Documentation/assoc_array.txt: convert to ReST markup

... and move to Documentation/core-api folder.

Signed-off-by: Silvio Fricke <[e-mail]>
Reviewed-by: Mauro Carvalho Chehab <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/assoc_array.txt

···
-		   ========================================
-		   GENERIC ASSOCIATIVE ARRAY IMPLEMENTATION
-		   ========================================
-
-Contents:
-
- - Overview.
-
- - The public API.
-   - Edit script.
-   - Operations table.
-   - Manipulation functions.
-   - Access functions.
-   - Index key form.
-
- - Internal workings.
-   - Basic internal tree layout.
-   - Shortcuts.
-   - Splitting and collapsing nodes.
-   - Non-recursive iteration.
-   - Simultaneous alteration and iteration.
-
-
-========
-OVERVIEW
-========
-
-This associative array implementation is an object container with the following
-properties:
-
- (1) Objects are opaque pointers.  The implementation does not care where they
-     point (if anywhere) or what they point to (if anything).
-
-     [!] NOTE: Pointers to objects _must_ be zero in the least significant bit.
-
- (2) Objects do not need to contain linkage blocks for use by the array.  This
-     permits an object to be located in multiple arrays simultaneously.
-     Rather, the array is made up of metadata blocks that point to objects.
-
- (3) Objects require index keys to locate them within the array.
-
- (4) Index keys must be unique.  Inserting an object with the same key as one
-     already in the array will replace the old object.
-
- (5) Index keys can be of any length and can be of different lengths.
-
- (6) Index keys should encode the length early on, before any variation due to
-     length is seen.
-
- (7) Index keys can include a hash to scatter objects throughout the array.
-
- (8) The array can iterated over.  The objects will not necessarily come out in
-     key order.
-
- (9) The array can be iterated over whilst it is being modified, provided the
-     RCU readlock is being held by the iterator.  Note, however, under these
-     circumstances, some objects may be seen more than once.  If this is a
-     problem, the iterator should lock against modification.  Objects will not
-     be missed, however, unless deleted.
-
-(10) Objects in the array can be looked up by means of their index key.
-
-(11) Objects can be looked up whilst the array is being modified, provided the
-     RCU readlock is being held by the thread doing the look up.
-
-The implementation uses a tree of 16-pointer nodes internally that are indexed
-on each level by nibbles from the index key in the same manner as in a radix
-tree.  To improve memory efficiency, shortcuts can be emplaced to skip over
-what would otherwise be a series of single-occupancy nodes.  Further, nodes
-pack leaf object pointers into spare space in the node rather than making an
-extra branch until as such time an object needs to be added to a full node.
-
-
-==============
-THE PUBLIC API
-==============
-
-The public API can be found in <linux/assoc_array.h>.  The associative array is
-rooted on the following structure:
-
-	struct assoc_array {
-		...
-	};
-
-The code is selected by enabling CONFIG_ASSOCIATIVE_ARRAY.
-
-
-EDIT SCRIPT
------------
-
-The insertion and deletion functions produce an 'edit script' that can later be
-applied to effect the changes without risking ENOMEM.  This retains the
-preallocated metadata blocks that will be installed in the internal tree and
-keeps track of the metadata blocks that will be removed from the tree when the
-script is applied.
-
-This is also used to keep track of dead blocks and dead objects after the
-script has been applied so that they can be freed later.  The freeing is done
-after an RCU grace period has passed - thus allowing access functions to
-proceed under the RCU read lock.
-
-The script appears as outside of the API as a pointer of the type:
-
-	struct assoc_array_edit;
-
-There are two functions for dealing with the script:
-
- (1) Apply an edit script.
-
-	void assoc_array_apply_edit(struct assoc_array_edit *edit);
-
-     This will perform the edit functions, interpolating various write barriers
-     to permit accesses under the RCU read lock to continue.  The edit script
-     will then be passed to call_rcu() to free it and any dead stuff it points
-     to.
-
- (2) Cancel an edit script.
-
-	void assoc_array_cancel_edit(struct assoc_array_edit *edit);
-
-     This frees the edit script and all preallocated memory immediately.  If
-     this was for insertion, the new object is _not_ released by this function,
-     but must rather be released by the caller.
-
-These functions are guaranteed not to fail.
-
-
-OPERATIONS TABLE
-----------------
-
-Various functions take a table of operations:
-
-	struct assoc_array_ops {
-		...
-	};
-
-This points to a number of methods, all of which need to be provided:
-
- (1) Get a chunk of index key from caller data:
-
-	unsigned long (*get_key_chunk)(const void *index_key, int level);
-
-     This should return a chunk of caller-supplied index key starting at the
-     *bit* position given by the level argument.  The level argument will be a
-     multiple of ASSOC_ARRAY_KEY_CHUNK_SIZE and the function should return
-     ASSOC_ARRAY_KEY_CHUNK_SIZE bits.  No error is possible.
