text: /* Generic associative array implementation.
   *
   * See Documentation/core-api/assoc_array.rst for information.
   *
   * Copyright (C) 2013 Red Hat, Inc. All Rights Reserved.
   * Written by David Howells (dhowells@redhat.com)
   *
   * This program is free software; you can redistribute it and/or
   * modify it under the terms of the GNU General Public Licence
   * as published by the Free Software Foundation; either version
   * 2 of the Licence, or (at your option) any later version.
   */
  //#define DEBUG
  #include <linux/rcupdate.h>
  #include <linux/slab.h>
  #include <linux/err.h>
  #include <linux/assoc_array_priv.h>
  
  /*
   * Iterate over an associative array.  The caller must hold the RCU read lock
   * or better.
   */
  static int assoc_array_subtree_iterate(const struct assoc_array_ptr *root,
  				       const struct assoc_array_ptr *stop,
  				       int (*iterator)(const void *leaf,
  						       void *iterator_data),
  				       void *iterator_data)
  {
  	const struct assoc_array_shortcut *shortcut;
  	const struct assoc_array_node *node;
  	const struct assoc_array_ptr *cursor, *ptr, *parent;
  	unsigned long has_meta;
  	int slot, ret;
  
  	cursor = root;
  
  begin_node:
  	if (assoc_array_ptr_is_shortcut(cursor)) {
  		/* Descend through a shortcut */
  		shortcut = assoc_array_ptr_to_shortcut(cursor);
  		smp_read_barrier_depends();
  		cursor = READ_ONCE(shortcut->next_node);
  	}
  
  	node = assoc_array_ptr_to_node(cursor);
  	smp_read_barrier_depends();
  	slot = 0;
  
  	/* We perform two passes of each node.
  	 *
  	 * The first pass does all the leaves in this node.  This means we
  	 * don't miss any leaves if the node is split up by insertion whilst
  	 * we're iterating over the branches rooted here (we may, however, see
  	 * some leaves twice).
  	 */
  	has_meta = 0;
  	for (; slot < ASSOC_ARRAY_FAN_OUT; slot++) {
  		ptr = READ_ONCE(node->slots[slot]);
  		has_meta |= (unsigned long)ptr;
  		if (ptr && assoc_array_ptr_is_leaf(ptr)) {
  			/* We need a barrier between the read of the pointer
  			 * and dereferencing the pointer - but only if we are
  			 * actually going to dereference it.
  			 */
  			smp_read_barrier_depends();
  
  			/* Invoke the callback */
  			ret = iterator(assoc_array_ptr_to_leaf(ptr),
  				       iterator_data);
  			if (ret)
  				return ret;
  		}
  	}
  
  	/* The second pass attends to all the metadata pointers.  If we follow
  	 * one of these we may find that we don't come back here, but rather go
  	 * back to a replacement node with the leaves in a different layout.
  	 *
  	 * We are guaranteed to make progress, however, as the slot number for
  	 * a particular portion of the key space cannot change - and we
  	 * continue at the back pointer + 1.
  	 */
  	if (!(has_meta & ASSOC_ARRAY_PTR_META_TYPE))
  		goto finished_node;
  	slot = 0;
  
  continue_node:
  	node = assoc_array_ptr_to_node(cursor);
  	smp_read_barrier_depends();
  
  	for (; slot < ASSOC_ARRAY_FAN_OUT; slot++) {
  		ptr = READ_ONCE(node->slots[slot]);
  		if (assoc_array_ptr_is_meta(ptr)) {
  			cursor = ptr;
  			goto begin_node;
  		}
  	}
  
  finished_node:
  	/* Move up to the parent (may need to skip back over a shortcut) */
  	parent = READ_ONCE(node->back_pointer);
  	slot = node->parent_slot;
  	if (parent == stop)
  		return 0;
  
  	if (assoc_array_ptr_is_shortcut(parent)) {
  		shortcut = assoc_array_ptr_to_shortcut(parent);
  		smp_read_barrier_depends();
  		cursor = parent;
  		parent = READ_ONCE(shortcut->back_pointer);
  		slot = shortcut->parent_slot;
  		if (parent == stop)
  			return 0;
  	}
  
  	/* Ascend to next slot in parent node */
  	cursor = parent;
  	slot++;
  	goto continue_node;
  }
  
  /**
   * assoc_array_iterate - Pass all objects in the array to a callback
   * @array: The array to iterate over.
   * @iterator: The callback function.
   * @iterator_data: Private data for the callback function.
   *
   * Iterate over all the objects in an associative array.  Each one will be
   * presented to the iterator function.
   *
   * If the array is being modified concurrently with the iteration then it is
   * possible that some objects in the array will be passed to the iterator
   * callback more than once - though every object should be passed at least
   * once.  If this is undesirable then the caller must lock against modification
   * for the duration of this function.
   *
   * The function will return 0 if no objects were in the array or else it will
   * return the result of the last iterator function called.  Iteration stops
   * immediately if any call to the iteration function results in a non-zero
   * return.
   *
   * The caller should hold the RCU read lock or better if concurrent
   * modification is possible.
   */
  int assoc_array_iterate(const struct assoc_array *array,
  			int (*iterator)(const void *object,
  					void *iterator_data),
  			void *iterator_data)
  {
  	struct assoc_array_ptr *root = READ_ONCE(array->root);
  
  	if (!root)
  		return 0;
  	return assoc_array_subtree_iterate(root, NULL, iterator, iterator_data);
  }
  
  enum assoc_array_walk_status {
  	assoc_array_walk_tree_empty,
  	assoc_array_walk_found_terminal_node,
  	assoc_array_walk_found_wrong_shortcut,
  };
  
  struct assoc_array_walk_result {
  	struct {
  		struct assoc_array_node	*node;	/* Node in which leaf might be found */
  		int		level;
  		int		slot;
  	} terminal_node;
  	struct {
  		struct assoc_array_shortcut *shortcut;
  		int		level;
  		int		sc_level;
  		unsigned long	sc_segments;
  		unsigned long	dissimilarity;
  	} wrong_shortcut;
  };
  
  /*
   * Navigate through the internal tree looking for the closest node to the key.
   */
  static enum assoc_array_walk_status
  assoc_array_walk(const struct assoc_array *array,
  		 const struct assoc_array_ops *ops,
  		 const void *index_key,
  		 struct assoc_array_walk_result *result)
  {
  	struct assoc_array_shortcut *shortcut;
  	struct assoc_array_node *node;
  	struct assoc_array_ptr *cursor, *ptr;
  	unsigned long sc_segments, dissimilarity;
  	unsigned long segments;
  	int level, sc_level, next_sc_level;
  	int slot;
  
  	pr_devel("-->%s()\n", __func__);
  
  	cursor = READ_ONCE(array->root);
  	if (!cursor)
  		return assoc_array_walk_tree_empty;
  
  	level = 0;
  
  	/* Use segments from the key for the new leaf to navigate through the
  	 * internal tree, skipping through nodes and shortcuts that are on
  	 * route to the destination.  Eventually we'll come to a slot that is
  	 * either empty or contains a leaf at which point we've found a node in
  	 * which the leaf we're looking for might be found or into which it
  	 * should be inserted.
  	 */
  jumped:
  	segments = ops->get_key_chunk(index_key, level);
  	pr_devel("segments[%d]: %lx\n", level, segments);
  
  	if (assoc_array_ptr_is_shortcut(cursor))
  		goto follow_shortcut;
  
  consider_node:
  	node = assoc_array_ptr_to_node(cursor);
  	smp_read_barrier_depends();
  
  	slot = segments >> (level & ASSOC_ARRAY_KEY_CHUNK_MASK);
  	slot &= ASSOC_ARRAY_FAN_MASK;
  	ptr = READ_ONCE(node->slots[slot]);
  
  	pr_devel("consider slot %x [ix=%d type=%lu]\n",
  		 slot, level, (unsigned long)ptr & 3);
  
  	if (!assoc_array_ptr_is_meta(ptr)) {
  		/* The node doesn't have a node/shortcut pointer in the slot
  		 * corresponding to the index key that we have to follow.
  		 */
  		result->terminal_node.node = node;
  		result->terminal_node.level = level;
  		result->terminal_node.slot = slot;
  		pr_devel("<--%s() = terminal_node\n", __func__);
  		return assoc_array_walk_found_terminal_node;
  	}
  
  	if (assoc_array_ptr_is_node(ptr)) {
  		/* There is a pointer to a node in the slot corresponding to
  		 * this index key segment, so we need to follow it.
  		 */
  		cursor = ptr;
  		level += ASSOC_ARRAY_LEVEL_STEP;
  		if ((level & ASSOC_ARRAY_KEY_CHUNK_MASK) != 0)
  			goto consider_node;
  		goto jumped;
  	}
  
