xref: /linux/include/linux/rculist.h (revision 100ceb4817a2ac650e29f107cf97161ce3e2289a)

/* SPDX-License-Identifier: GPL-2.0 */
#ifndef _LINUX_RCULIST_H
#define _LINUX_RCULIST_H

#ifdef __KERNEL__

/*
 * RCU-protected list version
 */
#include <linux/list.h>
#include <linux/rcupdate.h>

/*
 * INIT_LIST_HEAD_RCU - Initialize a list_head visible to RCU readers
 * @list: list to be initialized
 *
 * You should instead use INIT_LIST_HEAD() for normal initialization and
 * cleanup tasks, when readers have no access to the list being initialized.
 * However, if the list being initialized is visible to readers, you
 * need to keep the compiler from being too mischievous.
 */
static inline void INIT_LIST_HEAD_RCU(struct list_head *list)
{
	WRITE_ONCE(list->next, list);
	WRITE_ONCE(list->prev, list);
}

/*
 * return the ->next pointer of a list_head in an rcu safe
 * way, we must not access it directly
 */
#define list_next_rcu(list)	(*((struct list_head __rcu **)(&(list)->next)))
/*
 * Return the ->prev pointer of a list_head in an rcu safe way. Don't
 * access it directly.
 *
 * Any list traversed with list_bidir_prev_rcu() must never use
 * list_del_rcu().  Doing so will poison the ->prev pointer that
 * list_bidir_prev_rcu() relies on, which will result in segfaults.
 * To prevent these segfaults, use list_bidir_del_rcu() instead
 * of list_del_rcu().
 */
#define list_bidir_prev_rcu(list) (*((struct list_head __rcu **)(&(list)->prev)))

/**
 * list_tail_rcu - returns the prev pointer of the head of the list
 * @head: the head of the list
 *
 * Note: This should only be used with the list header, and even then
 * only if list_del() and similar primitives are not also used on the
 * list header.
 */
#define list_tail_rcu(head)	(*((struct list_head __rcu **)(&(head)->prev)))

/*
 * Check during list traversal that we are within an RCU reader
 */

#define check_arg_count_one(dummy)

#ifdef CONFIG_PROVE_RCU_LIST
#define __list_check_rcu(dummy, cond, extra...)				\
	({								\
	check_arg_count_one(extra);					\
	RCU_LOCKDEP_WARN(!(cond) && !rcu_read_lock_any_held(),		\
			 "RCU-list traversed in non-reader section!");	\
	})

#define __list_check_srcu(cond)					 \
	({								 \
	RCU_LOCKDEP_WARN(!(cond),					 \
		"RCU-list traversed without holding the required lock!");\
	})
#else
#define __list_check_rcu(dummy, cond, extra...)				\
	({ check_arg_count_one(extra); })

#define __list_check_srcu(cond) ({ })
#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_rcu(struct list_head *new,
		struct list_head *prev, struct list_head *next)
{
	if (!__list_add_valid(new, prev, next))
		return;

	new->next = next;
	new->prev = prev;
	rcu_assign_pointer(list_next_rcu(prev), new);
	next->prev = new;
}

/**
 * list_add_rcu - add a new entry to rcu-protected list
 * @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.
 *
 * The caller must take whatever precautions are necessary
 * (such as holding appropriate locks) to avoid racing
 * with another list-mutation primitive, such as list_add_rcu()
 * or list_del_rcu(), running on this same list.
 * However, it is perfectly legal to run concurrently with
 * the _rcu list-traversal primitives, such as
 * list_for_each_entry_rcu().
 */
static inline void list_add_rcu(struct list_head *new, struct list_head *head)
{
	__list_add_rcu(new, head, head->next);
}

