/* 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);

        blank = ULONG_MAX << (level & ASSOC_ARRAY_KEY_CHUNK_MASK);
        pr_devel("blank off [%zu] %d: %lx\n", keylen - 1, level, blank);
        new_s0->index_key[keylen - 1] &= ~blank;

        /* This now reduces to a node splitting exercise for which we'll need
         * to regenerate the disparity table.
         */
        for (i = 0; i < ASSOC_ARRAY_FAN_OUT; i++) {
                ptr = node->slots[i];
                base_seg = ops->get_object_key_chunk(assoc_array_ptr_to_leaf(ptr),
                                                     level);
                base_seg >>= level & ASSOC_ARRAY_KEY_CHUNK_MASK;
                edit->segment_cache[i] = base_seg & ASSOC_ARRAY_FAN_MASK;
        }

        base_seg = ops->get_key_chunk(index_key, level);
        base_seg >>= level & ASSOC_ARRAY_KEY_CHUNK_MASK;
        edit->segment_cache[ASSOC_ARRAY_FAN_OUT] = base_seg & ASSOC_ARRAY_FAN_MASK;
        goto do_split_node;
}

/*
 * Handle insertion into the middle of a shortcut.
 */
static bool assoc_array_insert_mid_shortcut(struct assoc_array_edit *edit,
                                            const struct assoc_array_ops *ops,
                                            struct assoc_array_walk_result *result)
{
        struct assoc_array_shortcut *shortcut, *new_s0, *new_s1;
        struct assoc_array_node *node, *new_n0, *side;
        unsigned long sc_segments, dissimilarity, blank;
        size_t keylen;
        int level, sc_level, diff;
        int sc_slot;

        shortcut        = result->wrong_shortcut.shortcut;
        level           = result->wrong_shortcut.level;
        sc_level        = result->wrong_shortcut.sc_level;
        sc_segments     = result->wrong_shortcut.sc_segments;
        dissimilarity   = result->wrong_shortcut.dissimilarity;

        pr_devel("-->%s(ix=%d dis=%lx scix=%d)\n",
                 __func__, level, dissimilarity, sc_level);

        /* We need to split a shortcut and insert a node between the two
         * pieces.  Zero-length pieces will be dispensed with entirely.
         *
         * First of all, we need to find out in which level the first
         * difference was.
         */
        diff = __ffs(dissimilarity);
        diff &= ~ASSOC_ARRAY_LEVEL_STEP_MASK;
        diff += sc_level & ~ASSOC_ARRAY_KEY_CHUNK_MASK;
        pr_devel("diff=%d\n", diff);

        if (!shortcut->back_pointer) {
                edit->set[0].ptr = &edit->array->root;
        } else if (assoc_array_ptr_is_node(shortcut->back_pointer)) {
                node = assoc_array_ptr_to_node(shortcut->back_pointer);
                edit->set[0].ptr = &node->slots[shortcut->parent_slot];
        } else {
                BUG();
        }

        edit->excised_meta[0] = assoc_array_shortcut_to_ptr(shortcut);

        /* Create a new node now since we're going to need it anyway */
        new_n0 = kzalloc(sizeof(struct assoc_array_node), GFP_KERNEL);
        if (!new_n0)
                return false;
        edit->new_meta[0] = assoc_array_node_to_ptr(new_n0);
        edit->adjust_count_on = new_n0;

        /* Insert a new shortcut before the new node if this segment isn't of
         * zero length - otherwise we just connect the new node directly to the
         * parent.
         */
        level += ASSOC_ARRAY_LEVEL_STEP;
        if (diff > level) {
                pr_devel("pre-shortcut %d...%d\n", level, diff);
                keylen = round_up(diff, ASSOC_ARRAY_KEY_CHUNK_SIZE);
                keylen >>= ASSOC_ARRAY_KEY_CHUNK_SHIFT;

                new_s0 = kzalloc(sizeof(struct assoc_array_shortcut) +
                                 keylen * sizeof(unsigned long), GFP_KERNEL);
                if (!new_s0)
                        return false;
                edit->new_meta[1] = assoc_array_shortcut_to_ptr(new_s0);
                edit->set[0].to = assoc_array_shortcut_to_ptr(new_s0);
                new_s0->back_pointer = shortcut->back_pointer;
                new_s0->parent_slot = shortcut->parent_slot;
                new_s0->next_node = assoc_array_node_to_ptr(new_n0);
                new_s0->skip_to_level = diff;

