text: /*
   * klist.c - Routines for manipulating klists.
   *
   * Copyright (C) 2005 Patrick Mochel
   *
   * This file is released under the GPL v2.
   *
   * This klist interface provides a couple of structures that wrap around
   * struct list_head to provide explicit list "head" (struct klist) and list
   * "node" (struct klist_node) objects. For struct klist, a spinlock is
   * included that protects access to the actual list itself. struct
   * klist_node provides a pointer to the klist that owns it and a kref
   * reference count that indicates the number of current users of that node
   * in the list.
   *
   * The entire point is to provide an interface for iterating over a list
   * that is safe and allows for modification of the list during the
   * iteration (e.g. insertion and removal), including modification of the
   * current node on the list.
   *
   * It works using a 3rd object type - struct klist_iter - that is declared
   * and initialized before an iteration. klist_next() is used to acquire the
   * next element in the list. It returns NULL if there are no more items.
   * Internally, that routine takes the klist's lock, decrements the
   * reference count of the previous klist_node and increments the count of
   * the next klist_node. It then drops the lock and returns.
   *
   * There are primitives for adding and removing nodes to/from a klist.
   * When deleting, klist_del() will simply decrement the reference count.
   * Only when the count goes to 0 is the node removed from the list.
   * klist_remove() will try to delete the node from the list and block until
   * it is actually removed. This is useful for objects (like devices) that
   * have been removed from the system and must be freed (but must wait until
   * all accessors have finished).
   */
  
  #include <linux/klist.h>
  #include <linux/export.h>
  #include <linux/sched.h>
  
  /*
   * Use the lowest bit of n_klist to mark deleted nodes and exclude
   * dead ones from iteration.
   */
  #define KNODE_DEAD		1LU
  #define KNODE_KLIST_MASK	~KNODE_DEAD
  
  static struct klist *knode_klist(struct klist_node *knode)
  {
  	return (struct klist *)
  		((unsigned long)knode->n_klist & KNODE_KLIST_MASK);
  }
  
  static bool knode_dead(struct klist_node *knode)
  {
  	return (unsigned long)knode->n_klist & KNODE_DEAD;
  }
  
  static void knode_set_klist(struct klist_node *knode, struct klist *klist)
  {
  	knode->n_klist = klist;
  	/* no knode deserves to start its life dead */
  	WARN_ON(knode_dead(knode));
  }
  
  static void knode_kill(struct klist_node *knode)
  {
  	/* and no knode should die twice ever either, see we're very humane */
  	WARN_ON(knode_dead(knode));
  	*(unsigned long *)&knode->n_klist |= KNODE_DEAD;
  }
  
  /**
   * klist_init - Initialize a klist structure.
   * @k: The klist we're initializing.
   * @get: The get function for the embedding object (NULL if none)
   * @put: The put function for the embedding object (NULL if none)
   *
   * Initialises the klist structure.  If the klist_node structures are
   * going to be embedded in refcounted objects (necessary for safe
   * deletion) then the get/put arguments are used to initialise
   * functions that take and release references on the embedding
   * objects.
   */
  void klist_init(struct klist *k, void (*get)(struct klist_node *),
  		void (*put)(struct klist_node *))
  {
  	INIT_LIST_HEAD(&k->k_list);
  	spin_lock_init(&k->k_lock);
  	k->get = get;
  	k->put = put;
  }
  EXPORT_SYMBOL_GPL(klist_init);
  
  static void add_head(struct klist *k, struct klist_node *n)
  {
  	spin_lock(&k->k_lock);
  	list_add(&n->n_node, &k->k_list);
  	spin_unlock(&k->k_lock);
  }
  
  static void add_tail(struct klist *k, struct klist_node *n)
  {
  	spin_lock(&k->k_lock);
  	list_add_tail(&n->n_node, &k->k_list);
  	spin_unlock(&k->k_lock);
  }
  
  static void klist_node_init(struct klist *k, struct klist_node *n)
  {
  	INIT_LIST_HEAD(&n->n_node);
  	kref_init(&n->n_ref);
  	knode_set_klist(n, k);
  	if (k->get)
  		k->get(n);
  }
  