-
-
- (2) Get a chunk of an object's index key.
-
-	unsigned long (*get_object_key_chunk)(const void *object, int level);
-
-     As the previous function, but gets its data from an object in the array
-     rather than from a caller-supplied index key.
-
-
- (3) See if this is the object we're looking for.
-
-	bool (*compare_object)(const void *object, const void *index_key);
-
-     Compare the object against an index key and return true if it matches and
-     false if it doesn't.
-
-
- (4) Diff the index keys of two objects.
-
-	int (*diff_objects)(const void *object, const void *index_key);
-
-     Return the bit position at which the index key of the specified object
-     differs from the given index key or -1 if they are the same.
-
-
- (5) Free an object.
-
-	void (*free_object)(void *object);
-
-     Free the specified object.  Note that this may be called an RCU grace
-     period after assoc_array_apply_edit() was called, so synchronize_rcu() may
-     be necessary on module unloading.
-
-
-MANIPULATION FUNCTIONS
-----------------------
-
-There are a number of functions for manipulating an associative array:
-
- (1) Initialise an associative array.
-
-	void assoc_array_init(struct assoc_array *array);
-
-     This initialises the base structure for an associative array.  It can't
-     fail.
-
-
- (2) Insert/replace an object in an associative array.
-
-	struct assoc_array_edit *
-	assoc_array_insert(struct assoc_array *array,
-			   const struct assoc_array_ops *ops,
-			   const void *index_key,
-			   void *object);
-
-     This inserts the given object into the array.  Note that the least
-     significant bit of the pointer must be zero as it's used to type-mark
-     pointers internally.
-
-     If an object already exists for that key then it will be replaced with the
-     new object and the old one will be freed automatically.
-
-     The index_key argument should hold index key information and is
-     passed to the methods in the ops table when they are called.
-
-     This function makes no alteration to the array itself, but rather returns
-     an edit script that must be applied.  -ENOMEM is returned in the case of
-     an out-of-memory error.
-
-     The caller should lock exclusively against other modifiers of the array.
-
-
- (3) Delete an object from an associative array.
-
-	struct assoc_array_edit *
-	assoc_array_delete(struct assoc_array *array,
-			   const struct assoc_array_ops *ops,
-			   const void *index_key);
-
-     This deletes an object that matches the specified data from the array.
-
-     The index_key argument should hold index key information and is
-     passed to the methods in the ops table when they are called.
-
-     This function makes no alteration to the array itself, but rather returns
-     an edit script that must be applied.  -ENOMEM is returned in the case of
-     an out-of-memory error.  NULL will be returned if the specified object is
-     not found within the array.
-
-     The caller should lock exclusively against other modifiers of the array.
-
-
- (4) Delete all objects from an associative array.
-
-	struct assoc_array_edit *
-	assoc_array_clear(struct assoc_array *array,
-			  const struct assoc_array_ops *ops);
-
-     This deletes all the objects from an associative array and leaves it
-     completely empty.
-
-     This function makes no alteration to the array itself, but rather returns
-     an edit script that must be applied.  -ENOMEM is returned in the case of
-     an out-of-memory error.
-
-     The caller should lock exclusively against other modifiers of the array.
-
-
- (5) Destroy an associative array, deleting all objects.
-
-	void assoc_array_destroy(struct assoc_array *array,
-				 const struct assoc_array_ops *ops);
-
-     This destroys the contents of the associative array and leaves it
-     completely empty.  It is not permitted for another thread to be traversing
-     the array under the RCU read lock at the same time as this function is
-     destroying it as no RCU deferral is performed on memory release -
-     something that would require memory to be allocated.
-
-     The caller should lock exclusively against other modifiers and accessors
-     of the array.
-
-
- (6) Garbage collect an associative array.
-
-	int assoc_array_gc(struct assoc_array *array,
-			   const struct assoc_array_ops *ops,
-			   bool (*iterator)(void *object, void *iterator_data),
-			   void *iterator_data);
-
-     This iterates over the objects in an associative array and passes each one
-     to iterator().  If iterator() returns true, the object is kept.  If it
-     returns false, the object will be freed.  If the iterator() function
-     returns true, it must perform any appropriate refcount incrementing on the
-     object before returning.
-
-     The internal tree will be packed down if possible as part of the iteration
-     to reduce the number of nodes in it.
-
-     The iterator_data is passed directly to iterator() and is otherwise
-     ignored by the function.
-
-     The function will return 0 if successful and -ENOMEM if there wasn't
-     enough memory.
-
-     It is possible for other threads to iterate over or search the array under
-     the RCU read lock whilst this function is in progress.  The caller should
-     lock exclusively against other modifiers of the array.
-
-
-ACCESS FUNCTIONS
-----------------
-
-There are two functions for accessing an associative array:
-
- (1) Iterate over all the objects in an associative array.