                new_n0->back_pointer = assoc_array_shortcut_to_ptr(new_s0);
                new_n0->parent_slot = 0;

                memcpy(new_s0->index_key, shortcut->index_key,
                       keylen * sizeof(unsigned long));

                blank = ULONG_MAX << (diff & ASSOC_ARRAY_KEY_CHUNK_MASK);
                pr_devel("blank off [%zu] %d: %lx\n", keylen - 1, diff, blank);
                new_s0->index_key[keylen - 1] &= ~blank;
        } else {
                pr_devel("no pre-shortcut\n");
                edit->set[0].to = assoc_array_node_to_ptr(new_n0);
                new_n0->back_pointer = shortcut->back_pointer;
                new_n0->parent_slot = shortcut->parent_slot;
        }

        side = assoc_array_ptr_to_node(shortcut->next_node);
        new_n0->nr_leaves_on_branch = side->nr_leaves_on_branch;

        /* We need to know which slot in the new node is going to take a
         * metadata pointer.
         */
        sc_slot = sc_segments >> (diff & ASSOC_ARRAY_KEY_CHUNK_MASK);
        sc_slot &= ASSOC_ARRAY_FAN_MASK;

        pr_devel("new slot %lx >> %d -> %d\n",
                 sc_segments, diff & ASSOC_ARRAY_KEY_CHUNK_MASK, sc_slot);

        /* Determine whether we need to follow the new node with a replacement
         * for the current shortcut.  We could in theory reuse the current
         * shortcut if its parent slot number doesn't change - but that's a
         * 1-in-16 chance so not worth expending the code upon.
         */
        level = diff + ASSOC_ARRAY_LEVEL_STEP;
        if (level < shortcut->skip_to_level) {
                pr_devel("post-shortcut %d...%d\n", level, shortcut->skip_to_level);
                keylen = round_up(shortcut->skip_to_level, ASSOC_ARRAY_KEY_CHUNK_SIZE);
                keylen >>= ASSOC_ARRAY_KEY_CHUNK_SHIFT;

                new_s1 = kzalloc(sizeof(struct assoc_array_shortcut) +
                                 keylen * sizeof(unsigned long), GFP_KERNEL);
                if (!new_s1)
                        return false;
                edit->new_meta[2] = assoc_array_shortcut_to_ptr(new_s1);

                new_s1->back_pointer = assoc_array_node_to_ptr(new_n0);
                new_s1->parent_slot = sc_slot;
                new_s1->next_node = shortcut->next_node;
                new_s1->skip_to_level = shortcut->skip_to_level;

                new_n0->slots[sc_slot] = assoc_array_shortcut_to_ptr(new_s1);

                memcpy(new_s1->index_key, shortcut->index_key,
                       keylen * sizeof(unsigned long));

                edit->set[1].ptr = &side->back_pointer;
                edit->set[1].to = assoc_array_shortcut_to_ptr(new_s1);
        } else {
                pr_devel("no post-shortcut\n");

                /* We don't have to replace the pointed-to node as long as we
                 * use memory barriers to make sure the parent slot number is
                 * changed before the back pointer (the parent slot number is
                 * irrelevant to the old parent shortcut).
                 */
                new_n0->slots[sc_slot] = shortcut->next_node;
                edit->set_parent_slot[0].p = &side->parent_slot;
                edit->set_parent_slot[0].to = sc_slot;
                edit->set[1].ptr = &side->back_pointer;
                edit->set[1].to = assoc_array_node_to_ptr(new_n0);
        }

        /* Install the new leaf in a spare slot in the new node. */
        if (sc_slot == 0)
                edit->leaf_p = &new_n0->slots[1];
        else
                edit->leaf_p = &new_n0->slots[0];

        pr_devel("<--%s() = ok [split shortcut]\n", __func__);
        return edit;
}

/**
 * assoc_array_insert - Script insertion of an object into an associative array
 * @array: The array to insert into.
 * @ops: The operations to use.
 * @index_key: The key to insert at.
 * @object: The object to insert.
 *
 * Precalculate and preallocate a script for the insertion or replacement of an
 * object in an associative array.  This results in an edit script that can
 * either be applied or cancelled.
 *
 * The function returns a pointer to an edit script or -ENOMEM.
 *
 * The caller should lock against other modifications and must continue to hold
 * the lock until assoc_array_apply_edit() has been called.
 *
 * Accesses to the tree may take place concurrently with this function,
 * provided they hold the RCU read lock.
 */
struct assoc_array_edit *assoc_array_insert(struct assoc_array *array,
                                            const struct assoc_array_ops *ops,
                                            const void *index_key,
                                            void *object)
{
        struct assoc_array_walk_result result;
        struct assoc_array_edit *edit;

        pr_devel("-->%s()\n", __func__);