  /**
   * klist_add_head - Initialize a klist_node and add it to front.
   * @n: node we're adding.
   * @k: klist it's going on.
   */
  void klist_add_head(struct klist_node *n, struct klist *k)
  {
  	klist_node_init(k, n);
  	add_head(k, n);
  }
  EXPORT_SYMBOL_GPL(klist_add_head);
  
  /**
   * klist_add_tail - Initialize a klist_node and add it to back.
   * @n: node we're adding.
   * @k: klist it's going on.
   */
  void klist_add_tail(struct klist_node *n, struct klist *k)
  {
  	klist_node_init(k, n);
  	add_tail(k, n);
  }
  EXPORT_SYMBOL_GPL(klist_add_tail);
  
  /**
   * klist_add_after - Init a klist_node and add it after an existing node
   * @n: node we're adding.
   * @pos: node to put @n after
   */
  void klist_add_after(struct klist_node *n, struct klist_node *pos)
  {
  	struct klist *k = knode_klist(pos);
  
  	klist_node_init(k, n);
  	spin_lock(&k->k_lock);
  	list_add(&n->n_node, &pos->n_node);
  	spin_unlock(&k->k_lock);
  }
  EXPORT_SYMBOL_GPL(klist_add_after);
  
  /**
   * klist_add_before - Init a klist_node and add it before an existing node
   * @n: node we're adding.
   * @pos: node to put @n after
   */
  void klist_add_before(struct klist_node *n, struct klist_node *pos)
  {
  	struct klist *k = knode_klist(pos);
  
  	klist_node_init(k, n);
  	spin_lock(&k->k_lock);
  	list_add_tail(&n->n_node, &pos->n_node);
  	spin_unlock(&k->k_lock);
  }
  EXPORT_SYMBOL_GPL(klist_add_before);
  
  struct klist_waiter {
  	struct list_head list;
  	struct klist_node *node;
  	struct task_struct *process;
  	int woken;
  };
  
  static DEFINE_SPINLOCK(klist_remove_lock);
  static LIST_HEAD(klist_remove_waiters);
  
  static void klist_release(struct kref *kref)
  {
  	struct klist_waiter *waiter, *tmp;
  	struct klist_node *n = container_of(kref, struct klist_node, n_ref);
  
  	WARN_ON(!knode_dead(n));
  	list_del(&n->n_node);
  	spin_lock(&klist_remove_lock);
  	list_for_each_entry_safe(waiter, tmp, &klist_remove_waiters, list) {
  		if (waiter->node != n)
  			continue;
  
  		list_del(&waiter->list);
  		waiter->woken = 1;
  		mb();
  		wake_up_process(waiter->process);
  	}
  	spin_unlock(&klist_remove_lock);
  	knode_set_klist(n, NULL);
  }
  
  static int klist_dec_and_del(struct klist_node *n)
  {
  	return kref_put(&n->n_ref, klist_release);
  }
  
  static void klist_put(struct klist_node *n, bool kill)
  {
  	struct klist *k = knode_klist(n);
  	void (*put)(struct klist_node *) = k->put;
  
  	spin_lock(&k->k_lock);
  	if (kill)
  		knode_kill(n);
  	if (!klist_dec_and_del(n))
  		put = NULL;
  	spin_unlock(&k->k_lock);
  	if (put)
  		put(n);
  }
  
  /**
   * klist_del - Decrement the reference count of node and try to remove.
   * @n: node we're deleting.
   */
  void klist_del(struct klist_node *n)
  {
  	klist_put(n, true);
  }
  EXPORT_SYMBOL_GPL(klist_del);
  