-
-	int assoc_array_iterate(const struct assoc_array *array,
-				int (*iterator)(const void *object,
-						void *iterator_data),
-				void *iterator_data);
-
-     This passes each object in the array to the iterator callback function.
-     iterator_data is private data for that function.
-
-     This may be used on an array at the same time as the array is being
-     modified, provided the RCU read lock is held.  Under such circumstances,
-     it is possible for the iteration function to see some objects twice.  If
-     this is a problem, then modification should be locked against.  The
-     iteration algorithm should not, however, miss any objects.
-
-     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.
-
-
- (2) Find an object in an associative array.
-
-	void *assoc_array_find(const struct assoc_array *array,
-			       const struct assoc_array_ops *ops,
-			       const void *index_key);
-
-     This walks through the array's internal tree directly to the object
-     specified by the index key..
-
-     This may be used on an array at the same time as the array is being
-     modified, provided the RCU read lock is held.
-
-     The function will return the object if found (and set *_type to the object
-     type) or will return NULL if the object was not found.
-
-
-INDEX KEY FORM
---------------
-
-The index key can be of any form, but since the algorithms aren't told how long
-the key is, it is strongly recommended that the index key includes its length
-very early on before any variation due to the length would have an effect on
-comparisons.
-
-This will cause leaves with different length keys to scatter away from each
-other - and those with the same length keys to cluster together.
-
-It is also recommended that the index key begin with a hash of the rest of the
-key to maximise scattering throughout keyspace.
-
-The better the scattering, the wider and lower the internal tree will be.
-
-Poor scattering isn't too much of a problem as there are shortcuts and nodes
-can contain mixtures of leaves and metadata pointers.
-
-The index key is read in chunks of machine word.  Each chunk is subdivided into
-one nibble (4 bits) per level, so on a 32-bit CPU this is good for 8 levels and
-on a 64-bit CPU, 16 levels.  Unless the scattering is really poor, it is
-unlikely that more than one word of any particular index key will have to be
-used.
-
-
-=================
-INTERNAL WORKINGS
-=================
-
-The associative array data structure has an internal tree.  This tree is
-constructed of two types of metadata blocks: nodes and shortcuts.
-
-A node is an array of slots.  Each slot can contain one of four things:
-
- (*) A NULL pointer, indicating that the slot is empty.
-
- (*) A pointer to an object (a leaf).
-
- (*) A pointer to a node at the next level.
-
- (*) A pointer to a shortcut.
-
-
-BASIC INTERNAL TREE LAYOUT
---------------------------
-
-Ignoring shortcuts for the moment, the nodes form a multilevel tree.  The index
-key space is strictly subdivided by the nodes in the tree and nodes occur on
-fixed levels.  For example:
-
- Level:	0		1		2		3
-	===============	===============	===============	===============
-							NODE D
-			NODE B		NODE C	+------>+---+
-		+------>+---+	+------>+---+	|	| 0 |
-	NODE A	|	| 0 |	|	| 0 |	|	+---+
-	+---+	|	+---+	|	+---+	|	:   :
-	| 0 |	|	:   :	|	:   :	|	+---+
-	+---+	|	+---+	|	+---+	|	| f |
-	| 1 |---+	| 3 |---+	| 7 |---+	+---+
-	+---+		+---+		+---+
-	:   :		:   :		| 8 |---+
-	+---+		+---+		+---+	|	NODE E
-	| e |---+	| f |		:   :   +------>+---+
-	+---+	|	+---+		+---+		| 0 |
-	| f |	|			| f |		+---+
-	+---+	|			+---+		:   :
-		|	NODE F				+---+
-		+------>+---+				| f |
-			| 0 |		NODE G		+---+
-			+---+	+------>+---+
-			:   :	|	| 0 |
-			+---+	|	+---+
-			| 6 |---+	:   :
-			+---+		+---+
-			:   :		| f |
-			+---+		+---+
-			| f |
-			+---+
-
-In the above example, there are 7 nodes (A-G), each with 16 slots (0-f).
-Assuming no other meta data nodes in the tree, the key space is divided thusly:
-
-	KEY PREFIX	NODE
-	==========	====
-	137*		D
-	138*		E
-	13[0-69-f]*	C
-	1[0-24-f]*	B
-	e6*		G
-	e[0-57-f]*	F
-	[02-df]*	A
-
-So, for instance, keys with the following example index keys will be found in
-the appropriate nodes:
-
-	INDEX KEY	PREFIX	NODE
-	===============	=======	====
-	13694892892489	13	C
-	13795289025897	137	D
-	13889dde88793	138	E
-	138bbb89003093	138	E
-	1394879524789	12	C
-	1458952489	1	B
-	9431809de993ba	-	A
-	b4542910809cd	-	A
-	e5284310def98	e	F
-	e68428974237	e6	G
-	e7fffcbd443	e	F
-	f3842239082	-	A
-
-To save memory, if a node can hold all the leaves in its portion of keyspace,
-then the node will have all those leaves in it and will not have any metadata
-pointers - even if some of those leaves would like to be in the same slot.