/**
 * list_add_tail_rcu - add a new entry to rcu-protected list
 * @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.
 *
 * The caller must take whatever precautions are necessary
 * (such as holding appropriate locks) to avoid racing
 * with another list-mutation primitive, such as list_add_tail_rcu()
 * or list_del_rcu(), running on this same list.
 * However, it is perfectly legal to run concurrently with
 * the _rcu list-traversal primitives, such as
 * list_for_each_entry_rcu().
 */
static inline void list_add_tail_rcu(struct list_head *new,
					struct list_head *head)
{
	__list_add_rcu(new, head->prev, head);
}

/**
 * list_del_rcu - deletes entry from list without re-initialization
 * @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. 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 list.
 *
 * The caller must take whatever precautions are necessary
 * (such as holding appropriate locks) to avoid racing
 * with another list-mutation primitive, such as list_del_rcu()
 * or list_add_rcu(), running on this same list.
 * However, it is perfectly legal to run concurrently with
 * the _rcu list-traversal primitives, such as
 * list_for_each_entry_rcu().
 *
 * Note that the caller is not permitted to immediately free
 * the newly deleted entry.  Instead, either synchronize_rcu()
 * or call_rcu() must be used to defer freeing until an RCU
 * grace period has elapsed.
 */
static inline void list_del_rcu(struct list_head *entry)
{
	__list_del_entry(entry);
	entry->prev = LIST_POISON2;
}

/**
 * list_bidir_del_rcu - deletes entry from list without re-initialization
 * @entry: the element to delete from the list.
 *
 * In contrast to list_del_rcu() doesn't poison the prev pointer thus
 * allowing backwards traversal via list_bidir_prev_rcu().
 *
 * Note: list_empty() on entry does not return true after this because
 * the entry is in a special undefined state that permits RCU-based
 * lockfree reverse traversal. In particular this means that we can not
 * poison the forward and backwards pointers that may still be used for
 * walking the list.
 *
 * The caller must take whatever precautions are necessary (such as
 * holding appropriate locks) to avoid racing with another list-mutation
 * primitive, such as list_bidir_del_rcu() or list_add_rcu(), running on
 * this same list. However, it is perfectly legal to run concurrently
 * with the _rcu list-traversal primitives, such as
 * list_for_each_entry_rcu().
 *
 * Note that list_del_rcu() and list_bidir_del_rcu() must not be used on
 * the same list.
 *
 * Note that the caller is not permitted to immediately free
 * the newly deleted entry.  Instead, either synchronize_rcu()
 * or call_rcu() must be used to defer freeing until an RCU
 * grace period has elapsed.
 */
static inline void list_bidir_del_rcu(struct list_head *entry)
{
	__list_del_entry(entry);
}

/**
 * hlist_del_init_rcu - deletes entry from hash list with re-initialization
 * @n: the element to delete from the hash list.
 *
 * Note: list_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_add_head_rcu() or
 * hlist_del_rcu(), running on this same list.  However, it is
 * perfectly legal to run concurrently with the _rcu list-traversal
 * primitives, such as hlist_for_each_entry_rcu().
 */
static inline void hlist_del_init_rcu(struct hlist_node *n)
{
	if (!hlist_unhashed(n)) {
		__hlist_del(n);
		WRITE_ONCE(n->pprev, NULL);
	}
}

/**
 * list_replace_rcu - replace old entry by new one
 * @old : the element to be replaced
 * @new : the new element to insert
 *
 * The @old entry will be replaced with the @new entry atomically from
 * the perspective of concurrent readers.  It is the caller's responsibility
 * to synchronize with concurrent updaters, if any.
 *
 * Note: @old should not be empty.
 */
static inline void list_replace_rcu(struct list_head *old,
				struct list_head *new)
{
	new->next = old->next;
	new->prev = old->prev;
	rcu_assign_pointer(list_next_rcu(new->prev), new);
	new->next->prev = new;
	old->prev = LIST_POISON2;
}

