (pos = list_last_entry(head, typeof(*pos), member),     \
         n = list_prev_entry(pos, member);           \
          !list_entry_is_head(pos, head, member);            \
          pos = n, n = list_prev_entry(n, member))
 
 /**
  * list_safe_reset_next - reset a stale list_for_each_entry_safe loop
  * @pos:    the loop cursor used in the list_for_each_entry_safe loop
  * @n:      temporary storage used in list_for_each_entry_safe
  * @member: the name of the list_head within the struct.
  *
  * list_safe_reset_next is not safe to use in general if the list may be
  * modified concurrently (eg. the lock is dropped in the loop body). An
  * exception to this is if the cursor element (pos) is pinned in the list,
  * and list_safe_reset_next is called after re-taking the lock and before
  * completing the current iteration of the loop body.
  */
 #define list_safe_reset_next(pos, n, member)                \
     n = list_next_entry(pos, member)
 
 /*
  * Double linked lists with a single pointer list head.
  * Mostly useful for hash tables where the two pointer list head is
  * too wasteful.
  * You lose the ability to access the tail in O(1).
  */
 
 #define HLIST_HEAD_INIT { .first = NULL }
 #define HLIST_HEAD(name) struct hlist_head name = {  .first = NULL }
 #define INIT_HLIST_HEAD(ptr) ((ptr)->first = NULL)
 static inline void INIT_HLIST_NODE(struct hlist_node *h)
 {
     h->next = NULL;
     h->pprev = NULL;
 }
 
 /**
  * hlist_unhashed - Has node been removed from list and reinitialized?
  * @h: Node to be checked
  *
  * Not that not all removal functions will leave a node in unhashed
  * state.  For example, hlist_nulls_del_init_rcu() does leave the
  * node in unhashed state, but hlist_nulls_del() does not.
  */
 static inline int hlist_unhashed(const struct hlist_node *h)
 {
     return !h->pprev;
 }
 
 /**
  * hlist_unhashed_lockless - Version of hlist_unhashed for lockless use
  * @h: Node to be checked
  *
  * This variant of hlist_unhashed() must be used in lockless contexts
  * to avoid potential load-tearing.  The READ_ONCE() is paired with the
  * various WRITE_ONCE() in hlist helpers that are defined below.
  */
 static inline int hlist_unhashed_lockless(const struct hlist_node *h)
 {
     return !READ_ONCE(h->pprev);
 }
 
 /**
  * hlist_empty - Is the specified hlist_head structure an empty hlist?
  * @h: Structure to check.
  */
 static inline int hlist_empty(const struct hlist_head *h)
 {
     return !READ_ONCE(h->first);
 }
 
 static inline void __hlist_del(struct hlist_node *n)
 {
     struct hlist_node *next = n->next;
     struct hlist_node **pprev = n->pprev;
 
     WRITE_ONCE(*pprev, next);
     if (next)
         WRITE_ONCE(next->pprev, pprev);
 }
 
 /**
  * hlist_del - Delete the specified hlist_node from its list
  * @n: Node to delete.
  *
  * Note that this function leaves the node in hashed state.  Use
  * hlist_del_init() or similar instead to unhash @n.
  */
 static inline void hlist_del(struct hlist_node *n)
 {
     __hlist_del(n);
     n->next = LIST_POISON1;
     n->pprev = LIST_POISON2;
 }
 
 /**
  * hlist_del_init - Delete the specified hlist_node from its list and initialize
  * @n: Node to delete.
  *
  * Note that this function leaves the node in unhashed state.
  */
 static inline void hlist_del_init(struct hlist_node *n)
 {
     if (!hlist_unhashed(n)) {
         __hlist_del(n);
         INIT_HLIST_NODE(n);
     }
 }
 
 /**
  * hlist_add_head - add a new entry at the beginning of the hlist
  * @n: new entry to be added
  * @h: hlist head to add it after
  *
  * Insert a new entry after the specified head.
  * This is good for implementing stacks.
  */
 static inline void hlist_add_head(struct hlist_node *n, struct hlist_head *h)
 {
     struct hlist_node *first = h->first;
     WRITE_ONCE(n->next, first);
     if (first)
         WRITE_ONCE(first->pprev, &n->next);
     WRITE_ONCE(h->first, n);
     WRITE_ONCE(n->pprev, &h->first);
 }
 
 /**
  * hlist_add_before - add a new entry before the one specified
  * @n: new entry to be added
  * @next: hlist node to add it before, which must be non-NULL
  */
 static inline void hlist_add_before(struct hlist_node *n,
                     struct hlist_node *next)
 {
     WRITE_ONCE(n->pprev, next->pprev);
     WRITE_ONCE(n->next, next);
     WRITE_ONCE(next->pprev, &n->next);
     WRITE_ONCE(*(n->pprev), n);
 }
 