  	/* There is a shortcut in the slot corresponding to the index key
  	 * segment.  We follow the shortcut if its partial index key matches
  	 * this leaf's.  Otherwise we need to split the shortcut.
  	 */
  	cursor = ptr;
  follow_shortcut:
  	shortcut = assoc_array_ptr_to_shortcut(cursor);
  	smp_read_barrier_depends();
  	pr_devel("shortcut to %d\n", shortcut->skip_to_level);
  	sc_level = level + ASSOC_ARRAY_LEVEL_STEP;
  	BUG_ON(sc_level > shortcut->skip_to_level);
  
  	do {
  		/* Check the leaf against the shortcut's index key a word at a
  		 * time, trimming the final word (the shortcut stores the index
  		 * key completely from the root to the shortcut's target).
  		 */
  		if ((sc_level & ASSOC_ARRAY_KEY_CHUNK_MASK) == 0)
  			segments = ops->get_key_chunk(index_key, sc_level);
  
  		sc_segments = shortcut->index_key[sc_level >> ASSOC_ARRAY_KEY_CHUNK_SHIFT];
  		dissimilarity = segments ^ sc_segments;
  
  		if (round_up(sc_level, ASSOC_ARRAY_KEY_CHUNK_SIZE) > shortcut->skip_to_level) {
  			/* Trim segments that are beyond the shortcut */
  			int shift = shortcut->skip_to_level & ASSOC_ARRAY_KEY_CHUNK_MASK;
  			dissimilarity &= ~(ULONG_MAX << shift);
  			next_sc_level = shortcut->skip_to_level;
  		} else {
  			next_sc_level = sc_level + ASSOC_ARRAY_KEY_CHUNK_SIZE;
  			next_sc_level = round_down(next_sc_level, ASSOC_ARRAY_KEY_CHUNK_SIZE);
  		}
  
  		if (dissimilarity != 0) {
  			/* This shortcut points elsewhere */
  			result->wrong_shortcut.shortcut = shortcut;
  			result->wrong_shortcut.level = level;
  			result->wrong_shortcut.sc_level = sc_level;
  			result->wrong_shortcut.sc_segments = sc_segments;
  			result->wrong_shortcut.dissimilarity = dissimilarity;
  			return assoc_array_walk_found_wrong_shortcut;
  		}
  
  		sc_level = next_sc_level;
  	} while (sc_level < shortcut->skip_to_level);
  
  	/* The shortcut matches the leaf's index to this point. */
  	cursor = READ_ONCE(shortcut->next_node);
  	if (((level ^ sc_level) & ~ASSOC_ARRAY_KEY_CHUNK_MASK) != 0) {
  		level = sc_level;
  		goto jumped;
  	} else {
  		level = sc_level;
  		goto consider_node;
  	}
  }
  
  /**
   * assoc_array_find - Find an object by index key
   * @array: The associative array to search.
   * @ops: The operations to use.
   * @index_key: The key to the object.
   *
   * Find an object in an associative array by walking through the internal tree
   * to the node that should contain the object and then searching the leaves
   * there.  NULL is returned if the requested object was not found in the array.
   *
   * The caller must hold the RCU read lock or better.
   */
  void *assoc_array_find(const struct assoc_array *array,
  		       const struct assoc_array_ops *ops,
  		       const void *index_key)
  {
  	struct assoc_array_walk_result result;
  	const struct assoc_array_node *node;
  	const struct assoc_array_ptr *ptr;
  	const void *leaf;
  	int slot;
  
  	if (assoc_array_walk(array, ops, index_key, &result) !=
  	    assoc_array_walk_found_terminal_node)
  		return NULL;
  
  	node = result.terminal_node.node;
  	smp_read_barrier_depends();
  
  	/* If the target key is available to us, it's has to be pointed to by
  	 * the terminal node.
  	 */
  	for (slot = 0; slot < ASSOC_ARRAY_FAN_OUT; slot++) {
  		ptr = READ_ONCE(node->slots[slot]);
  		if (ptr && assoc_array_ptr_is_leaf(ptr)) {
  			/* We need a barrier between the read of the pointer
  			 * and dereferencing the pointer - but only if we are
  			 * actually going to dereference it.
  			 */
  			leaf = assoc_array_ptr_to_leaf(ptr);
  			smp_read_barrier_depends();
  			if (ops->compare_object(leaf, index_key))
  				return (void *)leaf;
  		}
  	}
  
  	return NULL;
  }
  
  /*
   * Destructively iterate over an associative array.  The caller must prevent
   * other simultaneous accesses.
   */
  static void assoc_array_destroy_subtree(struct assoc_array_ptr *root,
  					const struct assoc_array_ops *ops)
  {
  	struct assoc_array_shortcut *shortcut;
  	struct assoc_array_node *node;
  	struct assoc_array_ptr *cursor, *parent = NULL;
  	int slot = -1;
  
  	pr_devel("-->%s()\n", __func__);
  
  	cursor = root;
  	if (!cursor) {
  		pr_devel("empty\n");
  		return;
  	}
  
  move_to_meta:
  	if (assoc_array_ptr_is_shortcut(cursor)) {
  		/* Descend through a shortcut */
  		pr_devel("[%d] shortcut\n", slot);
  		BUG_ON(!assoc_array_ptr_is_shortcut(cursor));
  		shortcut = assoc_array_ptr_to_shortcut(cursor);
  		BUG_ON(shortcut->back_pointer != parent);
  		BUG_ON(slot != -1 && shortcut->parent_slot != slot);
  		parent = cursor;
  		cursor = shortcut->next_node;
  		slot = -1;
  		BUG_ON(!assoc_array_ptr_is_node(cursor));
  	}
  
  	pr_devel("[%d] node\n", slot);
  	node = assoc_array_ptr_to_node(cursor);
  	BUG_ON(node->back_pointer != parent);
  	BUG_ON(slot != -1 && node->parent_slot != slot);
  	slot = 0;
  
  continue_node:
  	pr_devel("Node %p [back=%p]\n", node, node->back_pointer);
  	for (; slot < ASSOC_ARRAY_FAN_OUT; slot++) {
  		struct assoc_array_ptr *ptr = node->slots[slot];
  		if (!ptr)
  			continue;
  		if (assoc_array_ptr_is_meta(ptr)) {
  			parent = cursor;
  			cursor = ptr;
  			goto move_to_meta;
  		}
  
  		if (ops) {
  			pr_devel("[%d] free leaf\n", slot);
  			ops->free_object(assoc_array_ptr_to_leaf(ptr));
  		}
  	}
  
  	parent = node->back_pointer;
  	slot = node->parent_slot;
  	pr_devel("free node\n");
  	kfree(node);
  	if (!parent)
  		return; /* Done */
  
  	/* Move back up to the parent (may need to free a shortcut on
  	 * the way up) */
  	if (assoc_array_ptr_is_shortcut(parent)) {
  		shortcut = assoc_array_ptr_to_shortcut(parent);
  		BUG_ON(shortcut->next_node != cursor);
  		cursor = parent;
  		parent = shortcut->back_pointer;
  		slot = shortcut->parent_slot;
  		pr_devel("free shortcut\n");
  		kfree(shortcut);
  		if (!parent)
  			return;
  
  		BUG_ON(!assoc_array_ptr_is_node(parent));
  	}
  
  	/* Ascend to next slot in parent node */
  	pr_devel("ascend to %p[%d]\n", parent, slot);
  	cursor = parent;
  	node = assoc_array_ptr_to_node(cursor);
  	slot++;
  	goto continue_node;
  }
  
  /**
   * assoc_array_destroy - Destroy an associative array
   * @array: The array to destroy.
   * @ops: The operations to use.
   *
   * Discard all metadata and free all objects in an associative array.  The
   * array will be empty and ready to use again upon completion.  This function
   * cannot fail.
   *
   * The caller must prevent all other accesses whilst this takes place as no
   * attempt is made to adjust pointers gracefully to permit RCU readlock-holding
   * accesses to continue.  On the other hand, no memory allocation is required.
   */
  void assoc_array_destroy(struct assoc_array *array,
  			 const struct assoc_array_ops *ops)
  {
  	assoc_array_destroy_subtree(array->root, ops);
  	array->root = NULL;
  }
  