/**
 * __list_splice_init_rcu - join an RCU-protected list into an existing list.
 * @list:	the RCU-protected list to splice
 * @prev:	points to the last element of the existing list
 * @next:	points to the first element of the existing list
 * @sync:	synchronize_rcu, synchronize_rcu_expedited, ...
 *
 * The list pointed to by @prev and @next can be RCU-read traversed
 * concurrently with this function.
 *
 * Note that this function blocks.
 *
 * Important note: the caller must take whatever action is necessary to prevent
 * any other updates to the existing list.  In principle, it is possible to
 * modify the list as soon as sync() begins execution. If this sort of thing
 * becomes necessary, an alternative version based on call_rcu() could be
 * created.  But only if -really- needed -- there is no shortage of RCU API
 * members.
 */
static inline void __list_splice_init_rcu(struct list_head *list,
					  struct list_head *prev,
					  struct list_head *next,
					  void (*sync)(void))
{
	struct list_head *first = list->next;
	struct list_head *last = list->prev;

	/*
	 * "first" and "last" tracking list, so initialize it.  RCU readers
	 * have access to this list, so we must use INIT_LIST_HEAD_RCU()
	 * instead of INIT_LIST_HEAD().
	 */

	INIT_LIST_HEAD_RCU(list);

	/*
	 * At this point, the list body still points to the source list.
	 * Wait for any readers to finish using the list before splicing
	 * the list body into the new list.  Any new readers will see
	 * an empty list.
	 */

	sync();
	ASSERT_EXCLUSIVE_ACCESS(*first);
	ASSERT_EXCLUSIVE_ACCESS(*last);

	/*
	 * Readers are finished with the source list, so perform splice.
	 * The order is important if the new list is global and accessible
	 * to concurrent RCU readers.  Note that RCU readers are not
	 * permitted to traverse the prev pointers without excluding
	 * this function.
	 */

	last->next = next;
	rcu_assign_pointer(list_next_rcu(prev), first);
	first->prev = prev;
	next->prev = last;
}

/**
 * list_splice_init_rcu - splice an RCU-protected list into an existing list,
 *                        designed for stacks.
 * @list:	the RCU-protected list to splice
 * @head:	the place in the existing list to splice the first list into
 * @sync:	synchronize_rcu, synchronize_rcu_expedited, ...
 */
static inline void list_splice_init_rcu(struct list_head *list,
					struct list_head *head,
					void (*sync)(void))
{
	if (!list_empty(list))
		__list_splice_init_rcu(list, head, head->next, sync);
}

/**
 * list_splice_tail_init_rcu - splice an RCU-protected list into an existing
 *                             list, designed for queues.
 * @list:	the RCU-protected list to splice
 * @head:	the place in the existing list to splice the first list into
 * @sync:	synchronize_rcu, synchronize_rcu_expedited, ...
 */
static inline void list_splice_tail_init_rcu(struct list_head *list,
					     struct list_head *head,
					     void (*sync)(void))
{
	if (!list_empty(list))
		__list_splice_init_rcu(list, head->prev, head, sync);
}

/**
 * list_entry_rcu - 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.
 *
 * This primitive may safely run concurrently with the _rcu list-mutation
 * primitives such as list_add_rcu() as long as it's guarded by rcu_read_lock().
 */
#define list_entry_rcu(ptr, type, member) \
	container_of(READ_ONCE(ptr), type, member)

/*
 * Where are list_empty_rcu() and list_first_entry_rcu()?
 *
 * They do not exist because they would lead to subtle race conditions:
 *
 * if (!list_empty_rcu(mylist)) {
 *	struct foo *bar = list_first_entry_rcu(mylist, struct foo, list_member);
 *	do_something(bar);
 * }
 *
 * The list might be non-empty when list_empty_rcu() checks it, but it
 * might have become empty by the time that list_first_entry_rcu() rereads
 * the ->next pointer, which would result in a SEGV.
 *
 * When not using RCU, it is OK for list_first_entry() to re-read that
 * pointer because both functions should be protected by some lock that
 * blocks writers.
 *
 * When using RCU, list_empty() uses READ_ONCE() to fetch the
 * RCU-protected ->next pointer and then compares it to the address of the
 * list head.  However, it neither dereferences this pointer nor provides
 * this pointer to its caller.  Thus, READ_ONCE() suffices (that is,
 * rcu_dereference() is not needed), which means that list_empty() can be
 * used anywhere you would want to use list_empty_rcu().  Just don't
 * expect anything useful to happen if you do a subsequent lockless
 * call to list_first_entry_rcu()!!!
 *
 * See list_first_or_null_rcu for an alternative.
 */