-
-A node can contain a heterogeneous mix of leaves and metadata pointers.
-Metadata pointers must be in the slots that match their subdivisions of key
-space.  The leaves can be in any slot not occupied by a metadata pointer.  It
-is guaranteed that none of the leaves in a node will match a slot occupied by a
-metadata pointer.  If the metadata pointer is there, any leaf whose key matches
-the metadata key prefix must be in the subtree that the metadata pointer points
-to.
-
-In the above example list of index keys, node A will contain:
-
-	SLOT	CONTENT		INDEX KEY (PREFIX)
-	====	===============	==================
-	1	PTR TO NODE B	1*
-	any	LEAF		9431809de993ba
-	any	LEAF		b4542910809cd
-	e	PTR TO NODE F	e*
-	any	LEAF		f3842239082
-
-and node B:
-
-	3	PTR TO NODE C	13*
-	any	LEAF		1458952489
-
-
-SHORTCUTS
----------
-
-Shortcuts are metadata records that jump over a piece of keyspace.  A shortcut
-is a replacement for a series of single-occupancy nodes ascending through the
-levels.  Shortcuts exist to save memory and to speed up traversal.
-
-It is possible for the root of the tree to be a shortcut - say, for example,
-the tree contains at least 17 nodes all with key prefix '1111'.  The insertion
-algorithm will insert a shortcut to skip over the '1111' keyspace in a single
-bound and get to the fourth level where these actually become different.
-
-
-SPLITTING AND COLLAPSING NODES
-------------------------------
-
-Each node has a maximum capacity of 16 leaves and metadata pointers.  If the
-insertion algorithm finds that it is trying to insert a 17th object into a
-node, that node will be split such that at least two leaves that have a common
-key segment at that level end up in a separate node rooted on that slot for
-that common key segment.
-
-If the leaves in a full node and the leaf that is being inserted are
-sufficiently similar, then a shortcut will be inserted into the tree.
-
-When the number of objects in the subtree rooted at a node falls to 16 or
-fewer, then the subtree will be collapsed down to a single node - and this will
-ripple towards the root if possible.
-
-
-NON-RECURSIVE ITERATION
------------------------
-
-Each node and shortcut contains a back pointer to its parent and the number of
-slot in that parent that points to it.  None-recursive iteration uses these to
-proceed rootwards through the tree, going to the parent node, slot N + 1 to
-make sure progress is made without the need for a stack.
-
-The backpointers, however, make simultaneous alteration and iteration tricky.
-
-
-SIMULTANEOUS ALTERATION AND ITERATION
--------------------------------------
-
-There are a number of cases to consider:
-
- (1) Simple insert/replace.  This involves simply replacing a NULL or old
-     matching leaf pointer with the pointer to the new leaf after a barrier.
-     The metadata blocks don't change otherwise.  An old leaf won't be freed
-     until after the RCU grace period.
-
- (2) Simple delete.  This involves just clearing an old matching leaf.  The
-     metadata blocks don't change otherwise.  The old leaf won't be freed until
-     after the RCU grace period.
-
- (3) Insertion replacing part of a subtree that we haven't yet entered.  This
-     may involve replacement of part of that subtree - but that won't affect
-     the iteration as we won't have reached the pointer to it yet and the
-     ancestry blocks are not replaced (the layout of those does not change).
-
- (4) Insertion replacing nodes that we're actively processing.  This isn't a
-     problem as we've passed the anchoring pointer and won't switch onto the
-     new layout until we follow the back pointers - at which point we've
-     already examined the leaves in the replaced node (we iterate over all the
-     leaves in a node before following any of its metadata pointers).
-
-     We might, however, re-see some leaves that have been split out into a new
-     branch that's in a slot further along than we were at.
-
- (5) Insertion replacing nodes that we're processing a dependent branch of.
-     This won't affect us until we follow the back pointers.  Similar to (4).
-
- (6) Deletion collapsing a branch under us.  This doesn't affect us because the
-     back pointers will get us back to the parent of the new node before we
-     could see the new node.  The entire collapsed subtree is thrown away
-     unchanged - and will still be rooted on the same slot, so we shouldn't
-     process it a second time as we'll go back to slot + 1.
-
-Note:
-
- (*) Under some circumstances, we need to simultaneously change the parent
-     pointer and the parent slot pointer on a node (say, for example, we
-     inserted another node before it and moved it up a level).  We cannot do
-     this without locking against a read - so we have to replace that node too.
-
-     However, when we're changing a shortcut into a node this isn't a problem
-     as shortcuts only have one slot and so the parent slot number isn't used
-     when traversing backwards over one.  This means that it's okay to change
-     the slot number first - provided suitable barriers are used to make sure
-     the parent slot number is read after the back pointer.