  /*
   * Handle insertion into an empty tree.
   */
  static bool assoc_array_insert_in_empty_tree(struct assoc_array_edit *edit)
  {
  	struct assoc_array_node *new_n0;
  
  	pr_devel("-->%s()\n", __func__);
  
  	new_n0 = kzalloc(sizeof(struct assoc_array_node), GFP_KERNEL);
  	if (!new_n0)
  		return false;
  
  	edit->new_meta[0] = assoc_array_node_to_ptr(new_n0);
  	edit->leaf_p = &new_n0->slots[0];
  	edit->adjust_count_on = new_n0;
  	edit->set[0].ptr = &edit->array->root;
  	edit->set[0].to = assoc_array_node_to_ptr(new_n0);
  
  	pr_devel("<--%s() = ok [no root]\n", __func__);
  	return true;
  }
  
  /*
   * Handle insertion into a terminal node.
   */
  static bool assoc_array_insert_into_terminal_node(struct assoc_array_edit *edit,
  						  const struct assoc_array_ops *ops,
  						  const void *index_key,
  						  struct assoc_array_walk_result *result)
  {
  	struct assoc_array_shortcut *shortcut, *new_s0;
  	struct assoc_array_node *node, *new_n0, *new_n1, *side;
  	struct assoc_array_ptr *ptr;
  	unsigned long dissimilarity, base_seg, blank;
  	size_t keylen;
  	bool have_meta;
  	int level, diff;
  	int slot, next_slot, free_slot, i, j;
  
  	node	= result->terminal_node.node;
  	level	= result->terminal_node.level;
  	edit->segment_cache[ASSOC_ARRAY_FAN_OUT] = result->terminal_node.slot;
  
  	pr_devel("-->%s()\n", __func__);
  
  	/* We arrived at a node which doesn't have an onward node or shortcut
  	 * pointer that we have to follow.  This means that (a) the leaf we
  	 * want must go here (either by insertion or replacement) or (b) we
  	 * need to split this node and insert in one of the fragments.
  	 */
  	free_slot = -1;
  
  	/* Firstly, we have to check the leaves in this node to see if there's
  	 * a matching one we should replace in place.
  	 */
  	for (i = 0; i < ASSOC_ARRAY_FAN_OUT; i++) {
  		ptr = node->slots[i];
  		if (!ptr) {
  			free_slot = i;
  			continue;
  		}
  		if (assoc_array_ptr_is_leaf(ptr) &&
  		    ops->compare_object(assoc_array_ptr_to_leaf(ptr),
  					index_key)) {
  			pr_devel("replace in slot %d\n", i);
  			edit->leaf_p = &node->slots[i];
  			edit->dead_leaf = node->slots[i];
  			pr_devel("<--%s() = ok [replace]\n", __func__);
  			return true;
  		}
  	}
  
  	/* If there is a free slot in this node then we can just insert the
  	 * leaf here.
  	 */
  	if (free_slot >= 0) {
  		pr_devel("insert in free slot %d\n", free_slot);
  		edit->leaf_p = &node->slots[free_slot];
  		edit->adjust_count_on = node;
  		pr_devel("<--%s() = ok [insert]\n", __func__);
  		return true;
  	}
  
  	/* The node has no spare slots - so we're either going to have to split
  	 * it or insert another node before it.
  	 *
  	 * Whatever, we're going to need at least two new nodes - so allocate
  	 * those now.  We may also need a new shortcut, but we deal with that
  	 * when we need it.
  	 */
  	new_n0 = kzalloc(sizeof(struct assoc_array_node), GFP_KERNEL);
  	if (!new_n0)
  		return false;
  	edit->new_meta[0] = assoc_array_node_to_ptr(new_n0);
  	new_n1 = kzalloc(sizeof(struct assoc_array_node), GFP_KERNEL);
  	if (!new_n1)
  		return false;
  	edit->new_meta[1] = assoc_array_node_to_ptr(new_n1);
  
  	/* We need to find out how similar the leaves are. */
  	pr_devel("no spare slots\n");
  	have_meta = false;
  	for (i = 0; i < ASSOC_ARRAY_FAN_OUT; i++) {
  		ptr = node->slots[i];
  		if (assoc_array_ptr_is_meta(ptr)) {
  			edit->segment_cache[i] = 0xff;
  			have_meta = true;
  			continue;
  		}
  		base_seg = ops->get_object_key_chunk(
  			assoc_array_ptr_to_leaf(ptr), level);
  		base_seg >>= level & ASSOC_ARRAY_KEY_CHUNK_MASK;
  		edit->segment_cache[i] = base_seg & ASSOC_ARRAY_FAN_MASK;
  	}
  
  	if (have_meta) {
  		pr_devel("have meta\n");
  		goto split_node;
  	}
  
  	/* The node contains only leaves */
  	dissimilarity = 0;
  	base_seg = edit->segment_cache[0];
  	for (i = 1; i < ASSOC_ARRAY_FAN_OUT; i++)
  		dissimilarity |= edit->segment_cache[i] ^ base_seg;
  
  	pr_devel("only leaves; dissimilarity=%lx\n", dissimilarity);
  
  	if ((dissimilarity & ASSOC_ARRAY_FAN_MASK) == 0) {
  		/* The old leaves all cluster in the same slot.  We will need
  		 * to insert a shortcut if the new node wants to cluster with them.
  		 */
  		if ((edit->segment_cache[ASSOC_ARRAY_FAN_OUT] ^ base_seg) == 0)
  			goto all_leaves_cluster_together;
  
  		/* Otherwise all the old leaves cluster in the same slot, but
  		 * the new leaf wants to go into a different slot - so we
  		 * create a new node (n0) to hold the new leaf and a pointer to
  		 * a new node (n1) holding all the old leaves.
  		 *
  		 * This can be done by falling through to the node splitting
  		 * path.
  		 */
  		pr_devel("present leaves cluster but not new leaf\n");
  	}
  
  split_node:
  	pr_devel("split node\n");
  
  	/* We need to split the current node.  The node must contain anything
  	 * from a single leaf (in the one leaf case, this leaf will cluster
  	 * with the new leaf) and the rest meta-pointers, to all leaves, some
  	 * of which may cluster.
  	 *
  	 * It won't contain the case in which all the current leaves plus the
  	 * new leaves want to cluster in the same slot.
  	 *
  	 * We need to expel at least two leaves out of a set consisting of the
  	 * leaves in the node and the new leaf.  The current meta pointers can
  	 * just be copied as they shouldn't cluster with any of the leaves.
  	 *
  	 * We need a new node (n0) to replace the current one and a new node to
  	 * take the expelled nodes (n1).
  	 */
  	edit->set[0].to = assoc_array_node_to_ptr(new_n0);
  	new_n0->back_pointer = node->back_pointer;
  	new_n0->parent_slot = node->parent_slot;
  	new_n1->back_pointer = assoc_array_node_to_ptr(new_n0);
  	new_n1->parent_slot = -1; /* Need to calculate this */
  
  do_split_node:
  	pr_devel("do_split_node\n");
  
  	new_n0->nr_leaves_on_branch = node->nr_leaves_on_branch;
  	new_n1->nr_leaves_on_branch = 0;
  
  	/* Begin by finding two matching leaves.  There have to be at least two
  	 * that match - even if there are meta pointers - because any leaf that
  	 * would match a slot with a meta pointer in it must be somewhere
  	 * behind that meta pointer and cannot be here.  Further, given N
  	 * remaining leaf slots, we now have N+1 leaves to go in them.
  	 */
  	for (i = 0; i < ASSOC_ARRAY_FAN_OUT; i++) {
  		slot = edit->segment_cache[i];
  		if (slot != 0xff)
  			for (j = i + 1; j < ASSOC_ARRAY_FAN_OUT + 1; j++)
  				if (edit->segment_cache[j] == slot)
  					goto found_slot_for_multiple_occupancy;
  	}
  found_slot_for_multiple_occupancy:
  	pr_devel("same slot: %x %x [%02x]\n", i, j, slot);
  	BUG_ON(i >= ASSOC_ARRAY_FAN_OUT);
  	BUG_ON(j >= ASSOC_ARRAY_FAN_OUT + 1);
  	BUG_ON(slot >= ASSOC_ARRAY_FAN_OUT);
  
  	new_n1->parent_slot = slot;
  
  	/* Metadata pointers cannot change slot */
  	for (i = 0; i < ASSOC_ARRAY_FAN_OUT; i++)
  		if (assoc_array_ptr_is_meta(node->slots[i]))
  			new_n0->slots[i] = node->slots[i];
  		else
  			new_n0->slots[i] = NULL;
  	BUG_ON(new_n0->slots[slot] != NULL);
  	new_n0->slots[slot] = assoc_array_node_to_ptr(new_n1);
  
