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Message-Id: <c9cfb821c62feb914eab6d604d08f811193a0a10.1479991208.git-series.silvio.fricke@gmail.com>
Date: Thu, 24 Nov 2016 13:42:17 +0100
From: Silvio Fricke <silvio.fricke@...il.com>
To: linux-doc@...r.kernel.org
Cc: Jonathan Corbet <corbet@....net>,
Ming Lei <ming.lei@...onical.com>,
"Luis R . Rodriguez" <mcgrof@...nel.org>,
Mauro Carvalho Chehab <mchehab@...nel.org>,
linux-kernel@...r.kernel.org,
Silvio Fricke <silvio.fricke@...il.com>
Subject: [PATCH 1/4] Documentation/assoc_array.txt: convert to ReST markup
... and move to Documentation/core-api folder.
Signed-off-by: Silvio Fricke <silvio.fricke@...il.com>
---
Documentation/assoc_array.txt | 574 +--------------------------
Documentation/core-api/assoc_array.rst | 549 +++++++++++++++++++++++++-
Documentation/core-api/index.rst | 1 +-
3 files changed, 550 insertions(+), 574 deletions(-)
delete mode 100644 Documentation/assoc_array.txt
create mode 100644 Documentation/core-api/assoc_array.rst
diff --git a/Documentation/assoc_array.txt b/Documentation/assoc_array.txt
deleted file mode 100644
index 2f2c6cd..0000000
--- a/Documentation/assoc_array.txt
+++ /dev/null
@@ -1,574 +0,0 @@
- ========================================
- 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.
diff --git a/Documentation/core-api/assoc_array.rst b/Documentation/core-api/assoc_array.rst
new file mode 100644
index 0000000..67a3a50
--- /dev/null
+++ b/Documentation/core-api/assoc_array.rst
@@ -0,0 +1,549 @@
+========================================
+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``.
+
+
+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.
diff --git a/Documentation/core-api/index.rst b/Documentation/core-api/index.rst
index f7ef7fd..480d9a3 100644
--- a/Documentation/core-api/index.rst
+++ b/Documentation/core-api/index.rst
@@ -7,6 +7,7 @@ Kernel and driver related documentation.
.. toctree::
:maxdepth: 1
+ assoc_array
workqueue
.. only:: subproject
--
git-series 0.9.1
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