/**
 * list_first_or_null_rcu - 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.
 *
 * This primitive may safely run concurrently with the _rcu list-mutation
 * primitives such as list_add_rcu() as long as it's guarded by rcu_read_lock().
 */
#define list_first_or_null_rcu(ptr, type, member) \
({ \
	struct list_head *__ptr = (ptr); \
	struct list_head *__next = READ_ONCE(__ptr->next); \
	likely(__ptr != __next) ? list_entry_rcu(__next, type, member) : NULL; \
})

/**
 * list_next_or_null_rcu - get the next element from a list
 * @head:	the head for the list.
 * @ptr:        the list head to take the next 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 ptr is at the end of the list, NULL is returned.
 *
 * This primitive may safely run concurrently with the _rcu list-mutation
 * primitives such as list_add_rcu() as long as it's guarded by rcu_read_lock().
 */
#define list_next_or_null_rcu(head, ptr, type, member) \
({ \
	struct list_head *__head = (head); \
	struct list_head *__ptr = (ptr); \
	struct list_head *__next = READ_ONCE(__ptr->next); \
	likely(__next != __head) ? list_entry_rcu(__next, type, \
						  member) : NULL; \
})

/**
 * list_for_each_entry_rcu	-	iterate over rcu 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.
 * @cond:	optional lockdep expression if called from non-RCU protection.
 *
 * This list-traversal primitive may safely run concurrently with
 * the _rcu list-mutation primitives such as list_add_rcu()
 * as long as the traversal is guarded by rcu_read_lock().
 */
#define list_for_each_entry_rcu(pos, head, member, cond...)		\
	for (__list_check_rcu(dummy, ## cond, 0),			\
	     pos = list_entry_rcu((head)->next, typeof(*pos), member);	\
		&pos->member != (head);					\
		pos = list_entry_rcu(pos->member.next, typeof(*pos), member))

/**
 * list_for_each_entry_srcu	-	iterate over rcu 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.
 * @cond:	lockdep expression for the lock required to traverse the list.
 *
 * This list-traversal primitive may safely run concurrently with
 * the _rcu list-mutation primitives such as list_add_rcu()
 * as long as the traversal is guarded by srcu_read_lock().
 * The lockdep expression srcu_read_lock_held() can be passed as the
 * cond argument from read side.
 */
#define list_for_each_entry_srcu(pos, head, member, cond)		\
	for (__list_check_srcu(cond),					\
	     pos = list_entry_rcu((head)->next, typeof(*pos), member);	\
		&pos->member != (head);					\
		pos = list_entry_rcu(pos->member.next, typeof(*pos), member))

/**
 * list_entry_lockless - 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.
 *
 * This primitive may safely run concurrently with the _rcu
 * list-mutation primitives such as list_add_rcu(), but requires some
 * implicit RCU read-side guarding.  One example is running within a special
 * exception-time environment where preemption is disabled and where lockdep
 * cannot be invoked.  Another example is when items are added to the list,
 * but never deleted.
 */
#define list_entry_lockless(ptr, type, member) \
	container_of((typeof(ptr))READ_ONCE(ptr), type, member)

/**
 * list_for_each_entry_lockless - iterate over rcu 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_struct within the struct.
 *
 * This primitive may safely run concurrently with the _rcu
 * list-mutation primitives such as list_add_rcu(), but requires some
 * implicit RCU read-side guarding.  One example is running within a special
 * exception-time environment where preemption is disabled and where lockdep
 * cannot be invoked.  Another example is when items are added to the list,
 * but never deleted.
 */
#define list_for_each_entry_lockless(pos, head, member) \
	for (pos = list_entry_lockless((head)->next, typeof(*pos), member); \
	     &pos->member != (head); \
	     pos = list_entry_lockless(pos->member.next, typeof(*pos), member))