  	/* Filter the leaf pointers between the new nodes */
  	free_slot = -1;
  	next_slot = 0;
  	for (i = 0; i < ASSOC_ARRAY_FAN_OUT; i++) {
  		if (assoc_array_ptr_is_meta(node->slots[i]))
  			continue;
  		if (edit->segment_cache[i] == slot) {
  			new_n1->slots[next_slot++] = node->slots[i];
  			new_n1->nr_leaves_on_branch++;
  		} else {
  			do {
  				free_slot++;
  			} while (new_n0->slots[free_slot] != NULL);
  			new_n0->slots[free_slot] = node->slots[i];
  		}
  	}
  
  	pr_devel("filtered: f=%x n=%x\n", free_slot, next_slot);
  
  	if (edit->segment_cache[ASSOC_ARRAY_FAN_OUT] != slot) {
  		do {
  			free_slot++;
  		} while (new_n0->slots[free_slot] != NULL);
  		edit->leaf_p = &new_n0->slots[free_slot];
  		edit->adjust_count_on = new_n0;
  	} else {
  		edit->leaf_p = &new_n1->slots[next_slot++];
  		edit->adjust_count_on = new_n1;
  	}
  
  	BUG_ON(next_slot <= 1);
  
  	edit->set_backpointers_to = assoc_array_node_to_ptr(new_n0);
  	for (i = 0; i < ASSOC_ARRAY_FAN_OUT; i++) {
  		if (edit->segment_cache[i] == 0xff) {
  			ptr = node->slots[i];
  			BUG_ON(assoc_array_ptr_is_leaf(ptr));
  			if (assoc_array_ptr_is_node(ptr)) {
  				side = assoc_array_ptr_to_node(ptr);
  				edit->set_backpointers[i] = &side->back_pointer;
  			} else {
  				shortcut = assoc_array_ptr_to_shortcut(ptr);
  				edit->set_backpointers[i] = &shortcut->back_pointer;
  			}
  		}
  	}
  
  	ptr = node->back_pointer;
  	if (!ptr)
  		edit->set[0].ptr = &edit->array->root;
  	else if (assoc_array_ptr_is_node(ptr))
  		edit->set[0].ptr = &assoc_array_ptr_to_node(ptr)->slots[node->parent_slot];
  	else
  		edit->set[0].ptr = &assoc_array_ptr_to_shortcut(ptr)->next_node;
  	edit->excised_meta[0] = assoc_array_node_to_ptr(node);
  	pr_devel("<--%s() = ok [split node]\n", __func__);
  	return true;
  
  all_leaves_cluster_together:
  	/* All the leaves, new and old, want to cluster together in this node
  	 * in the same slot, so we have to replace this node with a shortcut to
  	 * skip over the identical parts of the key and then place a pair of
  	 * nodes, one inside the other, at the end of the shortcut and
  	 * distribute the keys between them.
  	 *
  	 * Firstly we need to work out where the leaves start diverging as a
  	 * bit position into their keys so that we know how big the shortcut
  	 * needs to be.
  	 *
  	 * We only need to make a single pass of N of the N+1 leaves because if
  	 * any keys differ between themselves at bit X then at least one of
  	 * them must also differ with the base key at bit X or before.
  	 */
  	pr_devel("all leaves cluster together\n");
  	diff = INT_MAX;
  	for (i = 0; i < ASSOC_ARRAY_FAN_OUT; i++) {
  		int x = ops->diff_objects(assoc_array_ptr_to_leaf(node->slots[i]),
  					  index_key);
  		if (x < diff) {
  			BUG_ON(x < 0);
  			diff = x;
  		}
  	}
  	BUG_ON(diff == INT_MAX);
  	BUG_ON(diff < level + ASSOC_ARRAY_LEVEL_STEP);
  
  	keylen = round_up(diff, ASSOC_ARRAY_KEY_CHUNK_SIZE);
  	keylen >>= ASSOC_ARRAY_KEY_CHUNK_SHIFT;
  
  	new_s0 = kzalloc(sizeof(struct assoc_array_shortcut) +
  			 keylen * sizeof(unsigned long), GFP_KERNEL);
  	if (!new_s0)
  		return false;
  	edit->new_meta[2] = assoc_array_shortcut_to_ptr(new_s0);
  
  	edit->set[0].to = assoc_array_shortcut_to_ptr(new_s0);
  	new_s0->back_pointer = node->back_pointer;
  	new_s0->parent_slot = node->parent_slot;
  	new_s0->next_node = assoc_array_node_to_ptr(new_n0);
  	new_n0->back_pointer = assoc_array_shortcut_to_ptr(new_s0);
  	new_n0->parent_slot = 0;
  	new_n1->back_pointer = assoc_array_node_to_ptr(new_n0);
  	new_n1->parent_slot = -1; /* Need to calculate this */
  
  	new_s0->skip_to_level = level = diff & ~ASSOC_ARRAY_LEVEL_STEP_MASK;
  	pr_devel("skip_to_level = %d [diff %d]\n", level, diff);
  	BUG_ON(level <= 0);
  
  	for (i = 0; i < keylen; i++)
  		new_s0->index_key[i] =
  			ops->get_key_chunk(index_key, i * ASSOC_ARRAY_KEY_CHUNK_SIZE);
  
  	blank = ULONG_MAX << (level & ASSOC_ARRAY_KEY_CHUNK_MASK);
  	pr_devel("blank off [%zu] %d: %lx\n", keylen - 1, level, blank);
  	new_s0->index_key[keylen - 1] &= ~blank;
  
  	/* This now reduces to a node splitting exercise for which we'll need
  	 * to regenerate the disparity table.
  	 */
  	for (i = 0; i < ASSOC_ARRAY_FAN_OUT; i++) {
  		ptr = node->slots[i];
  		base_seg = ops->get_object_key_chunk(assoc_array_ptr_to_leaf(ptr),
  						     level);
  		base_seg >>= level & ASSOC_ARRAY_KEY_CHUNK_MASK;
  		edit->segment_cache[i] = base_seg & ASSOC_ARRAY_FAN_MASK;
  	}
  
  	base_seg = ops->get_key_chunk(index_key, level);
  	base_seg >>= level & ASSOC_ARRAY_KEY_CHUNK_MASK;
  	edit->segment_cache[ASSOC_ARRAY_FAN_OUT] = base_seg & ASSOC_ARRAY_FAN_MASK;
  	goto do_split_node;
  }
  
  /*
   * Handle insertion into the middle of a shortcut.
   */
  static bool assoc_array_insert_mid_shortcut(struct assoc_array_edit *edit,
  					    const struct assoc_array_ops *ops,
  					    struct assoc_array_walk_result *result)
  {
  	struct assoc_array_shortcut *shortcut, *new_s0, *new_s1;
  	struct assoc_array_node *node, *new_n0, *side;
  	unsigned long sc_segments, dissimilarity, blank;
  	size_t keylen;
  	int level, sc_level, diff;
  	int sc_slot;
  
  	shortcut	= result->wrong_shortcut.shortcut;
  	level		= result->wrong_shortcut.level;
  	sc_level	= result->wrong_shortcut.sc_level;
  	sc_segments	= result->wrong_shortcut.sc_segments;
  	dissimilarity	= result->wrong_shortcut.dissimilarity;
  
  	pr_devel("-->%s(ix=%d dis=%lx scix=%d)\n",
  		 __func__, level, dissimilarity, sc_level);
  
  	/* We need to split a shortcut and insert a node between the two
  	 * pieces.  Zero-length pieces will be dispensed with entirely.
  	 *
  	 * First of all, we need to find out in which level the first
  	 * difference was.
  	 */
  	diff = __ffs(dissimilarity);
  	diff &= ~ASSOC_ARRAY_LEVEL_STEP_MASK;
  	diff += sc_level & ~ASSOC_ARRAY_KEY_CHUNK_MASK;
  	pr_devel("diff=%d\n", diff);
  
  	if (!shortcut->back_pointer) {
  		edit->set[0].ptr = &edit->array->root;
  	} else if (assoc_array_ptr_is_node(shortcut->back_pointer)) {
  		node = assoc_array_ptr_to_node(shortcut->back_pointer);
  		edit->set[0].ptr = &node->slots[shortcut->parent_slot];
  	} else {
  		BUG();
  	}
  
  	edit->excised_meta[0] = assoc_array_shortcut_to_ptr(shortcut);
  
  	/* Create a new node now since we're going to need it anyway */
  	new_n0 = kzalloc(sizeof(struct assoc_array_node), GFP_KERNEL);
  	if (!new_n0)
  		return false;
  	edit->new_meta[0] = assoc_array_node_to_ptr(new_n0);
  	edit->adjust_count_on = new_n0;
  