        /* The leaf pointer we're given must not have the bottom bit set as we
         * use those for type-marking the pointer.  NULL pointers are also not
         * allowed as they indicate an empty slot but we have to allow them
         * here as they can be updated later.
         */
        BUG_ON(assoc_array_ptr_is_meta(object));

        edit = kzalloc(sizeof(struct assoc_array_edit), GFP_KERNEL);
        if (!edit)
                return ERR_PTR(-ENOMEM);
        edit->array = array;
        edit->ops = ops;
        edit->leaf = assoc_array_leaf_to_ptr(object);
        edit->adjust_count_by = 1;

        switch (assoc_array_walk(array, ops, index_key, &result)) {
        case assoc_array_walk_tree_empty:
                /* Allocate a root node if there isn't one yet */
                if (!assoc_array_insert_in_empty_tree(edit))
                        goto enomem;
                return edit;

        case assoc_array_walk_found_terminal_node:
                /* We found a node that doesn't have a node/shortcut pointer in
                 * the slot corresponding to the index key that we have to

                 * follow.
                 */
                if (!assoc_array_insert_into_terminal_node(edit, ops, index_key,
                                                           &result))
                        goto enomem;
                return edit;

        case assoc_array_walk_found_wrong_shortcut:
                /* We found a shortcut that didn't match our key in a slot we
                 * needed to follow.
                 */
                if (!assoc_array_insert_mid_shortcut(edit, ops, &result))
                        goto enomem;
                return edit;
        }

enomem:
        /* Clean up after an out of memory error */
        pr_devel("enomem\n");
        assoc_array_cancel_edit(edit);
        return ERR_PTR(-ENOMEM);
}

/**
 * assoc_array_insert_set_object - Set the new object pointer in an edit script
 * @edit: The edit script to modify.
 * @object: The object pointer to set.
 *
 * Change the object to be inserted in an edit script.  The object pointed to
 * by the old object is not freed.  This must be done prior to applying the
 * script.
 */
void assoc_array_insert_set_object(struct assoc_array_edit *edit, void *object)
{
        BUG_ON(!object);
        edit->leaf = assoc_array_leaf_to_ptr(object);
}

struct assoc_array_delete_collapse_context {
        struct assoc_array_node *node;
        const void              *skip_leaf;
        int                     slot;
};

/*
 * Subtree collapse to node iterator.
 */
static int assoc_array_delete_collapse_iterator(const void *leaf,
                                                void *iterator_data)
{
        struct assoc_array_delete_collapse_context *collapse = iterator_data;

        if (leaf == collapse->skip_leaf)
                return 0;

        BUG_ON(collapse->slot >= ASSOC_ARRAY_FAN_OUT);

        collapse->node->slots[collapse->slot++] = assoc_array_leaf_to_ptr(leaf);
        return 0;
}

/**
 * assoc_array_delete - Script deletion of an object from an associative array
 * @array: The array to search.
 * @ops: The operations to use.
 * @index_key: The key to the object.
 *
 * Precalculate and preallocate a script for the deletion of an object from an
 * associative array.  This results in an edit script that can either be
 * applied or cancelled.
 *
 * The function returns a pointer to an edit script if the object was found,
 * NULL if the object was not found or -ENOMEM.
 *
 * The caller should lock against other modifications and must continue to hold
 * the lock until assoc_array_apply_edit() has been called.
 *
 * Accesses to the tree may take place concurrently with this function,
 * provided they hold the RCU read lock.
 */
struct assoc_array_edit *assoc_array_delete(struct assoc_array *array,
                                            const struct assoc_array_ops *ops,
                                            const void *index_key)
{
        struct assoc_array_delete_collapse_context collapse;
        struct assoc_array_walk_result result;
        struct assoc_array_node *node, *new_n0;
        struct assoc_array_edit *edit;
        struct assoc_array_ptr *ptr;
        bool has_meta;
        int slot, i;

        pr_devel("-->%s()\n", __func__);

        edit = kzalloc(sizeof(struct assoc_array_edit), GFP_KERNEL);
        if (!edit)
                return ERR_PTR(-ENOMEM);
        edit->array = array;
        edit->ops = ops;
        edit->adjust_count_by = -1;