-
-Obsolete blocks and leaves are freed up after an RCU grace period has passed,
-so as long as anyone doing walking or iteration holds the RCU read lock, the
-old superstructure should not go away on them.

Documentation/core-api/assoc_array.rst

···
+========================================
+Generic Associative Array Implementation
+========================================
+
+Overview
+========
+
+This associative array implementation is an object container with the following
+properties:
+
+1. Objects are opaque pointers.  The implementation does not care where they
+   point (if anywhere) or what they point to (if anything).
+.. note:: Pointers to objects _must_ be zero in the least significant bit.**
+
+2. Objects do not need to contain linkage blocks for use by the array.  This
+   permits an object to be located in multiple arrays simultaneously.
+   Rather, the array is made up of metadata blocks that point to objects.
+
+3. Objects require index keys to locate them within the array.
+
+4. Index keys must be unique.  Inserting an object with the same key as one
+   already in the array will replace the old object.
+
+5. Index keys can be of any length and can be of different lengths.
+
+6. Index keys should encode the length early on, before any variation due to
+   length is seen.
+
+7. Index keys can include a hash to scatter objects throughout the array.
+
+8. The array can iterated over.  The objects will not necessarily come out in
+   key order.
+
+9. The array can be iterated over whilst it is being modified, provided the
+   RCU readlock is being held by the iterator.  Note, however, under these
+   circumstances, some objects may be seen more than once.  If this is a
+   problem, the iterator should lock against modification.  Objects will not
+   be missed, however, unless deleted.
+
+10. Objects in the array can be looked up by means of their index key.
+
+11. Objects can be looked up whilst the array is being modified, provided the
+    RCU readlock is being held by the thread doing the look up.
+
+The implementation uses a tree of 16-pointer nodes internally that are indexed
+on each level by nibbles from the index key in the same manner as in a radix
+tree.  To improve memory efficiency, shortcuts can be emplaced to skip over
+what would otherwise be a series of single-occupancy nodes.  Further, nodes
+pack leaf object pointers into spare space in the node rather than making an
+extra branch until as such time an object needs to be added to a full node.
+
+
+The Public API
+==============
+
+The public API can be found in ``<linux/assoc_array.h>``.  The associative
+array is rooted on the following structure::
+
+    struct assoc_array {
+            ...
+    };
+
+The code is selected by enabling ``CONFIG_ASSOCIATIVE_ARRAY`` with::
+
+    ./script/config -e ASSOCIATIVE_ARRAY
+
+
+Edit Script
+-----------
+
+The insertion and deletion functions produce an 'edit script' that can later be
+applied to effect the changes without risking ``ENOMEM``. This retains the
+preallocated metadata blocks that will be installed in the internal tree and
+keeps track of the metadata blocks that will be removed from the tree when the
+script is applied.
+
+This is also used to keep track of dead blocks and dead objects after the
+script has been applied so that they can be freed later.  The freeing is done
+after an RCU grace period has passed - thus allowing access functions to
+proceed under the RCU read lock.
+
+The script appears as outside of the API as a pointer of the type::
+
+    struct assoc_array_edit;
+
+There are two functions for dealing with the script:
+
+1. Apply an edit script::
+
+    void assoc_array_apply_edit(struct assoc_array_edit *edit);
+
+This will perform the edit functions, interpolating various write barriers
+to permit accesses under the RCU read lock to continue.  The edit script
+will then be passed to ``call_rcu()`` to free it and any dead stuff it points
+to.
+
+2. Cancel an edit script::
+
+    void assoc_array_cancel_edit(struct assoc_array_edit *edit);
+
+This frees the edit script and all preallocated memory immediately. If
+this was for insertion, the new object is _not_ released by this function,
+but must rather be released by the caller.
+
+These functions are guaranteed not to fail.
+
+
+Operations Table
+----------------
+
+Various functions take a table of operations::
+
+    struct assoc_array_ops {
+            ...
+    };
+
+This points to a number of methods, all of which need to be provided:
+
+1. Get a chunk of index key from caller data::
+
+    unsigned long (*get_key_chunk)(const void *index_key, int level);
+
+This should return a chunk of caller-supplied index key starting at the
+*bit* position given by the level argument.  The level argument will be a
+multiple of ``ASSOC_ARRAY_KEY_CHUNK_SIZE`` and the function should return
+``ASSOC_ARRAY_KEY_CHUNK_SIZE bits``.  No error is possible.
+
+
+2. Get a chunk of an object's index key::
+
+    unsigned long (*get_object_key_chunk)(const void *object, int level);
+
+As the previous function, but gets its data from an object in the array
+rather than from a caller-supplied index key.