  	/* Insert a new shortcut before the new node if this segment isn't of
  	 * zero length - otherwise we just connect the new node directly to the
  	 * parent.
  	 */
  	level += ASSOC_ARRAY_LEVEL_STEP;
  	if (diff > level) {
  		pr_devel("pre-shortcut %d...%d\n", level, diff);
  		keylen = round_up(diff, ASSOC_ARRAY_KEY_CHUNK_SIZE);
  		keylen >>= ASSOC_ARRAY_KEY_CHUNK_SHIFT;
  
  		new_s0 = kzalloc(sizeof(struct assoc_array_shortcut) +
  				 keylen * sizeof(unsigned long), GFP_KERNEL);
  		if (!new_s0)
  			return false;
  		edit->new_meta[1] = assoc_array_shortcut_to_ptr(new_s0);
  		edit->set[0].to = assoc_array_shortcut_to_ptr(new_s0);
  		new_s0->back_pointer = shortcut->back_pointer;
  		new_s0->parent_slot = shortcut->parent_slot;
  		new_s0->next_node = assoc_array_node_to_ptr(new_n0);
  		new_s0->skip_to_level = diff;
  
  		new_n0->back_pointer = assoc_array_shortcut_to_ptr(new_s0);
  		new_n0->parent_slot = 0;
  
  		memcpy(new_s0->index_key, shortcut->index_key,
  		       keylen * sizeof(unsigned long));
  
  		blank = ULONG_MAX << (diff & ASSOC_ARRAY_KEY_CHUNK_MASK);
  		pr_devel("blank off [%zu] %d: %lx\n", keylen - 1, diff, blank);
  		new_s0->index_key[keylen - 1] &= ~blank;
  	} else {
  		pr_devel("no pre-shortcut\n");
  		edit->set[0].to = assoc_array_node_to_ptr(new_n0);
  		new_n0->back_pointer = shortcut->back_pointer;
  		new_n0->parent_slot = shortcut->parent_slot;
  	}
  
  	side = assoc_array_ptr_to_node(shortcut->next_node);
  	new_n0->nr_leaves_on_branch = side->nr_leaves_on_branch;
  
  	/* We need to know which slot in the new node is going to take a
  	 * metadata pointer.
  	 */
  	sc_slot = sc_segments >> (diff & ASSOC_ARRAY_KEY_CHUNK_MASK);
  	sc_slot &= ASSOC_ARRAY_FAN_MASK;
  
  	pr_devel("new slot %lx >> %d -> %d\n",
  		 sc_segments, diff & ASSOC_ARRAY_KEY_CHUNK_MASK, sc_slot);
  
  	/* Determine whether we need to follow the new node with a replacement
  	 * for the current shortcut.  We could in theory reuse the current
  	 * shortcut if its parent slot number doesn't change - but that's a
  	 * 1-in-16 chance so not worth expending the code upon.
  	 */
  	level = diff + ASSOC_ARRAY_LEVEL_STEP;
  	if (level < shortcut->skip_to_level) {
  		pr_devel("post-shortcut %d...%d\n", level, shortcut->skip_to_level);
  		keylen = round_up(shortcut->skip_to_level, ASSOC_ARRAY_KEY_CHUNK_SIZE);
  		keylen >>= ASSOC_ARRAY_KEY_CHUNK_SHIFT;
  
  		new_s1 = kzalloc(sizeof(struct assoc_array_shortcut) +
  				 keylen * sizeof(unsigned long), GFP_KERNEL);
  		if (!new_s1)
  			return false;
  		edit->new_meta[2] = assoc_array_shortcut_to_ptr(new_s1);
  
  		new_s1->back_pointer = assoc_array_node_to_ptr(new_n0);
  		new_s1->parent_slot = sc_slot;
  		new_s1->next_node = shortcut->next_node;
  		new_s1->skip_to_level = shortcut->skip_to_level;
  
  		new_n0->slots[sc_slot] = assoc_array_shortcut_to_ptr(new_s1);
  
  		memcpy(new_s1->index_key, shortcut->index_key,
  		       keylen * sizeof(unsigned long));
  
  		edit->set[1].ptr = &side->back_pointer;
  		edit->set[1].to = assoc_array_shortcut_to_ptr(new_s1);
  	} else {
  		pr_devel("no post-shortcut\n");
  
  		/* We don't have to replace the pointed-to node as long as we
  		 * use memory barriers to make sure the parent slot number is
  		 * changed before the back pointer (the parent slot number is
  		 * irrelevant to the old parent shortcut).
  		 */
  		new_n0->slots[sc_slot] = shortcut->next_node;
  		edit->set_parent_slot[0].p = &side->parent_slot;
  		edit->set_parent_slot[0].to = sc_slot;
  		edit->set[1].ptr = &side->back_pointer;
  		edit->set[1].to = assoc_array_node_to_ptr(new_n0);
  	}
  
  	/* Install the new leaf in a spare slot in the new node. */
  	if (sc_slot == 0)
  		edit->leaf_p = &new_n0->slots[1];
  	else
  		edit->leaf_p = &new_n0->slots[0];
  
  	pr_devel("<--%s() = ok [split shortcut]\n", __func__);
  	return edit;
  }
  
  /**
   * assoc_array_insert - Script insertion of an object into an associative array
   * @array: The array to insert into.
   * @ops: The operations to use.
   * @index_key: The key to insert at.
   * @object: The object to insert.
   *
   * Precalculate and preallocate a script for the insertion or replacement of an
   * object in an associative array.  This results in an edit script that can
   * either be applied or cancelled.
   *
   * The function returns a pointer to an edit script or -ENOMEM.
   *
   * The caller should lock against other modifications and must continue to hold
   * the lock until assoc_array_apply_edit() has been called.
   *
   * Accesses to the tree may take place concurrently with this function,
   * provided they hold the RCU read lock.
   */
  struct assoc_array_edit *assoc_array_insert(struct assoc_array *array,
  					    const struct assoc_array_ops *ops,
  					    const void *index_key,
  					    void *object)
  {
  	struct assoc_array_walk_result result;
  	struct assoc_array_edit *edit;
  
  	pr_devel("-->%s()\n", __func__);
  
  	/* The leaf pointer we're given must not have the bottom bit set as we
  	 * use those for type-marking the pointer.  NULL pointers are also not
  	 * allowed as they indicate an empty slot but we have to allow them
  	 * here as they can be updated later.
  	 */
  	BUG_ON(assoc_array_ptr_is_meta(object));
  
  	edit = kzalloc(sizeof(struct assoc_array_edit), GFP_KERNEL);
  	if (!edit)
  		return ERR_PTR(-ENOMEM);
  	edit->array = array;
  	edit->ops = ops;
  	edit->leaf = assoc_array_leaf_to_ptr(object);
  	edit->adjust_count_by = 1;
  
  	switch (assoc_array_walk(array, ops, index_key, &result)) {
  	case assoc_array_walk_tree_empty:
  		/* Allocate a root node if there isn't one yet */
  		if (!assoc_array_insert_in_empty_tree(edit))
  			goto enomem;
  		return edit;
  
  	case assoc_array_walk_found_terminal_node:
  		/* We found a node that doesn't have a node/shortcut pointer in
  		 * the slot corresponding to the index key that we have to
  		 * follow.
  		 */
  		if (!assoc_array_insert_into_terminal_node(edit, ops, index_key,
  							   &result))
  			goto enomem;
  		return edit;
  
  	case assoc_array_walk_found_wrong_shortcut:
  		/* We found a shortcut that didn't match our key in a slot we
  		 * needed to follow.
  		 */
  		if (!assoc_array_insert_mid_shortcut(edit, ops, &result))
  			goto enomem;
  		return edit;
  	}
  
  enomem:
  	/* Clean up after an out of memory error */
  	pr_devel("enomem\n");
  	assoc_array_cancel_edit(edit);
  	return ERR_PTR(-ENOMEM);
  }
  
  /**
   * assoc_array_insert_set_object - Set the new object pointer in an edit script
   * @edit: The edit script to modify.
   * @object: The object pointer to set.
   *
   * Change the object to be inserted in an edit script.  The object pointed to
   * by the old object is not freed.  This must be done prior to applying the
   * script.
   */
  void assoc_array_insert_set_object(struct assoc_array_edit *edit, void *object)
  {
  	BUG_ON(!object);
  	edit->leaf = assoc_array_leaf_to_ptr(object);
  }
  
  struct assoc_array_delete_collapse_context {
  	struct assoc_array_node	*node;
  	const void		*skip_leaf;
  	int			slot;
  };
  
