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diff --git a/Documentation/assoc_array.txt b/Documentation/assoc_array.txt new file mode 100644 index 000000000000..f4faec0f66e4 --- /dev/null +++ b/Documentation/assoc_array.txt @@ -0,0 +1,574 @@ + ======================================== + 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 *a, const void *b); + + Return the bit position at which the index keys of two objects differ 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. |