        switch (assoc_array_walk(array, ops, index_key, &result)) {
        case assoc_array_walk_found_terminal_node:
                /* We found a node that should contain the leaf we've been
                 * asked to remove - *if* it's in the tree.
                 */
                pr_devel("terminal_node\n");
                node = result.terminal_node.node;

                for (slot = 0; slot < ASSOC_ARRAY_FAN_OUT; slot++) {
                        ptr = node->slots[slot];
                        if (ptr &&
                            assoc_array_ptr_is_leaf(ptr) &&
                            ops->compare_object(assoc_array_ptr_to_leaf(ptr),
                                                index_key))
                                goto found_leaf;
                }
        case assoc_array_walk_tree_empty:
        case assoc_array_walk_found_wrong_shortcut:
        default:
                assoc_array_cancel_edit(edit);
                pr_devel("not found\n");
                return NULL;
        }

found_leaf:
        BUG_ON(array->nr_leaves_on_tree <= 0);

        /* In the simplest form of deletion we just clear the slot and release
         * the leaf after a suitable interval.
         */
        edit->dead_leaf = node->slots[slot];
        edit->set[0].ptr = &node->slots[slot];
        edit->set[0].to = NULL;
        edit->adjust_count_on = node;

        /* If that concludes erasure of the last leaf, then delete the entire
         * internal array.
         */
        if (array->nr_leaves_on_tree == 1) {
                edit->set[1].ptr = &array->root;
                edit->set[1].to = NULL;
                edit->adjust_count_on = NULL;
                edit->excised_subtree = array->root;
                pr_devel("all gone\n");
                return edit;
        }

        /* However, we'd also like to clear up some metadata blocks if we
         * possibly can.
         *
         * We go for a simple algorithm of: if this node has FAN_OUT or fewer
         * leaves in it, then attempt to collapse it - and attempt to
         * recursively collapse up the tree.
         *
         * We could also try and collapse in partially filled subtrees to take
         * up space in this node.
         */
        if (node->nr_leaves_on_branch <= ASSOC_ARRAY_FAN_OUT + 1) {
                struct assoc_array_node *parent, *grandparent;
                struct assoc_array_ptr *ptr;

                /* First of all, we need to know if this node has metadata so
                 * that we don't try collapsing if all the leaves are already
                 * here.
                 */
                has_meta = false;
                for (i = 0; i < ASSOC_ARRAY_FAN_OUT; i++) {
                        ptr = node->slots[i];
                        if (assoc_array_ptr_is_meta(ptr)) {
                                has_meta = true;
                                break;
                        }
                }

                pr_devel("leaves: %ld [m=%d]\n",
                         node->nr_leaves_on_branch - 1, has_meta);

                /* Look further up the tree to see if we can collapse this node
                 * into a more proximal node too.
                 */
                parent = node;
        collapse_up:
                pr_devel("collapse subtree: %ld\n", parent->nr_leaves_on_branch);

                ptr = parent->back_pointer;
                if (!ptr)
                        goto do_collapse;
                if (assoc_array_ptr_is_shortcut(ptr)) {
                        struct assoc_array_shortcut *s = assoc_array_ptr_to_shortcut(ptr);
                        ptr = s->back_pointer;
                        if (!ptr)
                                goto do_collapse;
                }

                grandparent = assoc_array_ptr_to_node(ptr);
                if (grandparent->nr_leaves_on_branch <= ASSOC_ARRAY_FAN_OUT + 1) {
                        parent = grandparent;
                        goto collapse_up;
                }

        do_collapse:
                /* There's no point collapsing if the original node has no meta
                 * pointers to discard and if we didn't merge into one of that
                 * node's ancestry.
                 */
                if (has_meta || parent != node) {
                        node = parent;

                        /* Create a new node to collapse into */
                        new_n0 = kzalloc(sizeof(struct assoc_array_node), GFP_KERNEL);
                        if (!new_n0)
                                goto enomem;
                        edit->new_meta[0] = assoc_array_node_to_ptr(new_n0);

                        new_n0->back_pointer = node->back_pointer;
                        new_n0->parent_slot = node->parent_slot;
                        new_n0->nr_leaves_on_branch = node->nr_leaves_on_branch;
                        edit->adjust_count_on = new_n0;

                        collapse.node = new_n0;
                        collapse.skip_leaf = assoc_array_ptr_to_leaf(edit->dead_leaf);
                        collapse.slot = 0;
                        assoc_array_subtree_iterate(assoc_array_node_to_ptr(node),
                                                    node->back_pointer,
                                                    assoc_array_delete_collapse_iterator,
                                                    &collapse);
                        pr_devel("collapsed %d,%lu\n", collapse.slot, new_n0->nr_leaves_on_branch);
                        BUG_ON(collapse.slot != new_n0->nr_leaves_on_branch - 1);