/**
 * list_for_each_entry_continue_rcu - 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 which must have been in the list when the RCU read
 * lock was taken.
 * This would typically require either that you obtained the node from a
 * previous walk of the list in the same RCU read-side critical section, or
 * that you held some sort of non-RCU reference (such as a reference count)
 * to keep the node alive *and* in the list.
 *
 * This iterator is similar to list_for_each_entry_from_rcu() except
 * this starts after the given position and that one starts at the given
 * position.
 */
#define list_for_each_entry_continue_rcu(pos, head, member) 		\
	for (pos = list_entry_rcu(pos->member.next, typeof(*pos), member); \
	     &pos->member != (head);	\
	     pos = list_entry_rcu(pos->member.next, typeof(*pos), member))

/**
 * list_for_each_entry_from_rcu - iterate over a list from current point
 * @pos:	the type * to use as a loop cursor.
 * @head:	the head for your list.
 * @member:	the name of the list_node within the struct.
 *
 * Iterate over the tail of a list starting from a given position,
 * which must have been in the list when the RCU read lock was taken.
 * This would typically require either that you obtained the node from a
 * previous walk of the list in the same RCU read-side critical section, or
 * that you held some sort of non-RCU reference (such as a reference count)
 * to keep the node alive *and* in the list.
 *
 * This iterator is similar to list_for_each_entry_continue_rcu() except
 * this starts from the given position and that one starts from the position
 * after the given position.
 */
#define list_for_each_entry_from_rcu(pos, head, member)			\
	for (; &(pos)->member != (head);					\
		pos = list_entry_rcu(pos->member.next, typeof(*(pos)), member))

/**
 * hlist_del_rcu - deletes entry from hash list without re-initialization
 * @n: the element to delete from the hash list.
 *
 * Note: list_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_add_head_rcu()
 * or hlist_del_rcu(), running on this same list.
 * However, it is perfectly legal to run concurrently with
 * the _rcu list-traversal primitives, such as
 * hlist_for_each_entry().
 */
static inline void hlist_del_rcu(struct hlist_node *n)
{
	__hlist_del(n);
	WRITE_ONCE(n->pprev, LIST_POISON2);
}

/**
 * hlist_replace_rcu - replace old entry by new one
 * @old : the element to be replaced
 * @new : the new element to insert
 *
 * The @old entry will be replaced with the @new entry atomically from
 * the perspective of concurrent readers.  It is the caller's responsibility
 * to synchronize with concurrent updaters, if any.
 */
static inline void hlist_replace_rcu(struct hlist_node *old,
					struct hlist_node *new)
{
	struct hlist_node *next = old->next;

	new->next = next;
	WRITE_ONCE(new->pprev, old->pprev);
	rcu_assign_pointer(*(struct hlist_node __rcu **)new->pprev, new);
	if (next)
		WRITE_ONCE(new->next->pprev, &new->next);
	WRITE_ONCE(old->pprev, LIST_POISON2);
}

/**
 * hlists_swap_heads_rcu - swap the lists the hlist heads point to
 * @left:  The hlist head on the left
 * @right: The hlist head on the right
 *
 * The lists start out as [@left  ][node1 ... ] and
 *                        [@right ][node2 ... ]
 * The lists end up as    [@left  ][node2 ... ]
 *                        [@right ][node1 ... ]
 */
static inline void hlists_swap_heads_rcu(struct hlist_head *left, struct hlist_head *right)
{
	struct hlist_node *node1 = left->first;
	struct hlist_node *node2 = right->first;

	rcu_assign_pointer(left->first, node2);
	rcu_assign_pointer(right->first, node1);
	WRITE_ONCE(node2->pprev, &left->first);
	WRITE_ONCE(node1->pprev, &right->first);
}

