Merge branch 'silvio' into docs-next

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/atomic_ops.txt Documentation/core-api/atomic_ops.rst

···
-		Semantics and Behavior of Atomic and
-		         Bitmask Operations
+=======================================================
+Semantics and Behavior of Atomic and Bitmask Operations
+=======================================================
 
-			  David S. Miller	 
+:Author: David S. Miller
 
-	This document is intended to serve as a guide to Linux port
+This document is intended to serve as a guide to Linux port
 maintainers on how to implement atomic counter, bitops, and spinlock
 interfaces properly.
 
-	The atomic_t type should be defined as a signed integer and
+Atomic Type And Operations
+==========================
+
+The atomic_t type should be defined as a signed integer and
 the atomic_long_t type as a signed long integer.  Also, they should
 be made opaque such that any kind of cast to a normal C integer type
-will fail.  Something like the following should suffice:
+will fail.  Something like the following should suffice::
 
 	typedef struct { int counter; } atomic_t;
 	typedef struct { long counter; } atomic_long_t;
 
 Historically, counter has been declared volatile.  This is now discouraged.
-See Documentation/process/volatile-considered-harmful.rst for the complete rationale.
+See :ref:`Documentation/process/volatile-considered-harmful.rst
+<volatile_considered_harmful>` for the complete rationale.
 
 local_t is very similar to atomic_t. If the counter is per CPU and only
 updated by one CPU, local_t is probably more appropriate. Please see
-Documentation/local_ops.txt for the semantics of local_t.
+:ref:`Documentation/core-api/local_ops.rst <local_ops>` for the semantics of
+local_t.
 
 The first operations to implement for atomic_t's are the initializers and
-plain reads.
+plain reads. ::
 
 	#define ATOMIC_INIT(i)		{ (i) }
 	#define atomic_set(v, i)	((v)->counter = (i))
 
-The first macro is used in definitions, such as:
+The first macro is used in definitions, such as::
 
-static atomic_t my_counter = ATOMIC_INIT(1);
+	static atomic_t my_counter = ATOMIC_INIT(1);
 
 The initializer is atomic in that the return values of the atomic operations
 are guaranteed to be correct reflecting the initialized value if the
···
 proper implicit or explicit read memory barrier is needed before reading the
 value with atomic_read from another thread.
 
-As with all of the atomic_ interfaces, replace the leading "atomic_"
-with "atomic_long_" to operate on atomic_long_t.
+As with all of the ``atomic_`` interfaces, replace the leading ``atomic_``
+with ``atomic_long_`` to operate on atomic_long_t.
 
-The second interface can be used at runtime, as in:
+The second interface can be used at runtime, as in::
 
 	struct foo { atomic_t counter; };
 	...
···
 or explicit memory barrier is needed before the value set with the operation
 is guaranteed to be readable with atomic_read from another thread.
 
-Next, we have:
+Next, we have::
 
 	#define atomic_read(v)	((v)->counter)
 
···
 interface must take care of that with a proper implicit or explicit memory
 barrier.
 
-*** WARNING: atomic_read() and atomic_set() DO NOT IMPLY BARRIERS! ***
+.. warning::
 
-Some architectures may choose to use the volatile keyword, barriers, or inline
-assembly to guarantee some degree of immediacy for atomic_read() and
-atomic_set().  This is not uniformly guaranteed, and may change in the future,
-so all users of atomic_t should treat atomic_read() and atomic_set() as simple
-C statements that may be reordered or optimized away entirely by the compiler
-or processor, and explicitly invoke the appropriate compiler and/or memory
-barrier for each use case.  Failure to do so will result in code that may
-suddenly break when used with different architectures or compiler
-optimizations, or even changes in unrelated code which changes how the
-compiler optimizes the section accessing atomic_t variables.
+	``atomic_read()`` and ``atomic_set()`` DO NOT IMPLY BARRIERS!
 