                        if (!node->back_pointer) {
                                edit->set[1].ptr = &array->root;
                        } else if (assoc_array_ptr_is_leaf(node->back_pointer)) {
                                BUG();
                        } else if (assoc_array_ptr_is_node(node->back_pointer)) {
                                struct assoc_array_node *p =
                                        assoc_array_ptr_to_node(node->back_pointer);
                                edit->set[1].ptr = &p->slots[node->parent_slot];
                        } else if (assoc_array_ptr_is_shortcut(node->back_pointer)) {
                                struct assoc_array_shortcut *s =
                                        assoc_array_ptr_to_shortcut(node->back_pointer);
                                edit->set[1].ptr = &s->next_node;
                        }
                        edit->set[1].to = assoc_array_node_to_ptr(new_n0);
                        edit->excised_subtree = assoc_array_node_to_ptr(node);
                }
        }

        return edit;

enomem:
        /* Clean up after an out of memory error */
        pr_devel("enomem\n");
        assoc_array_cancel_edit(edit);
        return ERR_PTR(-ENOMEM);
}

/**
 * assoc_array_clear - Script deletion of all objects from an associative array
 * @array: The array to clear.
 * @ops: The operations to use.
 *
 * Precalculate and preallocate a script for the deletion of all the objects
 * from an associative array.  This results in an edit script that can either
 * be applied or cancelled.
 *
 * The function returns a pointer to an edit script if there are objects to be
 * deleted, NULL if there are no objects in the array or -ENOMEM.
 *
 * The caller should lock against other modifications and must continue to hold
 * the lock until assoc_array_apply_edit() has been called.
 *
 * Accesses to the tree may take place concurrently with this function,
 * provided they hold the RCU read lock.
 */
struct assoc_array_edit *assoc_array_clear(struct assoc_array *array,
                                           const struct assoc_array_ops *ops)
{
        struct assoc_array_edit *edit;

        pr_devel("-->%s()\n", __func__);

        if (!array->root)
                return NULL;

        edit = kzalloc(sizeof(struct assoc_array_edit), GFP_KERNEL);
        if (!edit)
                return ERR_PTR(-ENOMEM);
        edit->array = array;
        edit->ops = ops;
        edit->set[1].ptr = &array->root;
        edit->set[1].to = NULL;
        edit->excised_subtree = array->root;
        edit->ops_for_excised_subtree = ops;
        pr_devel("all gone\n");
        return edit;
}

/*
 * Handle the deferred destruction after an applied edit.
 */
static void assoc_array_rcu_cleanup(struct rcu_head *head)
{
        struct assoc_array_edit *edit =
                container_of(head, struct assoc_array_edit, rcu);
        int i;

        pr_devel("-->%s()\n", __func__);

        if (edit->dead_leaf)
                edit->ops->free_object(assoc_array_ptr_to_leaf(edit->dead_leaf));
        for (i = 0; i < ARRAY_SIZE(edit->excised_meta); i++)
                if (edit->excised_meta[i])
                        kfree(assoc_array_ptr_to_node(edit->excised_meta[i]));

        if (edit->excised_subtree) {
                BUG_ON(assoc_array_ptr_is_leaf(edit->excised_subtree));
                if (assoc_array_ptr_is_node(edit->excised_subtree)) {
                        struct assoc_array_node *n =
                                assoc_array_ptr_to_node(edit->excised_subtree);
                        n->back_pointer = NULL;
                } else {
                        struct assoc_array_shortcut *s =
                                assoc_array_ptr_to_shortcut(edit->excised_subtree);
                        s->back_pointer = NULL;
                }
                assoc_array_destroy_subtree(edit->excised_subtree,
                                            edit->ops_for_excised_subtree);
        }

        kfree(edit);
}

/**
 * assoc_array_apply_edit - Apply an edit script to an associative array
 * @edit: The script to apply.
 *
 * Apply an edit script to an associative array to effect an insertion,
 * deletion or clearance.  As the edit script includes preallocated memory,
 * this is guaranteed not to fail.
 *
 * The edit script, dead objects and dead metadata will be scheduled for
 * destruction after an RCU grace period to permit those doing read-only
 * accesses on the array to continue to do so under the RCU read lock whilst
 * the edit is taking place.
 */
void assoc_array_apply_edit(struct assoc_array_edit *edit)
{
        struct assoc_array_shortcut *shortcut;
        struct assoc_array_node *node;
        struct assoc_array_ptr *ptr;
        int i;