/*
 * return the first or the next element in an RCU protected hlist
 */
#define hlist_first_rcu(head)	(*((struct hlist_node __rcu **)(&(head)->first)))
#define hlist_next_rcu(node)	(*((struct hlist_node __rcu **)(&(node)->next)))
#define hlist_pprev_rcu(node)	(*((struct hlist_node __rcu **)((node)->pprev)))

/**
 * hlist_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,
 * 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_add_head_rcu()
 * or hlist_del_rcu(), running on this same list.
 * However, it is perfectly legal to run concurrently with
 * the _rcu list-traversal primitives, such as
 * hlist_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_add_head_rcu(struct hlist_node *n,
					struct hlist_head *h)
{
	struct hlist_node *first = h->first;

	n->next = first;
	WRITE_ONCE(n->pprev, &h->first);
	rcu_assign_pointer(hlist_first_rcu(h), n);
	if (first)
		WRITE_ONCE(first->pprev, &n->next);
}

/**
 * hlist_add_tail_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,
 * 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_add_head_rcu()
 * or hlist_del_rcu(), running on this same list.
 * However, it is perfectly legal to run concurrently with
 * the _rcu list-traversal primitives, such as
 * hlist_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_add_tail_rcu(struct hlist_node *n,
				      struct hlist_head *h)
{
	struct hlist_node *i, *last = NULL;

	/* Note: write side code, so rcu accessors are not needed. */
	for (i = h->first; i; i = i->next)
		last = i;

	if (last) {
		n->next = last->next;
		WRITE_ONCE(n->pprev, &last->next);
		rcu_assign_pointer(hlist_next_rcu(last), n);
	} else {
		hlist_add_head_rcu(n, h);
	}
}

/**
 * hlist_add_before_rcu
 * @n: the new element to add to the hash list.
 * @next: the existing element to add the new element before.
 *
 * Description:
 * Adds the specified element to the specified hlist
 * before the specified node 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_add_head_rcu()
 * or hlist_del_rcu(), running on this same list.
 * However, it is perfectly legal to run concurrently with
 * the _rcu list-traversal primitives, such as
 * hlist_for_each_entry_rcu(), used to prevent memory-consistency
 * problems on Alpha CPUs.
 */
static inline void hlist_add_before_rcu(struct hlist_node *n,
					struct hlist_node *next)
{
	WRITE_ONCE(n->pprev, next->pprev);
	n->next = next;
	rcu_assign_pointer(hlist_pprev_rcu(n), n);
	WRITE_ONCE(next->pprev, &n->next);
}

/**
 * hlist_add_behind_rcu
 * @n: the new element to add to the hash list.
 * @prev: the existing element to add the new element after.
 *
 * Description:
 * Adds the specified element to the specified hlist
 * after the specified node 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_add_head_rcu()
 * or hlist_del_rcu(), running on this same list.
 * However, it is perfectly legal to run concurrently with
 * the _rcu list-traversal primitives, such as
 * hlist_for_each_entry_rcu(), used to prevent memory-consistency
 * problems on Alpha CPUs.
 */
static inline void hlist_add_behind_rcu(struct hlist_node *n,
					struct hlist_node *prev)
{
	n->next = prev->next;
	WRITE_ONCE(n->pprev, &prev->next);
	rcu_assign_pointer(hlist_next_rcu(prev), n);
	if (n->next)
		WRITE_ONCE(n->next->pprev, &n->next);
}

#define __hlist_for_each_rcu(pos, head)				\
	for (pos = rcu_dereference(hlist_first_rcu(head));	\
	     pos;						\
	     pos = rcu_dereference(hlist_next_rcu(pos)))

/**
 * hlist_for_each_entry_rcu - iterate over rcu 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.
 * @cond:	optional lockdep expression if called from non-RCU protection.
 *
 * This list-traversal primitive may safely run concurrently with
 * the _rcu list-mutation primitives such as hlist_add_head_rcu()
 * as long as the traversal is guarded by rcu_read_lock().
 */
#define hlist_for_each_entry_rcu(pos, head, member, cond...)		\
	for (__list_check_rcu(dummy, ## cond, 0),			\
	     pos = hlist_entry_safe(rcu_dereference_raw(hlist_first_rcu(head)),\
			typeof(*(pos)), member);			\
		pos;							\
		pos = hlist_entry_safe(rcu_dereference_raw(hlist_next_rcu(\
			&(pos)->member)), typeof(*(pos)), member))