-*** YOU HAVE BEEN WARNED! ***
+	Some architectures may choose to use the volatile keyword, barriers, or
+	inline assembly to guarantee some degree of immediacy for atomic_read()
+	and atomic_set().  This is not uniformly guaranteed, and may change in
+	the future, so all users of atomic_t should treat atomic_read() and
+	atomic_set() as simple C statements that may be reordered or optimized
+	away entirely by the compiler or processor, and explicitly invoke the
+	appropriate compiler and/or memory barrier for each use case.  Failure
+	to do so will result in code that may suddenly break when used with
+	different architectures or compiler optimizations, or even changes in
+	unrelated code which changes how the compiler optimizes the section
+	accessing atomic_t variables.
 
 Properly aligned pointers, longs, ints, and chars (and unsigned
 equivalents) may be atomically loaded from and stored to in the same
···
 optimizations that might otherwise optimize accesses out of existence on
 the one hand, or that might create unsolicited accesses on the other.
 
-For example consider the following code:
+For example consider the following code::
 
 	while (a > 0)
 		do_something();
 
 If the compiler can prove that do_something() does not store to the
 variable a, then the compiler is within its rights transforming this to
-the following:
+the following::
 
 	tmp = a;
 	if (a > 0)
···
 			do_something();
 
 If you don't want the compiler to do this (and you probably don't), then
-you should use something like the following:
+you should use something like the following::
 
 	while (READ_ONCE(a) < 0)
 		do_something();
 
 Alternatively, you could place a barrier() call in the loop.
 
-For another example, consider the following code:
+For another example, consider the following code::
 
 	tmp_a = a;
 	do_something_with(tmp_a);
···
 
 If the compiler can prove that do_something_with() does not store to the
 variable a, then the compiler is within its rights to manufacture an
-additional load as follows:
+additional load as follows::
 
 	tmp_a = a;
 	do_something_with(tmp_a);
···
 do_something_with() was an inline function that made very heavy use
 of registers: reloading from variable a could save a flush to the
 stack and later reload.  To prevent the compiler from attacking your
-code in this manner, write the following:
+code in this manner, write the following::
 
 	tmp_a = READ_ONCE(a);
 	do_something_with(tmp_a);
···
 
 For a final example, consider the following code, assuming that the
 variable a is set at boot time before the second CPU is brought online
-and never changed later, so that memory barriers are not needed:
+and never changed later, so that memory barriers are not needed::
 
 	if (a)
 		b = 9;
···
 		b = 42;
 
 The compiler is within its rights to manufacture an additional store
-by transforming the above code into the following:
+by transforming the above code into the following::
 
 	b = 42;
 	if (a)
···
 
 This could come as a fatal surprise to other code running concurrently
 that expected b to never have the value 42 if a was zero.  To prevent
-the compiler from doing this, write something like:
+the compiler from doing this, write something like::
 
 	if (a)
 		WRITE_ONCE(b, 9);
···
 Don't even -think- about doing this without proper use of memory barriers,
 locks, or atomic operations if variable a can change at runtime!
 
-*** WARNING: READ_ONCE() OR WRITE_ONCE() DO NOT IMPLY A BARRIER! ***
+.. warning::
+
+	``READ_ONCE()`` OR ``WRITE_ONCE()`` DO NOT IMPLY A BARRIER!
 
 Now, we move onto the atomic operation interfaces typically implemented with
-the help of assembly code.
+the help of assembly code. ::
 
 	void atomic_add(int i, atomic_t *v);
 	void atomic_sub(int i, atomic_t *v);
···
 require any explicit memory barriers.  They need only perform the
 atomic_t counter update in an SMP safe manner.
 
-Next, we have:
+Next, we have::
 
 	int atomic_inc_return(atomic_t *v);
 	int atomic_dec_return(atomic_t *v);
···
 memory barrier semantics which satisfy the above requirements, that is
 fine as well.
 
-Let's move on:
+Let's move on::
 
 	int atomic_add_return(int i, atomic_t *v);
 	int atomic_sub_return(int i, atomic_t *v);
···
 This means that like atomic_{inc,dec}_return(), the memory barrier
 semantics are required.
 
-Next:
+Next::
 
 	int atomic_inc_and_test(atomic_t *v);
 	int atomic_dec_and_test(atomic_t *v);
···
 resulting counter value was zero or not.
 