+
+
+3. See if this is the object we're looking for::
+
+    bool (*compare_object)(const void *object, const void *index_key);
+
+Compare the object against an index key and return ``true`` if it matches and
+``false`` if it doesn't.
+
+
+4. Diff the index keys of two objects::
+
+    int (*diff_objects)(const void *object, const void *index_key);
+
+Return the bit position at which the index key of the specified object
+differs from the given index key or -1 if they are the same.
+
+
+5. Free an object::
+
+    void (*free_object)(void *object);
+
+Free the specified object.  Note that this may be called an RCU grace period
+after ``assoc_array_apply_edit()`` was called, so ``synchronize_rcu()`` may be
+necessary on module unloading.
+
+
+Manipulation Functions
+----------------------
+
+There are a number of functions for manipulating an associative array:
+
+1. Initialise an associative array::
+
+    void assoc_array_init(struct assoc_array *array);
+
+This initialises the base structure for an associative array.  It can't fail.
+
+
+2. Insert/replace an object in an associative array::
+
+    struct assoc_array_edit *
+    assoc_array_insert(struct assoc_array *array,
+                       const struct assoc_array_ops *ops,
+                       const void *index_key,
+                       void *object);
+
+This inserts the given object into the array.  Note that the least
+significant bit of the pointer must be zero as it's used to type-mark
+pointers internally.
+
+If an object already exists for that key then it will be replaced with the
+new object and the old one will be freed automatically.
+
+The ``index_key`` argument should hold index key information and is
+passed to the methods in the ops table when they are called.
+
+This function makes no alteration to the array itself, but rather returns
+an edit script that must be applied.  ``-ENOMEM`` is returned in the case of
+an out-of-memory error.
+
+The caller should lock exclusively against other modifiers of the array.
+
+
+3. Delete an object from an associative array::
+
+    struct assoc_array_edit *
+    assoc_array_delete(struct assoc_array *array,
+                       const struct assoc_array_ops *ops,
+                       const void *index_key);
+
+This deletes an object that matches the specified data from the array.
+
+The ``index_key`` argument should hold index key information and is
+passed to the methods in the ops table when they are called.
+
+This function makes no alteration to the array itself, but rather returns
+an edit script that must be applied.  ``-ENOMEM`` is returned in the case of
+an out-of-memory error.  ``NULL`` will be returned if the specified object is
+not found within the array.
+
+The caller should lock exclusively against other modifiers of the array.
+
+
+4. Delete all objects from an associative array::
+
+    struct assoc_array_edit *
+    assoc_array_clear(struct assoc_array *array,
+                      const struct assoc_array_ops *ops);
+
+This deletes all the objects from an associative array and leaves it
+completely empty.
+
+This function makes no alteration to the array itself, but rather returns
+an edit script that must be applied.  ``-ENOMEM`` is returned in the case of
+an out-of-memory error.
+
+The caller should lock exclusively against other modifiers of the array.
+
+
+5. Destroy an associative array, deleting all objects::
+
+    void assoc_array_destroy(struct assoc_array *array,
+                             const struct assoc_array_ops *ops);
+
+This destroys the contents of the associative array and leaves it
+completely empty.  It is not permitted for another thread to be traversing
+the array under the RCU read lock at the same time as this function is
+destroying it as no RCU deferral is performed on memory release -
+something that would require memory to be allocated.
+
+The caller should lock exclusively against other modifiers and accessors
+of the array.
+
+
+6. Garbage collect an associative array::
+
+    int assoc_array_gc(struct assoc_array *array,
+                       const struct assoc_array_ops *ops,
+                       bool (*iterator)(void *object, void *iterator_data),
+                       void *iterator_data);
+
+This iterates over the objects in an associative array and passes each one to
+``iterator()``.  If ``iterator()`` returns ``true``, the object is kept.  If it
+returns ``false``, the object will be freed.  If the ``iterator()`` function
+returns ``true``, it must perform any appropriate refcount incrementing on the
+object before returning.
+
+The internal tree will be packed down if possible as part of the iteration
+to reduce the number of nodes in it.
+
+The ``iterator_data`` is passed directly to ``iterator()`` and is otherwise
+ignored by the function.
+
+The function will return ``0`` if successful and ``-ENOMEM`` if there wasn't
+enough memory.
+
+It is possible for other threads to iterate over or search the array under
+the RCU read lock whilst this function is in progress.  The caller should
+lock exclusively against other modifiers of the array.
+
+
+Access Functions
+----------------
+
+There are two functions for accessing an associative array:
+
+1. Iterate over all the objects in an associative array::
+
+    int assoc_array_iterate(const struct assoc_array *array,
+                            int (*iterator)(const void *object,
+                                            void *iterator_data),
+                            void *iterator_data);
+
+This passes each object in the array to the iterator callback function.
+``iterator_data`` is private data for that function.