/**
 * hlist_for_each_entry_srcu - iterate over rcu 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.
 * @cond:	lockdep expression for the lock required to traverse the list.
 *
 * This list-traversal primitive may safely run concurrently with
 * the _rcu list-mutation primitives such as hlist_add_head_rcu()
 * as long as the traversal is guarded by srcu_read_lock().
 * The lockdep expression srcu_read_lock_held() can be passed as the
 * cond argument from read side.
 */
#define hlist_for_each_entry_srcu(pos, head, member, cond)		\
	for (__list_check_srcu(cond),					\
	     pos = hlist_entry_safe(rcu_dereference_raw(hlist_first_rcu(head)),\
			typeof(*(pos)), member);			\
		pos;							\
		pos = hlist_entry_safe(rcu_dereference_raw(hlist_next_rcu(\
			&(pos)->member)), typeof(*(pos)), member))

/**
 * hlist_for_each_entry_rcu_notrace - iterate over rcu list of given type (for tracing)
 * @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.
 *
 * This list-traversal primitive may safely run concurrently with
 * the _rcu list-mutation primitives such as hlist_add_head_rcu()
 * as long as the traversal is guarded by rcu_read_lock().
 *
 * This is the same as hlist_for_each_entry_rcu() except that it does
 * not do any RCU debugging or tracing.
 */
#define hlist_for_each_entry_rcu_notrace(pos, head, member)			\
	for (pos = hlist_entry_safe(rcu_dereference_raw_check(hlist_first_rcu(head)),\
			typeof(*(pos)), member);			\
		pos;							\
		pos = hlist_entry_safe(rcu_dereference_raw_check(hlist_next_rcu(\
			&(pos)->member)), typeof(*(pos)), member))

/**
 * hlist_for_each_entry_rcu_bh - iterate over rcu 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.
 *
 * This list-traversal primitive may safely run concurrently with
 * the _rcu list-mutation primitives such as hlist_add_head_rcu()
 * as long as the traversal is guarded by rcu_read_lock().
 */
#define hlist_for_each_entry_rcu_bh(pos, head, member)			\
	for (pos = hlist_entry_safe(rcu_dereference_bh(hlist_first_rcu(head)),\
			typeof(*(pos)), member);			\
		pos;							\
		pos = hlist_entry_safe(rcu_dereference_bh(hlist_next_rcu(\
			&(pos)->member)), typeof(*(pos)), member))

/**
 * hlist_for_each_entry_continue_rcu - 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_rcu(pos, member)			\
	for (pos = hlist_entry_safe(rcu_dereference_raw(hlist_next_rcu( \
			&(pos)->member)), typeof(*(pos)), member);	\
	     pos;							\
	     pos = hlist_entry_safe(rcu_dereference_raw(hlist_next_rcu(	\
			&(pos)->member)), typeof(*(pos)), member))

/**
 * hlist_for_each_entry_continue_rcu_bh - 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_rcu_bh(pos, member)		\
	for (pos = hlist_entry_safe(rcu_dereference_bh(hlist_next_rcu(  \
			&(pos)->member)), typeof(*(pos)), member);	\
	     pos;							\
	     pos = hlist_entry_safe(rcu_dereference_bh(hlist_next_rcu(	\
			&(pos)->member)), typeof(*(pos)), member))

/**
 * hlist_for_each_entry_from_rcu - 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_rcu(pos, member)			\
	for (; pos;							\
	     pos = hlist_entry_safe(rcu_dereference_raw(hlist_next_rcu(	\
			&(pos)->member)), typeof(*(pos)), member))

#endif	/* __KERNEL__ */
#endif