 Again, these primitives provide explicit memory barrier semantics around
-the atomic operation.
+the atomic operation::
 
 	int atomic_sub_and_test(int i, atomic_t *v);
 
 This is identical to atomic_dec_and_test() except that an explicit
 decrement is given instead of the implicit "1".  This primitive must
-provide explicit memory barrier semantics around the operation.
+provide explicit memory barrier semantics around the operation::
 
 	int atomic_add_negative(int i, atomic_t *v);
 
···
 This primitive must provide explicit memory barrier semantics around
 the operation.
 
-Then:
+Then::
 
 	int atomic_xchg(atomic_t *v, int new);
 
···
 the given new value.  It returns the old value that the atomic variable v had
 just before the operation.
 
-atomic_xchg must provide explicit memory barriers around the operation.
+atomic_xchg must provide explicit memory barriers around the operation. ::
 
 	int atomic_cmpxchg(atomic_t *v, int old, int new);
 
 This performs an atomic compare exchange operation on the atomic value v,
 with the given old and new values. Like all atomic_xxx operations,
 atomic_cmpxchg will only satisfy its atomicity semantics as long as all
-other accesses of *v are performed through atomic_xxx operations.
+other accesses of \*v are performed through atomic_xxx operations.
 
 atomic_cmpxchg must provide explicit memory barriers around the operation,
 although if the comparison fails then no memory ordering guarantees are
···
 The semantics for atomic_cmpxchg are the same as those defined for 'cas'
 below.
 
-Finally:
+Finally::
 
 	int atomic_add_unless(atomic_t *v, int a, int u);
 
···
 
 If a caller requires memory barrier semantics around an atomic_t
 operation which does not return a value, a set of interfaces are
-defined which accomplish this:
+defined which accomplish this::
 
 	void smp_mb__before_atomic(void);
 	void smp_mb__after_atomic(void);
 
-For example, smp_mb__before_atomic() can be used like so:
+For example, smp_mb__before_atomic() can be used like so::
 
 	obj->dead = 1;
 	smp_mb__before_atomic();
···
 an example, which follows a pattern occurring frequently in the Linux
 kernel.  It is the use of atomic counters to implement reference
 counting, and it works such that once the counter falls to zero it can
-be guaranteed that no other entity can be accessing the object:
+be guaranteed that no other entity can be accessing the object::
 
-static void obj_list_add(struct obj *obj, struct list_head *head)
-{
-	obj->active = 1;
-	list_add(&obj->list, head);
-}
+	static void obj_list_add(struct obj *obj, struct list_head *head)
+	{
+		obj->active = 1;
+		list_add(&obj->list, head);
+	}
 
-static void obj_list_del(struct obj *obj)
-{
-	list_del(&obj->list);
-	obj->active = 0;
-}
+	static void obj_list_del(struct obj *obj)
+	{
+		list_del(&obj->list);
+		obj->active = 0;
+	}
 
-static void obj_destroy(struct obj *obj)
-{
-	BUG_ON(obj->active);
-	kfree(obj);
-}
+	static void obj_destroy(struct obj *obj)
+	{
+		BUG_ON(obj->active);
+		kfree(obj);
+	}
 
-struct obj *obj_list_peek(struct list_head *head)
-{
-	if (!list_empty(head)) {
+	struct obj *obj_list_peek(struct list_head *head)
+	{
+		if (!list_empty(head)) {
+			struct obj *obj;
+
+			obj = list_entry(head->next, struct obj, list);
+			atomic_inc(&obj->refcnt);
+			return obj;
+		}
+		return NULL;
+	}
+
+	void obj_poke(void)
+	{
 		struct obj *obj;
 
-		obj = list_entry(head->next, struct obj, list);
-		atomic_inc(&obj->refcnt);
-		return obj;
+		spin_lock(&global_list_lock);
+		obj = obj_list_peek(&global_list);
+		spin_unlock(&global_list_lock);
+
+		if (obj) {
+			obj->ops->poke(obj);
+			if (atomic_dec_and_test(&obj->refcnt))
+				obj_destroy(obj);
+		}
 	}
-	return NULL;
-}
 
-void obj_poke(void)
-{
-	struct obj *obj;
+	void obj_timeout(struct obj *obj)
+	{
+		spin_lock(&global_list_lock);
+		obj_list_del(obj);
+		spin_unlock(&global_list_lock);
 
