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 // SPDX-License-Identifier: GPL-2.0-or-later
 /* 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)
  */
 //#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(struct_size(new_s0, index_key, keylen), 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(struct_size(new_s0, index_key, keylen),
                  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,
                flex_array_size(new_s0, index_key, keylen));
 
         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(struct_size(new_s1, index_key, keylen),
                  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,
                flex_array_size(new_s1, index_key, keylen));
 
         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;
         }
         fallthrough;
     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(struct_size(new_s, index_key, keylen),
                 GFP_KERNEL);
         if (!new_s)
             goto enomem;
         pr_devel("dup shortcut %p -> %p\n", shortcut, new_s);
         memcpy(new_s, shortcut, struct_size(new_s, index_key, keylen));
         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;
 }

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