xref: /linux-6.15/include/linux/list.h (revision cec78a59)

/* 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_LIST_HARDENED

#ifdef CONFIG_DEBUG_LIST
# define __list_valid_slowpath
#else
# define __list_valid_slowpath __cold __preserve_most
#endif

/*
 * Performs the full set of list corruption checks before __list_add().
 * On list corruption reports a warning, and returns false.
 */
extern bool __list_valid_slowpath __list_add_valid_or_report(struct list_head *new,
							     struct list_head *prev,
							     struct list_head *next);

/*
 * Performs list corruption checks before __list_add(). Returns false if a
 * corruption is detected, true otherwise.
 *
 * With CONFIG_LIST_HARDENED only, performs minimal list integrity checking
 * inline to catch non-faulting corruptions, and only if a corruption is
 * detected calls the reporting function __list_add_valid_or_report().
 */
static __always_inline bool __list_add_valid(struct list_head *new,
					     struct list_head *prev,
					     struct list_head *next)
{
	bool ret = true;

	if (!IS_ENABLED(CONFIG_DEBUG_LIST)) {
		/*
		 * With the hardening version, elide checking if next and prev
		 * are NULL, since the immediate dereference of them below would
		 * result in a fault if NULL.
		 *
		 * With the reduced set of checks, we can afford to inline the
		 * checks, which also gives the compiler a chance to elide some
		 * of them completely if they can be proven at compile-time. If
		 * one of the pre-conditions does not hold, the slow-path will
		 * show a report which pre-condition failed.
		 */
		if (likely(next->prev == prev && prev->next == next && new != prev && new != next))
			return true;
		ret = false;
	}

	ret &= __list_add_valid_or_report(new, prev, next);
	return ret;
}

/*
 * Performs the full set of list corruption checks before __list_del_entry().
 * On list corruption reports a warning, and returns false.
 */
extern bool __list_valid_slowpath __list_del_entry_valid_or_report(struct list_head *entry);

/*
 * Performs list corruption checks before __list_del_entry(). Returns false if a
 * corruption is detected, true otherwise.
 *
 * With CONFIG_LIST_HARDENED only, performs minimal list integrity checking
 * inline to catch non-faulting corruptions, and only if a corruption is
 * detected calls the reporting function __list_del_entry_valid_or_report().
 */
static __always_inline bool __list_del_entry_valid(struct list_head *entry)
{
	bool ret = true;

	if (!IS_ENABLED(CONFIG_DEBUG_LIST)) {
		struct list_head *prev = entry->prev;
		struct list_head *next = entry->next;

		/*
		 * With the hardening version, elide checking if next and prev
		 * are NULL, LIST_POISON1 or LIST_POISON2, since the immediate
		 * dereference of them below would result in a fault.
		 */
		if (likely(prev->next == entry && next->prev == entry))
			return true;
		ret = false;
	}

	ret &= __list_del_entry_valid_or_report(entry);
	return ret;
}
#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_count_nodes - count nodes in the list
 * @head:	the head for your list.
 */
static inline size_t list_count_nodes(struct list_head *head)
{
	struct list_head *pos;
	size_t count = 0;

	list_for_each(pos, head)
		count++;

	return count;
}

/**
 * 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)				\
	list_is_head(&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;
}

/**
 * hlist_splice_init() - move all entries from one list to another
 * @from: hlist_head from which entries will be moved
 * @last: last entry on the @from list
 * @to:   hlist_head to which entries will be moved
 *
 * @to can be empty, @from must contain at least @last.
 */
static inline void hlist_splice_init(struct hlist_head *from,
				     struct hlist_node *last,
				     struct hlist_head *to)
{
	if (to->first)
		to->first->pprev = &last->next;
	last->next = to->first;
	to->first = from->first;
	from->first->pprev = &to->first;
	from->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))

/**
 * hlist_count_nodes - count nodes in the hlist
 * @head:	the head for your hlist.
 */
static inline size_t hlist_count_nodes(struct hlist_head *head)
{
	struct hlist_node *pos;
	size_t count = 0;

	hlist_for_each(pos, head)
		count++;

	return count;
}

#endif