        pr_devel("-->%s()\n", __func__);

        smp_wmb();
        if (edit->leaf_p)
                *edit->leaf_p = edit->leaf;

        smp_wmb();
        for (i = 0; i < ARRAY_SIZE(edit->set_parent_slot); i++)
                if (edit->set_parent_slot[i].p)
                        *edit->set_parent_slot[i].p = edit->set_parent_slot[i].to;

        smp_wmb();
        for (i = 0; i < ARRAY_SIZE(edit->set_backpointers); i++)
                if (edit->set_backpointers[i])
                        *edit->set_backpointers[i] = edit->set_backpointers_to;

        smp_wmb();
        for (i = 0; i < ARRAY_SIZE(edit->set); i++)
                if (edit->set[i].ptr)
                        *edit->set[i].ptr = edit->set[i].to;

        if (edit->array->root == NULL) {
                edit->array->nr_leaves_on_tree = 0;
        } else if (edit->adjust_count_on) {
                node = edit->adjust_count_on;
                for (;;) {
                        node->nr_leaves_on_branch += edit->adjust_count_by;

                        ptr = node->back_pointer;
                        if (!ptr)
                                break;
                        if (assoc_array_ptr_is_shortcut(ptr)) {
                                shortcut = assoc_array_ptr_to_shortcut(ptr);
                                ptr = shortcut->back_pointer;
                                if (!ptr)
                                        break;
                        }
                        BUG_ON(!assoc_array_ptr_is_node(ptr));
                        node = assoc_array_ptr_to_node(ptr);
                }

                edit->array->nr_leaves_on_tree += edit->adjust_count_by;
        }

        call_rcu(&edit->rcu, assoc_array_rcu_cleanup);
}

/**
 * assoc_array_cancel_edit - Discard an edit script.
 * @edit: The script to discard.
 *
 * Free an edit script and all the preallocated data it holds without making
 * any changes to the associative array it was intended for.
 *
 * NOTE!  In the case of an insertion script, this does _not_ release the leaf
 * that was to be inserted.  That is left to the caller.
 */
void assoc_array_cancel_edit(struct assoc_array_edit *edit)
{
        struct assoc_array_ptr *ptr;
        int i;

        pr_devel("-->%s()\n", __func__);

        /* Clean up after an out of memory error */
        for (i = 0; i < ARRAY_SIZE(edit->new_meta); i++) {
                ptr = edit->new_meta[i];
                if (ptr) {
                        if (assoc_array_ptr_is_node(ptr))
                                kfree(assoc_array_ptr_to_node(ptr));
                        else
                                kfree(assoc_array_ptr_to_shortcut(ptr));
                }
        }
        kfree(edit);
}

/**
 * assoc_array_gc - Garbage collect an associative array.
 * @array: The array to clean.
 * @ops: The operations to use.
 * @iterator: A callback function to pass judgement on each object.
 * @iterator_data: Private data for the callback function.
 *
 * Collect garbage from an associative array and pack down the internal tree to
 * save memory.
 *
 * The iterator function is asked to pass judgement upon each object in the
 * array.  If it returns false, the object is discard and if it returns true,
 * the object is kept.  If it returns true, it must increment the object's
 * usage count (or whatever it needs to do to retain it) before returning.
 *
 * This function returns 0 if successful or -ENOMEM if out of memory.  In the
 * latter case, the array is not changed.
 *
 * The caller should lock against other modifications and must continue to hold
 * the lock until assoc_array_apply_edit() has been called.
 *
 * Accesses to the tree may take place concurrently with this function,
 * provided they hold the RCU read lock.
 */
int assoc_array_gc(struct assoc_array *array,
                   const struct assoc_array_ops *ops,
                   bool (*iterator)(void *object, void *iterator_data),
                   void *iterator_data)
{
        struct assoc_array_shortcut *shortcut, *new_s;
        struct assoc_array_node *node, *new_n;
        struct assoc_array_edit *edit;
        struct assoc_array_ptr *cursor, *ptr;
        struct assoc_array_ptr *new_root, *new_parent, **new_ptr_pp;
        unsigned long nr_leaves_on_tree;
        int keylen, slot, nr_free, next_slot, i;

        pr_devel("-->%s()\n", __func__);

        if (!array->root)
                return 0;