  /*
   * Subtree collapse to node iterator.
   */
  static int assoc_array_delete_collapse_iterator(const void *leaf,
  						void *iterator_data)
  {
  	struct assoc_array_delete_collapse_context *collapse = iterator_data;
  
  	if (leaf == collapse->skip_leaf)
  		return 0;
  
  	BUG_ON(collapse->slot >= ASSOC_ARRAY_FAN_OUT);
  
  	collapse->node->slots[collapse->slot++] = assoc_array_leaf_to_ptr(leaf);
  	return 0;
  }
  
  /**
   * assoc_array_delete - Script deletion of an object from an associative array
   * @array: The array to search.
   * @ops: The operations to use.
   * @index_key: The key to the object.
   *
   * Precalculate and preallocate a script for the deletion of an object from an
   * associative array.  This results in an edit script that can either be
   * applied or cancelled.
   *
   * The function returns a pointer to an edit script if the object was found,
   * NULL if the object was not found or -ENOMEM.
   *
   * The caller should lock against other modifications and must continue to hold
   * the lock until assoc_array_apply_edit() has been called.
   *
   * Accesses to the tree may take place concurrently with this function,
   * provided they hold the RCU read lock.
   */
  struct assoc_array_edit *assoc_array_delete(struct assoc_array *array,
  					    const struct assoc_array_ops *ops,
  					    const void *index_key)
  {
  	struct assoc_array_delete_collapse_context collapse;
  	struct assoc_array_walk_result result;
  	struct assoc_array_node *node, *new_n0;
  	struct assoc_array_edit *edit;
  	struct assoc_array_ptr *ptr;
  	bool has_meta;
  	int slot, i;
  
  	pr_devel("-->%s()\n", __func__);
  
  	edit = kzalloc(sizeof(struct assoc_array_edit), GFP_KERNEL);
  	if (!edit)
  		return ERR_PTR(-ENOMEM);
  	edit->array = array;
  	edit->ops = ops;
  	edit->adjust_count_by = -1;
  
  	switch (assoc_array_walk(array, ops, index_key, &result)) {
  	case assoc_array_walk_found_terminal_node:
  		/* We found a node that should contain the leaf we've been
  		 * asked to remove - *if* it's in the tree.
  		 */
  		pr_devel("terminal_node\n");
  		node = result.terminal_node.node;
  
  		for (slot = 0; slot < ASSOC_ARRAY_FAN_OUT; slot++) {
  			ptr = node->slots[slot];
  			if (ptr &&
  			    assoc_array_ptr_is_leaf(ptr) &&
  			    ops->compare_object(assoc_array_ptr_to_leaf(ptr),
  						index_key))
  				goto found_leaf;
  		}
  	case assoc_array_walk_tree_empty:
  	case assoc_array_walk_found_wrong_shortcut:
  	default:
  		assoc_array_cancel_edit(edit);
  		pr_devel("not found\n");
  		return NULL;
  	}
  
  found_leaf:
  	BUG_ON(array->nr_leaves_on_tree <= 0);
  
  	/* In the simplest form of deletion we just clear the slot and release
  	 * the leaf after a suitable interval.
  	 */
  	edit->dead_leaf = node->slots[slot];
  	edit->set[0].ptr = &node->slots[slot];
  	edit->set[0].to = NULL;
  	edit->adjust_count_on = node;
  
  	/* If that concludes erasure of the last leaf, then delete the entire
  	 * internal array.
  	 */
  	if (array->nr_leaves_on_tree == 1) {
  		edit->set[1].ptr = &array->root;
  		edit->set[1].to = NULL;
  		edit->adjust_count_on = NULL;
  		edit->excised_subtree = array->root;
  		pr_devel("all gone\n");
  		return edit;
  	}
  
  	/* However, we'd also like to clear up some metadata blocks if we
  	 * possibly can.
  	 *
  	 * We go for a simple algorithm of: if this node has FAN_OUT or fewer
  	 * leaves in it, then attempt to collapse it - and attempt to
  	 * recursively collapse up the tree.
  	 *
  	 * We could also try and collapse in partially filled subtrees to take
  	 * up space in this node.
  	 */
  	if (node->nr_leaves_on_branch <= ASSOC_ARRAY_FAN_OUT + 1) {
  		struct assoc_array_node *parent, *grandparent;
  		struct assoc_array_ptr *ptr;
  
  		/* First of all, we need to know if this node has metadata so
  		 * that we don't try collapsing if all the leaves are already
  		 * here.
  		 */
  		has_meta = false;
  		for (i = 0; i < ASSOC_ARRAY_FAN_OUT; i++) {
  			ptr = node->slots[i];
  			if (assoc_array_ptr_is_meta(ptr)) {
  				has_meta = true;
  				break;
  			}
  		}
  
  		pr_devel("leaves: %ld [m=%d]\n",
  			 node->nr_leaves_on_branch - 1, has_meta);
  
  		/* Look further up the tree to see if we can collapse this node
  		 * into a more proximal node too.
  		 */
  		parent = node;
  	collapse_up:
  		pr_devel("collapse subtree: %ld\n", parent->nr_leaves_on_branch);
  
  		ptr = parent->back_pointer;
  		if (!ptr)
  			goto do_collapse;
  		if (assoc_array_ptr_is_shortcut(ptr)) {
  			struct assoc_array_shortcut *s = assoc_array_ptr_to_shortcut(ptr);
  			ptr = s->back_pointer;
  			if (!ptr)
  				goto do_collapse;
  		}
  
  		grandparent = assoc_array_ptr_to_node(ptr);
  		if (grandparent->nr_leaves_on_branch <= ASSOC_ARRAY_FAN_OUT + 1) {
  			parent = grandparent;
  			goto collapse_up;
  		}
  
  	do_collapse:
  		/* There's no point collapsing if the original node has no meta
  		 * pointers to discard and if we didn't merge into one of that
  		 * node's ancestry.
  		 */
  		if (has_meta || parent != node) {
  			node = parent;
  
  			/* Create a new node to collapse into */
  			new_n0 = kzalloc(sizeof(struct assoc_array_node), GFP_KERNEL);
  			if (!new_n0)
  				goto enomem;
  			edit->new_meta[0] = assoc_array_node_to_ptr(new_n0);
  
  			new_n0->back_pointer = node->back_pointer;
  			new_n0->parent_slot = node->parent_slot;
  			new_n0->nr_leaves_on_branch = node->nr_leaves_on_branch;
  			edit->adjust_count_on = new_n0;
  
  			collapse.node = new_n0;
  			collapse.skip_leaf = assoc_array_ptr_to_leaf(edit->dead_leaf);
  			collapse.slot = 0;
  			assoc_array_subtree_iterate(assoc_array_node_to_ptr(node),
  						    node->back_pointer,
  						    assoc_array_delete_collapse_iterator,
  						    &collapse);
  			pr_devel("collapsed %d,%lu\n", collapse.slot, new_n0->nr_leaves_on_branch);
  			BUG_ON(collapse.slot != new_n0->nr_leaves_on_branch - 1);
  
  			if (!node->back_pointer) {
  				edit->set[1].ptr = &array->root;
  			} else if (assoc_array_ptr_is_leaf(node->back_pointer)) {
  				BUG();
  			} else if (assoc_array_ptr_is_node(node->back_pointer)) {
  				struct assoc_array_node *p =
  					assoc_array_ptr_to_node(node->back_pointer);
  				edit->set[1].ptr = &p->slots[node->parent_slot];
  			} else if (assoc_array_ptr_is_shortcut(node->back_pointer)) {
  				struct assoc_array_shortcut *s =
  					assoc_array_ptr_to_shortcut(node->back_pointer);
  				edit->set[1].ptr = &s->next_node;
  			}
  			edit->set[1].to = assoc_array_node_to_ptr(new_n0);
  			edit->excised_subtree = assoc_array_node_to_ptr(node);
  		}
  	}
  
  	return edit;
  
  enomem:
  	/* Clean up after an out of memory error */
  	pr_devel("enomem\n");
  	assoc_array_cancel_edit(edit);
  	return ERR_PTR(-ENOMEM);
  }
  