+
+This may be used on an array at the same time as the array is being
+modified, provided the RCU read lock is held.  Under such circumstances,
+it is possible for the iteration function to see some objects twice.  If
+this is a problem, then modification should be locked against.  The
+iteration algorithm should not, however, miss any objects.
+
+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.
+
+
+2. Find an object in an associative array::
+
+    void *assoc_array_find(const struct assoc_array *array,
+                           const struct assoc_array_ops *ops,
+                           const void *index_key);
+
+This walks through the array's internal tree directly to the object
+specified by the index key..
+
+This may be used on an array at the same time as the array is being
+modified, provided the RCU read lock is held.
+
+The function will return the object if found (and set ``*_type`` to the object
+type) or will return ``NULL`` if the object was not found.
+
+
+Index Key Form
+--------------
+
+The index key can be of any form, but since the algorithms aren't told how long
+the key is, it is strongly recommended that the index key includes its length
+very early on before any variation due to the length would have an effect on
+comparisons.
+
+This will cause leaves with different length keys to scatter away from each
+other - and those with the same length keys to cluster together.
+
+It is also recommended that the index key begin with a hash of the rest of the
+key to maximise scattering throughout keyspace.
+
+The better the scattering, the wider and lower the internal tree will be.
+
+Poor scattering isn't too much of a problem as there are shortcuts and nodes
+can contain mixtures of leaves and metadata pointers.
+
+The index key is read in chunks of machine word.  Each chunk is subdivided into
+one nibble (4 bits) per level, so on a 32-bit CPU this is good for 8 levels and
+on a 64-bit CPU, 16 levels.  Unless the scattering is really poor, it is
+unlikely that more than one word of any particular index key will have to be
+used.
+
+
+Internal Workings
+=================
+
+The associative array data structure has an internal tree.  This tree is
+constructed of two types of metadata blocks: nodes and shortcuts.
+
+A node is an array of slots.  Each slot can contain one of four things:
+
+* A NULL pointer, indicating that the slot is empty.
+* A pointer to an object (a leaf).
+* A pointer to a node at the next level.
+* A pointer to a shortcut.
+
+
+Basic Internal Tree Layout
+--------------------------
+
+Ignoring shortcuts for the moment, the nodes form a multilevel tree.  The index
+key space is strictly subdivided by the nodes in the tree and nodes occur on
+fixed levels.  For example::
+
+ Level: 0               1               2               3
+        =============== =============== =============== ===============
+                                                        NODE D
+                        NODE B          NODE C  +------>+---+
+                +------>+---+   +------>+---+   |       | 0 |
+        NODE A  |       | 0 |   |       | 0 |   |       +---+
+        +---+   |       +---+   |       +---+   |       :   :
+        | 0 |   |       :   :   |       :   :   |       +---+
+        +---+   |       +---+   |       +---+   |       | f |
+        | 1 |---+       | 3 |---+       | 7 |---+       +---+
+        +---+           +---+           +---+
+        :   :           :   :           | 8 |---+
+        +---+           +---+           +---+   |       NODE E
+        | e |---+       | f |           :   :   +------>+---+
+        +---+   |       +---+           +---+           | 0 |
+        | f |   |                       | f |           +---+
+        +---+   |                       +---+           :   :
+                |       NODE F                          +---+
+                +------>+---+                           | f |
+                        | 0 |           NODE G          +---+
+                        +---+   +------>+---+
+                        :   :   |       | 0 |
+                        +---+   |       +---+
+                        | 6 |---+       :   :
+                        +---+           +---+
+                        :   :           | f |
+                        +---+           +---+
+                        | f |
+                        +---+
+
+In the above example, there are 7 nodes (A-G), each with 16 slots (0-f).
+Assuming no other meta data nodes in the tree, the key space is divided
+thusly::
+
+    KEY PREFIX      NODE
+    ==========      ====
+    137*            D
+    138*            E
+    13[0-69-f]*     C
+    1[0-24-f]*      B
+    e6*             G
+    e[0-57-f]*      F
+    [02-df]*        A
+
+So, for instance, keys with the following example index keys will be found in
+the appropriate nodes::
+
+    INDEX KEY       PREFIX  NODE
+    =============== ======= ====
+    13694892892489  13      C
+    13795289025897  137     D
+    13889dde88793   138     E
+    138bbb89003093  138     E
+    1394879524789   12      C
+    1458952489      1       B
+    9431809de993ba  -       A
+    b4542910809cd   -       A
+    e5284310def98   e       F
+    e68428974237    e6      G
+    e7fffcbd443     e       F
+    f3842239082     -       A
+
+To save memory, if a node can hold all the leaves in its portion of keyspace,
+then the node will have all those leaves in it and will not have any metadata
+pointers - even if some of those leaves would like to be in the same slot.
+
+A node can contain a heterogeneous mix of leaves and metadata pointers.