-	spin_lock(&global_list_lock);
-	obj = obj_list_peek(&global_list);
-	spin_unlock(&global_list_lock);
-
-	if (obj) {
-		obj->ops->poke(obj);
 		if (atomic_dec_and_test(&obj->refcnt))
 			obj_destroy(obj);
 	}
-}
 
-void obj_timeout(struct obj *obj)
-{
-	spin_lock(&global_list_lock);
-	obj_list_del(obj);
-	spin_unlock(&global_list_lock);
+.. note::
 
-	if (atomic_dec_and_test(&obj->refcnt))
-		obj_destroy(obj);
-}
-
-(This is a simplification of the ARP queue management in the
- generic neighbour discover code of the networking.  Olaf Kirch
- found a bug wrt. memory barriers in kfree_skb() that exposed
- the atomic_t memory barrier requirements quite clearly.)
+	This is a simplification of the ARP queue management in the generic
+	neighbour discover code of the networking.  Olaf Kirch found a bug wrt.
+	memory barriers in kfree_skb() that exposed the atomic_t memory barrier
+	requirements quite clearly.
 
 Given the above scheme, it must be the case that the obj->active
 update done by the obj list deletion be visible to other processors
···
 
 Otherwise, the counter could fall to zero, yet obj->active would still
 be set, thus triggering the assertion in obj_destroy().  The error
-sequence looks like this:
+sequence looks like this::
 
 	cpu 0				cpu 1
 	obj_poke()			obj_timeout()
···
 Another note is that the atomic_t operations returning values are
 extremely slow on an old 386.
 
+
+Atomic Bitmask
+==============
+
 We will now cover the atomic bitmask operations.  You will find that
 their SMP and memory barrier semantics are similar in shape and scope
 to the atomic_t ops above.
···
 Native atomic bit operations are defined to operate on objects aligned
 to the size of an "unsigned long" C data type, and are least of that
 size.  The endianness of the bits within each "unsigned long" are the
-native endianness of the cpu.
+native endianness of the cpu. ::
 
 	void set_bit(unsigned long nr, volatile unsigned long *addr);
 	void clear_bit(unsigned long nr, volatile unsigned long *addr);
···
 indicated by "nr" on the bit mask pointed to by "ADDR".
 
 They must execute atomically, yet there are no implicit memory barrier
-semantics required of these interfaces.
+semantics required of these interfaces. ::
 
 	int test_and_set_bit(unsigned long nr, volatile unsigned long *addr);
 	int test_and_clear_bit(unsigned long nr, volatile unsigned long *addr);
···
 All memory operations before the atomic bit operation call must be
 made visible globally before the atomic bit operation is made visible.
 Likewise, the atomic bit operation must be visible globally before any
-subsequent memory operation is made visible.  For example:
+subsequent memory operation is made visible.  For example::
 
 	obj->dead = 1;
 	if (test_and_set_bit(0, &obj->flags))
···
 memory operation done by test_and_set_bit() must become visible before
 "obj->killed = 1;" is visible.
 
-Finally there is the basic operation:
+Finally there is the basic operation::
 
 	int test_bit(unsigned long nr, __const__ volatile unsigned long *addr);
 
···
 
 If explicit memory barriers are required around {set,clear}_bit() (which do
 not return a value, and thus does not need to provide memory barrier
-semantics), two interfaces are provided:
+semantics), two interfaces are provided::
 
 	void smp_mb__before_atomic(void);
 	void smp_mb__after_atomic(void);
 
 They are used as follows, and are akin to their atomic_t operation
-brothers:
+brothers::
 
 	/* All memory operations before this call will
 	 * be globally visible before the clear_bit().
···
 same as spinlocks). These operate in the same way as their non-_lock/unlock
 postfixed variants, except that they are to provide acquire/release semantics,
 respectively. This means they can be used for bit_spin_trylock and
-bit_spin_unlock type operations without specifying any more barriers.
+bit_spin_unlock type operations without specifying any more barriers. ::
 
 	int test_and_set_bit_lock(unsigned long nr, unsigned long *addr);
 	void clear_bit_unlock(unsigned long nr, unsigned long *addr);
···
 locking scheme is being used to protect the bitmask, and thus less
 expensive non-atomic operations may be used in the implementation.
 They have names similar to the above bitmask operation interfaces,
-except that two underscores are prefixed to the interface name.
+except that two underscores are prefixed to the interface name. ::
 
 	void __set_bit(unsigned long nr, volatile unsigned long *addr);
 	void __clear_bit(unsigned long nr, volatile unsigned long *addr);
···
 memory-barrier semantics as the atomic and bit operations returning
 values.
 