  /**
   * assoc_array_clear - Script deletion of all objects from an associative array
   * @array: The array to clear.
   * @ops: The operations to use.
   *
   * Precalculate and preallocate a script for the deletion of all the objects
   * from an associative array.  This results in an edit script that can either
   * be applied or cancelled.
   *
   * The function returns a pointer to an edit script if there are objects to be
   * deleted, NULL if there are no objects in the array or -ENOMEM.
   *
   * The caller should lock against other modifications and must continue to hold
   * the lock until assoc_array_apply_edit() has been called.
   *
   * Accesses to the tree may take place concurrently with this function,
   * provided they hold the RCU read lock.
   */
  struct assoc_array_edit *assoc_array_clear(struct assoc_array *array,
  					   const struct assoc_array_ops *ops)
  {
  	struct assoc_array_edit *edit;
  
  	pr_devel("-->%s()\n", __func__);
  
  	if (!array->root)
  		return NULL;
  
  	edit = kzalloc(sizeof(struct assoc_array_edit), GFP_KERNEL);
  	if (!edit)
  		return ERR_PTR(-ENOMEM);
  	edit->array = array;
  	edit->ops = ops;
  	edit->set[1].ptr = &array->root;
  	edit->set[1].to = NULL;
  	edit->excised_subtree = array->root;
  	edit->ops_for_excised_subtree = ops;
  	pr_devel("all gone\n");
  	return edit;
  }
  
  /*
   * Handle the deferred destruction after an applied edit.
   */
  static void assoc_array_rcu_cleanup(struct rcu_head *head)
  {
  	struct assoc_array_edit *edit =
  		container_of(head, struct assoc_array_edit, rcu);
  	int i;
  
  	pr_devel("-->%s()\n", __func__);
  
  	if (edit->dead_leaf)
  		edit->ops->free_object(assoc_array_ptr_to_leaf(edit->dead_leaf));
  	for (i = 0; i < ARRAY_SIZE(edit->excised_meta); i++)
  		if (edit->excised_meta[i])
  			kfree(assoc_array_ptr_to_node(edit->excised_meta[i]));
  
  	if (edit->excised_subtree) {
  		BUG_ON(assoc_array_ptr_is_leaf(edit->excised_subtree));
  		if (assoc_array_ptr_is_node(edit->excised_subtree)) {
  			struct assoc_array_node *n =
  				assoc_array_ptr_to_node(edit->excised_subtree);
  			n->back_pointer = NULL;
  		} else {
  			struct assoc_array_shortcut *s =
  				assoc_array_ptr_to_shortcut(edit->excised_subtree);
  			s->back_pointer = NULL;
  		}
  		assoc_array_destroy_subtree(edit->excised_subtree,
  					    edit->ops_for_excised_subtree);
  	}
  
  	kfree(edit);
  }
  
  /**
   * assoc_array_apply_edit - Apply an edit script to an associative array
   * @edit: The script to apply.
   *
   * Apply an edit script to an associative array to effect an insertion,
   * deletion or clearance.  As the edit script includes preallocated memory,
   * this is guaranteed not to fail.
   *
   * The edit script, dead objects and dead metadata will be scheduled for
   * destruction after an RCU grace period to permit those doing read-only
   * accesses on the array to continue to do so under the RCU read lock whilst
   * the edit is taking place.
   */
  void assoc_array_apply_edit(struct assoc_array_edit *edit)
  {
  	struct assoc_array_shortcut *shortcut;
  	struct assoc_array_node *node;
  	struct assoc_array_ptr *ptr;
  	int i;
  
  	pr_devel("-->%s()\n", __func__);
  
  	smp_wmb();
  	if (edit->leaf_p)
  		*edit->leaf_p = edit->leaf;
  
  	smp_wmb();
  	for (i = 0; i < ARRAY_SIZE(edit->set_parent_slot); i++)
  		if (edit->set_parent_slot[i].p)
  			*edit->set_parent_slot[i].p = edit->set_parent_slot[i].to;
  
  	smp_wmb();
  	for (i = 0; i < ARRAY_SIZE(edit->set_backpointers); i++)
  		if (edit->set_backpointers[i])
  			*edit->set_backpointers[i] = edit->set_backpointers_to;
  
  	smp_wmb();
  	for (i = 0; i < ARRAY_SIZE(edit->set); i++)
  		if (edit->set[i].ptr)
  			*edit->set[i].ptr = edit->set[i].to;
  
  	if (edit->array->root == NULL) {
  		edit->array->nr_leaves_on_tree = 0;
  	} else if (edit->adjust_count_on) {
  		node = edit->adjust_count_on;
  		for (;;) {
  			node->nr_leaves_on_branch += edit->adjust_count_by;
  
  			ptr = node->back_pointer;
  			if (!ptr)
  				break;
  			if (assoc_array_ptr_is_shortcut(ptr)) {
  				shortcut = assoc_array_ptr_to_shortcut(ptr);
  				ptr = shortcut->back_pointer;
  				if (!ptr)
  					break;
  			}
  			BUG_ON(!assoc_array_ptr_is_node(ptr));
  			node = assoc_array_ptr_to_node(ptr);
  		}
  
  		edit->array->nr_leaves_on_tree += edit->adjust_count_by;
  	}
  
  	call_rcu(&edit->rcu, assoc_array_rcu_cleanup);
  }
  
  /**
   * assoc_array_cancel_edit - Discard an edit script.
   * @edit: The script to discard.
   *
   * Free an edit script and all the preallocated data it holds without making
   * any changes to the associative array it was intended for.
   *
   * NOTE!  In the case of an insertion script, this does _not_ release the leaf
   * that was to be inserted.  That is left to the caller.
   */
  void assoc_array_cancel_edit(struct assoc_array_edit *edit)
  {
  	struct assoc_array_ptr *ptr;
  	int i;
  
  	pr_devel("-->%s()\n", __func__);
  
  	/* Clean up after an out of memory error */
  	for (i = 0; i < ARRAY_SIZE(edit->new_meta); i++) {
  		ptr = edit->new_meta[i];
  		if (ptr) {
  			if (assoc_array_ptr_is_node(ptr))
  				kfree(assoc_array_ptr_to_node(ptr));
  			else
  				kfree(assoc_array_ptr_to_shortcut(ptr));
  		}
  	}
  	kfree(edit);
  }
  
  /**
   * assoc_array_gc - Garbage collect an associative array.
   * @array: The array to clean.
   * @ops: The operations to use.
   * @iterator: A callback function to pass judgement on each object.
   * @iterator_data: Private data for the callback function.
   *
   * Collect garbage from an associative array and pack down the internal tree to
   * save memory.
   *
   * The iterator function is asked to pass judgement upon each object in the
   * array.  If it returns false, the object is discard and if it returns true,
   * the object is kept.  If it returns true, it must increment the object's
   * usage count (or whatever it needs to do to retain it) before returning.
   *
   * This function returns 0 if successful or -ENOMEM if out of memory.  In the
   * latter case, the array is not changed.
   *
   * The caller should lock against other modifications and must continue to hold
   * the lock until assoc_array_apply_edit() has been called.
   *
   * Accesses to the tree may take place concurrently with this function,
   * provided they hold the RCU read lock.
   */
  int assoc_array_gc(struct assoc_array *array,
  		   const struct assoc_array_ops *ops,
  		   bool (*iterator)(void *object, void *iterator_data),
  		   void *iterator_data)
  {
  	struct assoc_array_shortcut *shortcut, *new_s;
  	struct assoc_array_node *node, *new_n;
  	struct assoc_array_edit *edit;
  	struct assoc_array_ptr *cursor, *ptr;
  	struct assoc_array_ptr *new_root, *new_parent, **new_ptr_pp;
  	unsigned long nr_leaves_on_tree;
  	int keylen, slot, nr_free, next_slot, i;
  
  	pr_devel("-->%s()\n", __func__);
  
  	if (!array->root)
  		return 0;
  
  	edit = kzalloc(sizeof(struct assoc_array_edit), GFP_KERNEL);
  	if (!edit)
  		return -ENOMEM;
  	edit->array = array;
  	edit->ops = ops;
  	edit->ops_for_excised_subtree = ops;
  	edit->set[0].ptr = &array->root;
  	edit->excised_subtree = array->root;
  
  	new_root = new_parent = NULL;
  	new_ptr_pp = &new_root;
  	cursor = array->root;
  
  descend:
  	/* If this point is a shortcut, then we need to duplicate it and
  	 * advance the target cursor.
  	 */
  	if (assoc_array_ptr_is_shortcut(cursor)) {
  		shortcut = assoc_array_ptr_to_shortcut(cursor);
  		keylen = round_up(shortcut->skip_to_level, ASSOC_ARRAY_KEY_CHUNK_SIZE);
  		keylen >>= ASSOC_ARRAY_KEY_CHUNK_SHIFT;
  		new_s = kmalloc(sizeof(struct assoc_array_shortcut) +
  				keylen * sizeof(unsigned long), GFP_KERNEL);
  		if (!new_s)
  			goto enomem;
  		pr_devel("dup shortcut %p -> %p\n", shortcut, new_s);
  		memcpy(new_s, shortcut, (sizeof(struct assoc_array_shortcut) +
  					 keylen * sizeof(unsigned long)));
  		new_s->back_pointer = new_parent;
  		new_s->parent_slot = shortcut->parent_slot;
  		*new_ptr_pp = new_parent = assoc_array_shortcut_to_ptr(new_s);
  		new_ptr_pp = &new_s->next_node;
  		cursor = shortcut->next_node;
  	}
  
