assoc_array.c 52 KB
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/* Generic associative array implementation.
 *
 * See Documentation/assoc_array.txt 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
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#include <linux/rcupdate.h>
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#include <linux/slab.h>
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#include <linux/err.h>
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#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 = ACCESS_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 = ACCESS_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 = ACCESS_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 = ACCESS_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 = ACCESS_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 = ACCESS_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,
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};
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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 = ACCESS_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 = ACCESS_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 = ACCESS_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 = ACCESS_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;
		}
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		if (assoc_array_ptr_is_leaf(ptr) &&
		    ops->compare_object(assoc_array_ptr_to_leaf(ptr),
					index_key)) {
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			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;

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		/* 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.
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		 */
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		pr_devel("present leaves cluster but not new leaf\n");
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	}

split_node:
	pr_devel("split node\n");

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	/* 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.
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	 *
	 * We need to expel at least two leaves out of a set consisting of the
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	 * 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.
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	 *
	 * 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++) {
748 749
		int x = ops->diff_objects(assoc_array_ptr_to_leaf(node->slots[i]),
					  index_key);
750 751 752 753 754 755 756 757 758 759 760 761 762 763 764 765 766 767 768 769 770 771 772 773 774 775 776 777 778 779 780 781 782 783 784 785 786 787 788 789 790 791 792 793 794 795 796 797 798 799 800 801 802 803 804 805 806 807 808 809 810 811 812 813 814 815 816 817 818 819 820 821 822 823 824 825 826 827 828 829 830 831 832 833 834 835 836 837 838 839 840 841 842 843 844 845 846 847 848 849 850 851 852 853 854 855 856 857 858 859 860 861 862 863 864 865 866 867 868 869 870 871 872 873 874 875 876 877 878 879 880 881 882 883 884 885 886 887 888 889 890 891 892 893 894 895 896 897 898 899 900 901 902 903 904 905 906 907 908 909 910 911 912 913 914 915 916 917 918 919 920 921 922 923 924 925 926 927 928 929 930 931 932 933 934 935 936 937 938 939 940 941 942 943 944 945 946 947 948 949 950 951 952 953 954 955 956 957 958 959 960 961 962 963 964 965 966 967 968 969 970 971 972 973 974 975 976 977 978 979 980 981 982 983 984 985 986 987 988 989 990 991 992 993 994 995 996 997 998 999 1000 1001 1002 1003 1004 1005 1006 1007 1008 1009 1010 1011 1012 1013 1014 1015 1016 1017 1018 1019 1020 1021 1022 1023 1024 1025 1026 1027 1028 1029 1030 1031 1032 1033 1034 1035 1036 1037 1038 1039 1040 1041 1042 1043 1044 1045 1046 1047 1048 1049 1050 1051 1052 1053 1054 1055 1056 1057 