Adding upstream version 4.19.249.upstream/4.19.249

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 1731 insertions, 0 deletions

diff --git a/lib/assoc_array.c b/lib/assoc_array.c
new file mode 100644
index 000000000..3b1ff063c
--- /dev/null
+++ b/lib/assoc_array.c
@@ -0,0 +1,1731 @@
+/* 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);
+		cursor = READ_ONCE(shortcut->next_node); /* Address dependency. */
+	}
+
+	node = assoc_array_ptr_to_node(cursor);
+	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]); /* Address dependency. */
+		has_meta |= (unsigned long)ptr;
+		if (ptr && assoc_array_ptr_is_leaf(ptr)) {
+			/* We need a barrier between the read of the pointer,
+			 * which is supplied by the above READ_ONCE().
+			 */
+			/* 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);
+	for (; slot < ASSOC_ARRAY_FAN_OUT; slot++) {
+		ptr = READ_ONCE(node->slots[slot]); /* Address dependency. */
+		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); /* Address dependency. */
+	slot = node->parent_slot;
+	if (parent == stop)
+		return 0;
+
+	if (assoc_array_ptr_is_shortcut(parent)) {
+		shortcut = assoc_array_ptr_to_shortcut(parent);
+		cursor = parent;
+		parent = READ_ONCE(shortcut->back_pointer); /* Address dependency. */
+		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); /* Address dependency. */
+
+	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);  /* Address dependency. */
+	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);
+	slot = segments >> (level & ASSOC_ARRAY_KEY_CHUNK_MASK);
+	slot &= ASSOC_ARRAY_FAN_MASK;
+	ptr = READ_ONCE(node->slots[slot]); /* Address dependency. */
+
+	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);
+	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); /* Address dependency. */
+	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;
+
+	/* 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]); /* Address dependency. */
+		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);
+			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);
+
+	if (level & ASSOC_ARRAY_KEY_CHUNK_MASK) {
+		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;
+	bool retained;
+	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;
+	}
+
+retry_compress:
+	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 */
+	retained = false;
+	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);
+			retained = true;
+		}
+	}
+
+	if (retained && new_n->nr_leaves_on_branch <= ASSOC_ARRAY_FAN_OUT) {
+		pr_devel("internal nodes remain despite enough space, retrying\n");
+		goto retry_compress;
+	}
+	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;
+}