        edit = kzalloc(sizeof(struct assoc_array_edit), GFP_KERNEL);
        if (!edit)
                return -ENOMEM;
        edit->array = array;
        edit->ops = ops;
        edit->ops_for_excised_subtree = ops;
        edit->set[0].ptr = &array->root;
        edit->excised_subtree = array->root;

        new_root = new_parent = NULL;
        new_ptr_pp = &new_root;
        cursor = array->root;

descend:
        /* If this point is a shortcut, then we need to duplicate it and
         * advance the target cursor.
         */
        if (assoc_array_ptr_is_shortcut(cursor)) {
                shortcut = assoc_array_ptr_to_shortcut(cursor);
                keylen = round_up(shortcut->skip_to_level, ASSOC_ARRAY_KEY_CHUNK_SIZE);
                keylen >>= ASSOC_ARRAY_KEY_CHUNK_SHIFT;
                new_s = kmalloc(sizeof(struct assoc_array_shortcut) +
                                keylen * sizeof(unsigned long), GFP_KERNEL);
                if (!new_s)
                        goto enomem;
                pr_devel("dup shortcut %p -> %p\n", shortcut, new_s);
                memcpy(new_s, shortcut, (sizeof(struct assoc_array_shortcut) +
                                         keylen * sizeof(unsigned long)));
                new_s->back_pointer = new_parent;
                new_s->parent_slot = shortcut->parent_slot;
                *new_ptr_pp = new_parent = assoc_array_shortcut_to_ptr(new_s);
                new_ptr_pp = &new_s->next_node;
                cursor = shortcut->next_node;
        }

        /* Duplicate the node at this position */
        node = assoc_array_ptr_to_node(cursor);
        new_n = kzalloc(sizeof(struct assoc_array_node), GFP_KERNEL);
        if (!new_n)
                goto enomem;
        pr_devel("dup node %p -> %p\n", node, new_n);
        new_n->back_pointer = new_parent;
        new_n->parent_slot = node->parent_slot;
        *new_ptr_pp = new_parent = assoc_array_node_to_ptr(new_n);
        new_ptr_pp = NULL;
        slot = 0;

continue_node:
        /* Filter across any leaves and gc any subtrees */
        for (; slot < ASSOC_ARRAY_FAN_OUT; slot++) {
                ptr = node->slots[slot];
                if (!ptr)
                        continue;

                if (assoc_array_ptr_is_leaf(ptr)) {
                        if (iterator(assoc_array_ptr_to_leaf(ptr),
                                     iterator_data))
                                /* The iterator will have done any reference
                                 * counting on the object for us.
                                 */
                                new_n->slots[slot] = ptr;
                        continue;
                }

                new_ptr_pp = &new_n->slots[slot];
                cursor = ptr;
                goto descend;
        }

        pr_devel("-- compress node %p --\n", new_n);

        /* Count up the number of empty slots in this node and work out the
         * subtree leaf count.
         */
        new_n->nr_leaves_on_branch = 0;
        nr_free = 0;
        for (slot = 0; slot < ASSOC_ARRAY_FAN_OUT; slot++) {
                ptr = new_n->slots[slot];
                if (!ptr)
                        nr_free++;
                else if (assoc_array_ptr_is_leaf(ptr))
                        new_n->nr_leaves_on_branch++;
        }
        pr_devel("free=%d, leaves=%lu\n", nr_free, new_n->nr_leaves_on_branch);

        /* See what we can fold in */
        next_slot = 0;
        for (slot = 0; slot < ASSOC_ARRAY_FAN_OUT; slot++) {
                struct assoc_array_shortcut *s;
                struct assoc_array_node *child;

                ptr = new_n->slots[slot];
                if (!ptr || assoc_array_ptr_is_leaf(ptr))
                        continue;

                s = NULL;
                if (assoc_array_ptr_is_shortcut(ptr)) {
                        s = assoc_array_ptr_to_shortcut(ptr);
                        ptr = s->next_node;
                }

                child = assoc_array_ptr_to_node(ptr);
                new_n->nr_leaves_on_branch += child->nr_leaves_on_branch;

                if (child->nr_leaves_on_branch <= nr_free + 1) {
                        /* Fold the child node into this one */
                        pr_devel("[%d] fold node %lu/%d [nx %d]\n",
                                 slot, child->nr_leaves_on_branch, nr_free + 1,
                                 next_slot);

                        /* We would already have reaped an intervening shortcut
                         * on the way back up the tree.
                         */
                        BUG_ON(s);