+Metadata pointers must be in the slots that match their subdivisions of key
+space.  The leaves can be in any slot not occupied by a metadata pointer.  It
+is guaranteed that none of the leaves in a node will match a slot occupied by a
+metadata pointer.  If the metadata pointer is there, any leaf whose key matches
+the metadata key prefix must be in the subtree that the metadata pointer points
+to.
+
+In the above example list of index keys, node A will contain::
+
+    SLOT    CONTENT         INDEX KEY (PREFIX)
+    ====    =============== ==================
+    1       PTR TO NODE B   1*
+    any     LEAF            9431809de993ba
+    any     LEAF            b4542910809cd
+    e       PTR TO NODE F   e*
+    any     LEAF            f3842239082
+
+and node B::
+
+    3	PTR TO NODE C	13*
+    any	LEAF		1458952489
+
+
+Shortcuts
+---------
+
+Shortcuts are metadata records that jump over a piece of keyspace.  A shortcut
+is a replacement for a series of single-occupancy nodes ascending through the
+levels.  Shortcuts exist to save memory and to speed up traversal.
+
+It is possible for the root of the tree to be a shortcut - say, for example,
+the tree contains at least 17 nodes all with key prefix ``1111``.  The
+insertion algorithm will insert a shortcut to skip over the ``1111`` keyspace
+in a single bound and get to the fourth level where these actually become
+different.
+
+
+Splitting And Collapsing Nodes
+------------------------------
+
+Each node has a maximum capacity of 16 leaves and metadata pointers.  If the
+insertion algorithm finds that it is trying to insert a 17th object into a
+node, that node will be split such that at least two leaves that have a common
+key segment at that level end up in a separate node rooted on that slot for
+that common key segment.
+
+If the leaves in a full node and the leaf that is being inserted are
+sufficiently similar, then a shortcut will be inserted into the tree.
+
+When the number of objects in the subtree rooted at a node falls to 16 or
+fewer, then the subtree will be collapsed down to a single node - and this will
+ripple towards the root if possible.
+
+
+Non-Recursive Iteration
+-----------------------
+
+Each node and shortcut contains a back pointer to its parent and the number of
+slot in that parent that points to it.  None-recursive iteration uses these to
+proceed rootwards through the tree, going to the parent node, slot N + 1 to
+make sure progress is made without the need for a stack.
+
+The backpointers, however, make simultaneous alteration and iteration tricky.
+
+
+Simultaneous Alteration And Iteration
+-------------------------------------
+
+There are a number of cases to consider:
+
+1. Simple insert/replace.  This involves simply replacing a NULL or old
+   matching leaf pointer with the pointer to the new leaf after a barrier.
+   The metadata blocks don't change otherwise.  An old leaf won't be freed
+   until after the RCU grace period.
+
+2. Simple delete.  This involves just clearing an old matching leaf.  The
+   metadata blocks don't change otherwise.  The old leaf won't be freed until
+   after the RCU grace period.
+
+3. Insertion replacing part of a subtree that we haven't yet entered.  This
+   may involve replacement of part of that subtree - but that won't affect
+   the iteration as we won't have reached the pointer to it yet and the
+   ancestry blocks are not replaced (the layout of those does not change).
+
+4. Insertion replacing nodes that we're actively processing.  This isn't a
+   problem as we've passed the anchoring pointer and won't switch onto the
+   new layout until we follow the back pointers - at which point we've
+   already examined the leaves in the replaced node (we iterate over all the
+   leaves in a node before following any of its metadata pointers).
+
+   We might, however, re-see some leaves that have been split out into a new
+   branch that's in a slot further along than we were at.
+
+5. Insertion replacing nodes that we're processing a dependent branch of.
+   This won't affect us until we follow the back pointers.  Similar to (4).
+
+6. Deletion collapsing a branch under us.  This doesn't affect us because the
+   back pointers will get us back to the parent of the new node before we
+   could see the new node.  The entire collapsed subtree is thrown away
+   unchanged - and will still be rooted on the same slot, so we shouldn't
+   process it a second time as we'll go back to slot + 1.
+
+.. note::
+
+   Under some circumstances, we need to simultaneously change the parent
+   pointer and the parent slot pointer on a node (say, for example, we
+   inserted another node before it and moved it up a level).  We cannot do
+   this without locking against a read - so we have to replace that node too.
+
+   However, when we're changing a shortcut into a node this isn't a problem
+   as shortcuts only have one slot and so the parent slot number isn't used
+   when traversing backwards over one.  This means that it's okay to change
+   the slot number first - provided suitable barriers are used to make sure
+   the parent slot number is read after the back pointer.
+
+Obsolete blocks and leaves are freed up after an RCU grace period has passed,
+so as long as anyone doing walking or iteration holds the RCU read lock, the
+old superstructure should not go away on them.

Documentation/core-api/index.rst

···
 .. toctree::
    :maxdepth: 1
 
+   assoc_array
    workqueue
 
 .. only::  subproject