-Note: If someone wants to use xchg(), cmpxchg() and their variants,
-linux/atomic.h should be included rather than asm/cmpxchg.h, unless
-the code is in arch/* and can take care of itself.
+.. note::
+
+	If someone wants to use xchg(), cmpxchg() and their variants,
+	linux/atomic.h should be included rather than asm/cmpxchg.h, unless the
+	code is in arch/* and can take care of itself.
 
 Spinlocks and rwlocks have memory barrier expectations as well.
 The rule to follow is simple:
···
 
 Which finally brings us to _atomic_dec_and_lock().  There is an
 architecture-neutral version implemented in lib/dec_and_lock.c,
-but most platforms will wish to optimize this in assembler.
+but most platforms will wish to optimize this in assembler. ::
 
 	int _atomic_dec_and_lock(atomic_t *atomic, spinlock_t *lock);
 
···
 subsequent memory operation.
 
 We can demonstrate this operation more clearly if we define
-an abstract atomic operation:
+an abstract atomic operation::
 
 	long cas(long *mem, long old, long new);
 
···
 3) Regardless, the current value at "mem" is returned.
 
 As an example usage, here is what an atomic counter update
-might look like:
+might look like::
 
-void example_atomic_inc(long *counter)
-{
-	long old, new, ret;
+	void example_atomic_inc(long *counter)
+	{
+		long old, new, ret;
 
-	while (1) {
-		old = *counter;
-		new = old + 1;
+		while (1) {
+			old = *counter;
+			new = old + 1;
 
-		ret = cas(counter, old, new);
-		if (ret == old)
-			break;
-	}
-}
-
-Let's use cas() in order to build a pseudo-C atomic_dec_and_lock():
-
-int _atomic_dec_and_lock(atomic_t *atomic, spinlock_t *lock)
-{
-	long old, new, ret;
-	int went_to_zero;
-
-	went_to_zero = 0;
-	while (1) {
-		old = atomic_read(atomic);
-		new = old - 1;
-		if (new == 0) {
-			went_to_zero = 1;
-			spin_lock(lock);
-		}
-		ret = cas(atomic, old, new);
-		if (ret == old)
-			break;
-		if (went_to_zero) {
-			spin_unlock(lock);
-			went_to_zero = 0;
+			ret = cas(counter, old, new);
+			if (ret == old)
+				break;
 		}
 	}
 
-	return went_to_zero;
-}
+Let's use cas() in order to build a pseudo-C atomic_dec_and_lock()::
+
+	int _atomic_dec_and_lock(atomic_t *atomic, spinlock_t *lock)
+	{
+		long old, new, ret;
+		int went_to_zero;
+
+		went_to_zero = 0;
+		while (1) {
+			old = atomic_read(atomic);
+			new = old - 1;
+			if (new == 0) {
+				went_to_zero = 1;
+				spin_lock(lock);
+			}
+			ret = cas(atomic, old, new);
+			if (ret == old)
+				break;
+			if (went_to_zero) {
+				spin_unlock(lock);
+				went_to_zero = 0;
+			}
+		}
+
+		return went_to_zero;
+	}
 
 Now, as far as memory barriers go, as long as spin_lock()
 strictly orders all subsequent memory operations (including
···
 a counter dropping to zero is never made visible before the
 spinlock being acquired.
 
-Note that this also means that for the case where the counter
-is not dropping to zero, there are no memory ordering
-requirements.
+.. note::
+
+	Note that this also means that for the case where the counter is not
+	dropping to zero, there are no memory ordering requirements.