                        new_n->slots[slot] = NULL;
                        nr_free++;
                        if (slot < next_slot)
                                next_slot = slot;
                        for (i = 0; i < ASSOC_ARRAY_FAN_OUT; i++) {
                                struct assoc_array_ptr *p = child->slots[i];
                                if (!p)
                                        continue;
                                BUG_ON(assoc_array_ptr_is_meta(p));
                                while (new_n->slots[next_slot])
                                        next_slot++;
                                BUG_ON(next_slot >= ASSOC_ARRAY_FAN_OUT);
                                new_n->slots[next_slot++] = p;
                                nr_free--;
                        }
                        kfree(child);
                } else {
                        pr_devel("[%d] retain node %lu/%d [nx %d]\n",
                                 slot, child->nr_leaves_on_branch, nr_free + 1,
                                 next_slot);
                }
        }

        pr_devel("after: %lu\n", new_n->nr_leaves_on_branch);

        nr_leaves_on_tree = new_n->nr_leaves_on_branch;

        /* Excise this node if it is singly occupied by a shortcut */
        if (nr_free == ASSOC_ARRAY_FAN_OUT - 1) {
                for (slot = 0; slot < ASSOC_ARRAY_FAN_OUT; slot++)
                        if ((ptr = new_n->slots[slot]))
                                break;

                if (assoc_array_ptr_is_meta(ptr) &&
                    assoc_array_ptr_is_shortcut(ptr)) {
                        pr_devel("excise node %p with 1 shortcut\n", new_n);
                        new_s = assoc_array_ptr_to_shortcut(ptr);
                        new_parent = new_n->back_pointer;
                        slot = new_n->parent_slot;
                        kfree(new_n);
                        if (!new_parent) {
                                new_s->back_pointer = NULL;
                                new_s->parent_slot = 0;
                                new_root = ptr;
                                goto gc_complete;
                        }

                        if (assoc_array_ptr_is_shortcut(new_parent)) {
                                /* We can discard any preceding shortcut also */
                                struct assoc_array_shortcut *s =
                                        assoc_array_ptr_to_shortcut(new_parent);

                                pr_devel("excise preceding shortcut\n");

                                new_parent = new_s->back_pointer = s->back_pointer;
                                slot = new_s->parent_slot = s->parent_slot;
                                kfree(s);
                                if (!new_parent) {
                                        new_s->back_pointer = NULL;
                                        new_s->parent_slot = 0;
                                        new_root = ptr;
                                        goto gc_complete;
                                }
                        }

                        new_s->back_pointer = new_parent;
                        new_s->parent_slot = slot;
                        new_n = assoc_array_ptr_to_node(new_parent);
                        new_n->slots[slot] = ptr;
                        goto ascend_old_tree;
                }
        }

        /* Excise any shortcuts we might encounter that point to nodes that
         * only contain leaves.
         */
        ptr = new_n->back_pointer;
        if (!ptr)
                goto gc_complete;

        if (assoc_array_ptr_is_shortcut(ptr)) {
                new_s = assoc_array_ptr_to_shortcut(ptr);
                new_parent = new_s->back_pointer;
                slot = new_s->parent_slot;

                if (new_n->nr_leaves_on_branch <= ASSOC_ARRAY_FAN_OUT) {
                        struct assoc_array_node *n;

                        pr_devel("excise shortcut\n");
                        new_n->back_pointer = new_parent;
                        new_n->parent_slot = slot;
                        kfree(new_s);
                        if (!new_parent) {
                                new_root = assoc_array_node_to_ptr(new_n);
                                goto gc_complete;
                        }

                        n = assoc_array_ptr_to_node(new_parent);
                        n->slots[slot] = assoc_array_node_to_ptr(new_n);
                }
        } else {
                new_parent = ptr;
        }
        new_n = assoc_array_ptr_to_node(new_parent);

ascend_old_tree:
        ptr = node->back_pointer;
        if (assoc_array_ptr_is_shortcut(ptr)) {
                shortcut = assoc_array_ptr_to_shortcut(ptr);
                slot = shortcut->parent_slot;
                cursor = shortcut->back_pointer;
                if (!cursor)
                        goto gc_complete;
        } else {
                slot = node->parent_slot;
                cursor = ptr;
        }
        BUG_ON(!cursor);
        node = assoc_array_ptr_to_node(cursor);
        slot++;
        goto continue_node;

gc_complete:
        edit->set[0].to = new_root;
        assoc_array_apply_edit(edit);
        array->nr_leaves_on_tree = nr_leaves_on_tree;
        return 0;

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