  /**
   * klist_remove - Decrement the refcount of node and wait for it to go away.
   * @n: node we're removing.
   */
  void klist_remove(struct klist_node *n)
  {
  	struct klist_waiter waiter;
  
  	waiter.node = n;
  	waiter.process = current;
  	waiter.woken = 0;
  	spin_lock(&klist_remove_lock);
  	list_add(&waiter.list, &klist_remove_waiters);
  	spin_unlock(&klist_remove_lock);
  
  	klist_del(n);
  
  	for (;;) {
  		set_current_state(TASK_UNINTERRUPTIBLE);
  		if (waiter.woken)
  			break;
  		schedule();
  	}
  	__set_current_state(TASK_RUNNING);
  }
  EXPORT_SYMBOL_GPL(klist_remove);
  
  /**
   * klist_node_attached - Say whether a node is bound to a list or not.
   * @n: Node that we're testing.
   */
  int klist_node_attached(struct klist_node *n)
  {
  	return (n->n_klist != NULL);
  }
  EXPORT_SYMBOL_GPL(klist_node_attached);
  
  /**
   * klist_iter_init_node - Initialize a klist_iter structure.
   * @k: klist we're iterating.
   * @i: klist_iter we're filling.
   * @n: node to start with.
   *
   * Similar to klist_iter_init(), but starts the action off with @n,
   * instead of with the list head.
   */
  void klist_iter_init_node(struct klist *k, struct klist_iter *i,
  			  struct klist_node *n)
  {
  	i->i_klist = k;
  	i->i_cur = n;
  	if (n)
  		kref_get(&n->n_ref);
  }
  EXPORT_SYMBOL_GPL(klist_iter_init_node);
  
  /**
   * klist_iter_init - Iniitalize a klist_iter structure.
   * @k: klist we're iterating.
   * @i: klist_iter structure we're filling.
   *
   * Similar to klist_iter_init_node(), but start with the list head.
   */
  void klist_iter_init(struct klist *k, struct klist_iter *i)
  {
  	klist_iter_init_node(k, i, NULL);
  }
  EXPORT_SYMBOL_GPL(klist_iter_init);
  
  /**
   * klist_iter_exit - Finish a list iteration.
   * @i: Iterator structure.
   *
   * Must be called when done iterating over list, as it decrements the
   * refcount of the current node. Necessary in case iteration exited before
   * the end of the list was reached, and always good form.
   */
  void klist_iter_exit(struct klist_iter *i)
  {
  	if (i->i_cur) {
  		klist_put(i->i_cur, false);
  		i->i_cur = NULL;
  	}
  }
  EXPORT_SYMBOL_GPL(klist_iter_exit);
  
  static struct klist_node *to_klist_node(struct list_head *n)
  {
  	return container_of(n, struct klist_node, n_node);
  }
  
  /**
   * klist_next - Ante up next node in list.
   * @i: Iterator structure.
   *
   * First grab list lock. Decrement the reference count of the previous
   * node, if there was one. Grab the next node, increment its reference
   * count, drop the lock, and return that next node.
   */
  struct klist_node *klist_next(struct klist_iter *i)
  {
  	void (*put)(struct klist_node *) = i->i_klist->put;
  	struct klist_node *last = i->i_cur;
  	struct klist_node *next;
  
  	spin_lock(&i->i_klist->k_lock);
  
  	if (last) {
  		next = to_klist_node(last->n_node.next);
  		if (!klist_dec_and_del(last))
  			put = NULL;
  	} else
  		next = to_klist_node(i->i_klist->k_list.next);
  
  	i->i_cur = NULL;
  	while (next != to_klist_node(&i->i_klist->k_list)) {
  		if (likely(!knode_dead(next))) {
  			kref_get(&next->n_ref);
  			i->i_cur = next;
  			break;
  		}
  		next = to_klist_node(next->n_node.next);
  	}
  
  	spin_unlock(&i->i_klist->k_lock);
  
  	if (put && last)
  		put(last);
  	return i->i_cur;
  }
  EXPORT_SYMBOL_GPL(klist_next);
  