 /**
  * hlist_add_behind - add a new entry after the one specified
  * @n: new entry to be added
  * @prev: hlist node to add it after, which must be non-NULL
  */
 static inline void hlist_add_behind(struct hlist_node *n,
                     struct hlist_node *prev)
 {
     WRITE_ONCE(n->next, prev->next);
     WRITE_ONCE(prev->next, n);
     WRITE_ONCE(n->pprev, &prev->next);
 
     if (n->next)
         WRITE_ONCE(n->next->pprev, &n->next);
 }
 
 /**
  * hlist_add_fake - create a fake hlist consisting of a single headless node
  * @n: Node to make a fake list out of
  *
  * This makes @n appear to be its own predecessor on a headless hlist.
  * The point of this is to allow things like hlist_del() to work correctly
  * in cases where there is no list.
  */
 static inline void hlist_add_fake(struct hlist_node *n)
 {
     n->pprev = &n->next;
 }
 
 /**
  * hlist_fake: Is this node a fake hlist?
  * @h: Node to check for being a self-referential fake hlist.
  */
 static inline bool hlist_fake(struct hlist_node *h)
 {
     return h->pprev == &h->next;
 }
 
 /**
  * hlist_is_singular_node - is node the only element of the specified hlist?
  * @n: Node to check for singularity.
  * @h: Header for potentially singular list.
  *
  * Check whether the node is the only node of the head without
  * accessing head, thus avoiding unnecessary cache misses.
  */
 static inline bool
 hlist_is_singular_node(struct hlist_node *n, struct hlist_head *h)
 {
     return !n->next && n->pprev == &h->first;
 }
 
 /**
  * hlist_move_list - Move an hlist
  * @old: hlist_head for old list.
  * @new: hlist_head for new list.
  *
  * Move a list from one list head to another. Fixup the pprev
  * reference of the first entry if it exists.
  */
 static inline void hlist_move_list(struct hlist_head *old,
                    struct hlist_head *new)
 {
     new->first = old->first;
     if (new->first)
         new->first->pprev = &new->first;
     old->first = NULL;
 }
 
 #define hlist_entry(ptr, type, member) container_of(ptr,type,member)
 
 #define hlist_for_each(pos, head) \
     for (pos = (head)->first; pos ; pos = pos->next)
 
 #define hlist_for_each_safe(pos, n, head) \
     for (pos = (head)->first; pos && ({ n = pos->next; 1; }); \
          pos = n)
 
 #define hlist_entry_safe(ptr, type, member) \
     ({ typeof(ptr) ____ptr = (ptr); \
        ____ptr ? hlist_entry(____ptr, type, member) : NULL; \
     })
 
 /**
  * hlist_for_each_entry - iterate over list of given type
  * @pos:    the type * to use as a loop cursor.
  * @head:   the head for your list.
  * @member: the name of the hlist_node within the struct.
  */
 #define hlist_for_each_entry(pos, head, member)             \
     for (pos = hlist_entry_safe((head)->first, typeof(*(pos)), member);\
          pos;                           \
          pos = hlist_entry_safe((pos)->member.next, typeof(*(pos)), member))
 
 /**
  * hlist_for_each_entry_continue - iterate over a hlist continuing after current point
  * @pos:    the type * to use as a loop cursor.
  * @member: the name of the hlist_node within the struct.
  */
 #define hlist_for_each_entry_continue(pos, member)          \
     for (pos = hlist_entry_safe((pos)->member.next, typeof(*(pos)), member);\
          pos;                           \
          pos = hlist_entry_safe((pos)->member.next, typeof(*(pos)), member))
 
 /**
  * hlist_for_each_entry_from - iterate over a hlist continuing from current point
  * @pos:    the type * to use as a loop cursor.
  * @member: the name of the hlist_node within the struct.
  */
 #define hlist_for_each_entry_from(pos, member)              \
     for (; pos;                         \
          pos = hlist_entry_safe((pos)->member.next, typeof(*(pos)), member))
 
 /**
  * hlist_for_each_entry_safe - iterate over list of given type safe against removal of list entry
  * @pos:    the type * to use as a loop cursor.
  * @n:      a &struct hlist_node to use as temporary storage
  * @head:   the head for your list.
  * @member: the name of the hlist_node within the struct.
  */
 #define hlist_for_each_entry_safe(pos, n, head, member)         \
     for (pos = hlist_entry_safe((head)->first, typeof(*pos), member);\
          pos && ({ n = pos->member.next; 1; });         \
          pos = hlist_entry_safe(n, typeof(*pos), member))
 
 #endif

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