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diff --git a/Documentation/core-api/assoc_array.rst b/Documentation/core-api/assoc_array.rst new file mode 100644 index 000000000..792bbf993 --- /dev/null +++ b/Documentation/core-api/assoc_array.rst @@ -0,0 +1,554 @@ +======================================== +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 while 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 while 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 while 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. |