  	/* Duplicate the node at this position */
  	node = assoc_array_ptr_to_node(cursor);
  	new_n = kzalloc(sizeof(struct assoc_array_node), GFP_KERNEL);
  	if (!new_n)
  		goto enomem;
  	pr_devel("dup node %p -> %p\n", node, new_n);
  	new_n->back_pointer = new_parent;
  	new_n->parent_slot = node->parent_slot;
  	*new_ptr_pp = new_parent = assoc_array_node_to_ptr(new_n);
  	new_ptr_pp = NULL;
  	slot = 0;
  
  continue_node:
  	/* Filter across any leaves and gc any subtrees */
  	for (; slot < ASSOC_ARRAY_FAN_OUT; slot++) {
  		ptr = node->slots[slot];
  		if (!ptr)
  			continue;
  
  		if (assoc_array_ptr_is_leaf(ptr)) {
  			if (iterator(assoc_array_ptr_to_leaf(ptr),
  				     iterator_data))
  				/* The iterator will have done any reference
  				 * counting on the object for us.
  				 */
  				new_n->slots[slot] = ptr;
  			continue;
  		}
  
  		new_ptr_pp = &new_n->slots[slot];
  		cursor = ptr;
  		goto descend;
  	}
  
  	pr_devel("-- compress node %p --\n", new_n);
  
  	/* Count up the number of empty slots in this node and work out the
  	 * subtree leaf count.
  	 */
  	new_n->nr_leaves_on_branch = 0;
  	nr_free = 0;
  	for (slot = 0; slot < ASSOC_ARRAY_FAN_OUT; slot++) {
  		ptr = new_n->slots[slot];
  		if (!ptr)
  			nr_free++;
  		else if (assoc_array_ptr_is_leaf(ptr))
  			new_n->nr_leaves_on_branch++;
  	}
  	pr_devel("free=%d, leaves=%lu\n", nr_free, new_n->nr_leaves_on_branch);
  
  	/* See what we can fold in */
  	next_slot = 0;
  	for (slot = 0; slot < ASSOC_ARRAY_FAN_OUT; slot++) {
  		struct assoc_array_shortcut *s;
  		struct assoc_array_node *child;
  
  		ptr = new_n->slots[slot];
  		if (!ptr || assoc_array_ptr_is_leaf(ptr))
  			continue;
  
  		s = NULL;
  		if (assoc_array_ptr_is_shortcut(ptr)) {
  			s = assoc_array_ptr_to_shortcut(ptr);
  			ptr = s->next_node;
  		}
  
  		child = assoc_array_ptr_to_node(ptr);
  		new_n->nr_leaves_on_branch += child->nr_leaves_on_branch;
  
  		if (child->nr_leaves_on_branch <= nr_free + 1) {
  			/* Fold the child node into this one */
  			pr_devel("[%d] fold node %lu/%d [nx %d]\n",
  				 slot, child->nr_leaves_on_branch, nr_free + 1,
  				 next_slot);
  
  			/* We would already have reaped an intervening shortcut
  			 * on the way back up the tree.
  			 */
  			BUG_ON(s);
  
  			new_n->slots[slot] = NULL;
  			nr_free++;
  			if (slot < next_slot)
  				next_slot = slot;
  			for (i = 0; i < ASSOC_ARRAY_FAN_OUT; i++) {
  				struct assoc_array_ptr *p = child->slots[i];
  				if (!p)
  					continue;
  				BUG_ON(assoc_array_ptr_is_meta(p));
  				while (new_n->slots[next_slot])
  					next_slot++;
  				BUG_ON(next_slot >= ASSOC_ARRAY_FAN_OUT);
  				new_n->slots[next_slot++] = p;
  				nr_free--;
  			}
  			kfree(child);
  		} else {
  			pr_devel("[%d] retain node %lu/%d [nx %d]\n",
  				 slot, child->nr_leaves_on_branch, nr_free + 1,
  				 next_slot);
  		}
  	}
  
  	pr_devel("after: %lu\n", new_n->nr_leaves_on_branch);
  
  	nr_leaves_on_tree = new_n->nr_leaves_on_branch;
  
  	/* Excise this node if it is singly occupied by a shortcut */
  	if (nr_free == ASSOC_ARRAY_FAN_OUT - 1) {
  		for (slot = 0; slot < ASSOC_ARRAY_FAN_OUT; slot++)
  			if ((ptr = new_n->slots[slot]))
  				break;
  
  		if (assoc_array_ptr_is_meta(ptr) &&
  		    assoc_array_ptr_is_shortcut(ptr)) {
  			pr_devel("excise node %p with 1 shortcut\n", new_n);
  			new_s = assoc_array_ptr_to_shortcut(ptr);
  			new_parent = new_n->back_pointer;
  			slot = new_n->parent_slot;
  			kfree(new_n);
  			if (!new_parent) {
  				new_s->back_pointer = NULL;
  				new_s->parent_slot = 0;
  				new_root = ptr;
  				goto gc_complete;
  			}
  
  			if (assoc_array_ptr_is_shortcut(new_parent)) {
  				/* We can discard any preceding shortcut also */
  				struct assoc_array_shortcut *s =
  					assoc_array_ptr_to_shortcut(new_parent);
  
  				pr_devel("excise preceding shortcut\n");
  
  				new_parent = new_s->back_pointer = s->back_pointer;
  				slot = new_s->parent_slot = s->parent_slot;
  				kfree(s);
  				if (!new_parent) {
  					new_s->back_pointer = NULL;
  					new_s->parent_slot = 0;
  					new_root = ptr;
  					goto gc_complete;
  				}
  			}
  
  			new_s->back_pointer = new_parent;
  			new_s->parent_slot = slot;
  			new_n = assoc_array_ptr_to_node(new_parent);
  			new_n->slots[slot] = ptr;
  			goto ascend_old_tree;
  		}
  	}
  
  	/* Excise any shortcuts we might encounter that point to nodes that
  	 * only contain leaves.
  	 */
  	ptr = new_n->back_pointer;
  	if (!ptr)
  		goto gc_complete;
  
  	if (assoc_array_ptr_is_shortcut(ptr)) {
  		new_s = assoc_array_ptr_to_shortcut(ptr);
  		new_parent = new_s->back_pointer;
  		slot = new_s->parent_slot;
  
  		if (new_n->nr_leaves_on_branch <= ASSOC_ARRAY_FAN_OUT) {
  			struct assoc_array_node *n;
  
  			pr_devel("excise shortcut\n");
  			new_n->back_pointer = new_parent;
  			new_n->parent_slot = slot;
  			kfree(new_s);
  			if (!new_parent) {
  				new_root = assoc_array_node_to_ptr(new_n);
  				goto gc_complete;
  			}
  
  			n = assoc_array_ptr_to_node(new_parent);
  			n->slots[slot] = assoc_array_node_to_ptr(new_n);
  		}
  	} else {
  		new_parent = ptr;
  	}
  	new_n = assoc_array_ptr_to_node(new_parent);
  
  ascend_old_tree:
  	ptr = node->back_pointer;
  	if (assoc_array_ptr_is_shortcut(ptr)) {
  		shortcut = assoc_array_ptr_to_shortcut(ptr);
  		slot = shortcut->parent_slot;
  		cursor = shortcut->back_pointer;
  		if (!cursor)
  			goto gc_complete;
  	} else {
  		slot = node->parent_slot;
  		cursor = ptr;
  	}
  	BUG_ON(!cursor);
  	node = assoc_array_ptr_to_node(cursor);
  	slot++;
  	goto continue_node;
  
  gc_complete:
  	edit->set[0].to = new_root;
  	assoc_array_apply_edit(edit);
  	array->nr_leaves_on_tree = nr_leaves_on_tree;
  	return 0;
  
  enomem:
  	pr_devel("enomem\n");
  	assoc_array_destroy_subtree(new_root, edit->ops);
  	kfree(edit);
  	return -ENOMEM;
  }
  