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
+   atomic_ops
+   local_ops
    workqueue
 
 Interfaces for kernel debugging

Documentation/core-api/local_ops.rst

···
+
+.. _local_ops:
+
+=================================================
+Semantics and Behavior of Local Atomic Operations
+=================================================
+
+:Author: Mathieu Desnoyers
+
+
+This document explains the purpose of the local atomic operations, how
+to implement them for any given architecture and shows how they can be used
+properly. It also stresses on the precautions that must be taken when reading
+those local variables across CPUs when the order of memory writes matters.
+
+.. note::
+
+    Note that ``local_t`` based operations are not recommended for general
+    kernel use. Please use the ``this_cpu`` operations instead unless there is
+    really a special purpose. Most uses of ``local_t`` in the kernel have been
+    replaced by ``this_cpu`` operations. ``this_cpu`` operations combine the
+    relocation with the ``local_t`` like semantics in a single instruction and
+    yield more compact and faster executing code.
+
+
+Purpose of local atomic operations
+==================================
+
+Local atomic operations are meant to provide fast and highly reentrant per CPU
+counters. They minimize the performance cost of standard atomic operations by
+removing the LOCK prefix and memory barriers normally required to synchronize
+across CPUs.
+
+Having fast per CPU atomic counters is interesting in many cases: it does not
+require disabling interrupts to protect from interrupt handlers and it permits
+coherent counters in NMI handlers. It is especially useful for tracing purposes
+and for various performance monitoring counters.
+
+Local atomic operations only guarantee variable modification atomicity wrt the
+CPU which owns the data. Therefore, care must taken to make sure that only one
+CPU writes to the ``local_t`` data. This is done by using per cpu data and
+making sure that we modify it from within a preemption safe context. It is
+however permitted to read ``local_t`` data from any CPU: it will then appear to
+be written out of order wrt other memory writes by the owner CPU.
+
+
+Implementation for a given architecture
+=======================================
+
+It can be done by slightly modifying the standard atomic operations: only
+their UP variant must be kept. It typically means removing LOCK prefix (on
+i386 and x86_64) and any SMP synchronization barrier. If the architecture does
+not have a different behavior between SMP and UP, including
+``asm-generic/local.h`` in your architecture's ``local.h`` is sufficient.
+
+The ``local_t`` type is defined as an opaque ``signed long`` by embedding an
+``atomic_long_t`` inside a structure. This is made so a cast from this type to
+a ``long`` fails. The definition looks like::
+
+    typedef struct { atomic_long_t a; } local_t;
+
+
+Rules to follow when using local atomic operations
+==================================================
+
+* Variables touched by local ops must be per cpu variables.
+* *Only* the CPU owner of these variables must write to them.
+* This CPU can use local ops from any context (process, irq, softirq, nmi, ...)
+  to update its ``local_t`` variables.
+* Preemption (or interrupts) must be disabled when using local ops in
+  process context to make sure the process won't be migrated to a
+  different CPU between getting the per-cpu variable and doing the
+  actual local op.
+* When using local ops in interrupt context, no special care must be
+  taken on a mainline kernel, since they will run on the local CPU with
+  preemption already disabled. I suggest, however, to explicitly
+  disable preemption anyway to make sure it will still work correctly on
+  -rt kernels.
+* Reading the local cpu variable will provide the current copy of the
+  variable.
+* Reads of these variables can be done from any CPU, because updates to
+  "``long``", aligned, variables are always atomic. Since no memory
+  synchronization is done by the writer CPU, an outdated copy of the
+  variable can be read when reading some *other* cpu's variables.
+
+
+How to use local atomic operations
+==================================
+
+::
+
+    #include <linux/percpu.h>
+    #include <asm/local.h>
+
+    static DEFINE_PER_CPU(local_t, counters) = LOCAL_INIT(0);
+
+
+Counting
+========
+
+Counting is done on all the bits of a signed long.
+
+In preemptible context, use ``get_cpu_var()`` and ``put_cpu_var()`` around
+local atomic operations: it makes sure that preemption is disabled around write
+access to the per cpu variable. For instance::
+
+    local_inc(&get_cpu_var(counters));
+    put_cpu_var(counters);
+
+If you are already in a preemption-safe context, you can use
+``this_cpu_ptr()`` instead::
+
+    local_inc(this_cpu_ptr(&counters));
+
+
+
+Reading the counters
+====================
+
+Those local counters can be read from foreign CPUs to sum the count. Note that
+the data seen by local_read across CPUs must be considered to be out of order
+relatively to other memory writes happening on the CPU that owns the data::
+
+    long sum = 0;
+    for_each_online_cpu(cpu)
+            sum += local_read(&per_cpu(counters, cpu));
+
+If you want to use a remote local_read to synchronize access to a resource
+between CPUs, explicit ``smp_wmb()`` and ``smp_rmb()`` memory barriers must be used
+respectively on the writer and the reader CPUs. It would be the case if you use
+the ``local_t`` variable as a counter of bytes written in a buffer: there should
+be a ``smp_wmb()`` between the buffer write and the counter increment and also a
+``smp_rmb()`` between the counter read and the buffer read.
+
+
+Here is a sample module which implements a basic per cpu counter using
+``local.h``::
+
+    /* test-local.c
+     *
+     * Sample module for local.h usage.
+     */
+
+
+    #include <asm/local.h>
+    #include <linux/module.h>
+    #include <linux/timer.h>
+
+    static DEFINE_PER_CPU(local_t, counters) = LOCAL_INIT(0);
+
+    static struct timer_list test_timer;
+
+    /* IPI called on each CPU. */
+    static void test_each(void *info)
+    {
+            /* Increment the counter from a non preemptible context */
+            printk("Increment on cpu %d\n", smp_processor_id());
+            local_inc(this_cpu_ptr(&counters));
+
+            /* This is what incrementing the variable would look like within a
+             * preemptible context (it disables preemption) :
+             *
+             * local_inc(&get_cpu_var(counters));
+             * put_cpu_var(counters);
+             */
+    }
+
+    static void do_test_timer(unsigned long data)
+    {
+            int cpu;
+
+            /* Increment the counters */
+            on_each_cpu(test_each, NULL, 1);
+            /* Read all the counters */
+            printk("Counters read from CPU %d\n", smp_processor_id());
+            for_each_online_cpu(cpu) {
+                    printk("Read : CPU %d, count %ld\n", cpu,
+                            local_read(&per_cpu(counters, cpu)));
+            }
+            del_timer(&test_timer);
+            test_timer.expires = jiffies + 1000;
+            add_timer(&test_timer);
+    }
+
+    static int __init test_init(void)
+    {
+            /* initialize the timer that will increment the counter */
+            init_timer(&test_timer);
+            test_timer.function = do_test_timer;
+            test_timer.expires = jiffies + 1;
+            add_timer(&test_timer);
+
+            return 0;
+    }
+
+    static void __exit test_exit(void)
+    {
+            del_timer_sync(&test_timer);
+    }
+
+    module_init(test_init);
+    module_exit(test_exit);
+
+    MODULE_LICENSE("GPL");
+    MODULE_AUTHOR("Mathieu Desnoyers");
+    MODULE_DESCRIPTION("Local Atomic Ops");