1058 1059 1060 1061 1062 1063 1064 1065 1066 1067 1068 1069 1070 1071 1072 1073 1074 1075 1076 1077 1078 1079 1080 1081 1082 1083 1084 1085 1086 1087 1088 1089 1090 1091 1092 1093 1094 1095 1096 1097 1098 1099 1100 1101 1102 1103 1104 1105 1106 1107 1108 1109 1110 1111 1112 1113 1114 1115 1116 1117 1118 1119 1120 1121 1122 1123 1124 1125 1126 1127 1128 1129 1130 1131 1132 1133 1134 1135 1136 1137 1138 1139 1140 1141 1142 1143 1144 1145 1146 1147 1148 1149 1150 1151 1152 1153 1154 1155 1156 1157 1158 1159 1160 1161 1162 1163 1164 1165 1166 1167 1168 1169 1170 1171 1172 1173 1174 1175 1176 1177 1178 1179 1180 1181 1182 1183 1184 1185 1186 1187 1188 1189 1190 1191 1192 1193 1194 1195 1196 1197 1198 1199 1200 1201 1202 1203 1204 1205 1206 1207 1208 1209 1210 1211 1212 1213 1214 1215 1216 1217 1218 1219 1220 1221 1222 1223 1224 1225 1226 1227 1228 1229 1230 1231 1232 1233 1234 1235 1236 1237 1238 1239 1240 1241 1242 1243 1244 1245 1246 1247 1248 1249 1250 1251 1252 1253 1254 1255 1256 1257 1258 1259 1260 1261 1262 1263 1264 1265 1266 1267 1268 1269 1270 1271 1272 1273 1274 1275 1276 1277 1278 1279 1280 1281 1282 1283 1284 1285 1286 1287 1288 1289 1290 1291 1292 1293 1294 1295 1296 1297 1298 1299 1300 1301 1302 1303 1304 1305 1306 1307 1308 1309 1310 1311 1312 1313 1314 1315 1316 1317 1318 1319 1320 1321 1322 1323 1324 1325 1326 1327 1328 1329 1330 1331 1332 1333 1334 1335 1336 1337 1338 1339 1340 1341 1342 1343 1344 1345 1346 1347 1348 1349 1350 1351 1352 1353 1354 1355 1356 1357 1358 1359 1360 1361 1362 1363 1364 1365 1366 1367 1368 1369 1370 1371 1372 1373 1374 1375 1376 1377 1378 1379 1380 1381 1382 1383 1384 1385 1386 1387 1388 1389 1390 1391 1392 1393 1394 1395 1396 1397 1398 1399 1400 1401 1402 1403 1404 1405 1406 1407 1408 1409 1410 1411 1412 1413 1414 1415 1416 1417 1418 1419 1420 1421 1422 1423 1424 1425 1426 1427 1428 1429 1430 1431 1432 1433 1434 1435 1436 1437 1438 1439 1440 1441 1442 1443 1444 1445 1446 1447 1448 1449 1450 1451 1452 1453 1454 1455 1456 1457 1458 1459 1460 1461 1462 1463 1464 1465 1466 1467 1468 1469 1470 1471 1472 1473 1474 1475 1476 1477 1478 1479 1480 1481 1482 1483 1484 1485 1486 1487 1488 1489 1490 1491 1492 1493 1494 1495 1496 1497 1498 1499 1500 1501 1502 1503 1504 1505 1506 1507 1508 1509 1510 1511 1512 1513 1514 1515 1516 1517 1518 1519 1520 1521 1522 1523 1524 1525 1526 1527 1528 1529 1530 1531 1532 1533 1534 1535 1536 1537 1538 1539 1540 1541 1542 1543 1544 1545 1546 1547 1548 1549 1550 1551 1552 1553 1554 1555 1556 1557 1558 1559 1560 1561 1562 1563 1564 1565 1566 1567 1568 1569 1570 1571 1572 1573 1574 1575 1576 1577 1578 1579 1580 1581 1582 1583 1584 1585 1586 1587 1588 1589 1590 1591 1592 1593 1594 1595 1596 1597 1598 1599 1600 1601 1602 1603 1604 1605 1606 1607 1608 1609 1610 1611 1612 1613 1614 1615 1616 1617 1618 1619 1620 1621 1622 1623 1624 1625 1626 1627 1628 1629 1630 1631 1632 1633 1634 1635 1636 1637 1638 1639 1640 1641 1642 1643 1644 1645 1646 1647 1648 1649 1650 1651 1652 1653 1654 1655 1656 1657 1658 1659 1660 1661 1662 1663 1664 1665 1666 1667 1668 1669 1670 1671 1672 1673 1674 1675 1676 1677 1678 1679 1680 1681 1682 1683 1684 1685 1686 1687 1688 1689 1690 1691 1692 1693 1694 1695 1696 1697 1698 1699 1700 1701 1702 1703 1704 1705 1706 1707 1708 1709 1710 1711
		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;
1712 1713
		if (!cursor)
			goto gc_complete;
1714 1715 1716 1717
	} else {
		slot = node->parent_slot;
		cursor = ptr;
	}
1718
	BUG_ON(!cursor);
1719 1720 1721 1722 1723 1724 1725
	node = assoc_array_ptr_to_node(cursor);
	slot++;
	goto continue_node;

gc_complete:
	edit->set[0].to = new_root;
	assoc_array_apply_edit(edit);
1726
	array->nr_leaves_on_tree = nr_leaves_on_tree;
1727 1728 1729 1730 1731 1732 1733 1734
	return 0;

enomem:
	pr_devel("enomem\n");
	assoc_array_destroy_subtree(new_root, edit->ops);
	kfree(edit);
	return -ENOMEM;
}