Documentation/local_ops.txt

···
-	     Semantics and Behavior of Local Atomic Operations
-
-			    Mathieu Desnoyers
-
-
-	This document explains the purpose of the local atomic operations, how
-to implement them for any given architecture and shows how they can be used
-properly. It also stresses on the precautions that must be taken when reading
-those local variables across CPUs when the order of memory writes matters.
-
-Note that local_t based operations are not recommended for general kernel use.
-Please use the this_cpu operations instead unless there is really a special purpose.
-Most uses of local_t in the kernel have been replaced by this_cpu operations.
-this_cpu operations combine the relocation with the local_t like semantics in
-a single instruction and yield more compact and faster executing code.
-
-
-* Purpose of local atomic operations
-
-Local atomic operations are meant to provide fast and highly reentrant per CPU
-counters. They minimize the performance cost of standard atomic operations by
-removing the LOCK prefix and memory barriers normally required to synchronize
-across CPUs.
-
-Having fast per CPU atomic counters is interesting in many cases : it does not
-require disabling interrupts to protect from interrupt handlers and it permits
-coherent counters in NMI handlers. It is especially useful for tracing purposes
-and for various performance monitoring counters.
-
-Local atomic operations only guarantee variable modification atomicity wrt the
-CPU which owns the data. Therefore, care must taken to make sure that only one
-CPU writes to the local_t data. This is done by using per cpu data and making
-sure that we modify it from within a preemption safe context. It is however
-permitted to read local_t data from any CPU : it will then appear to be written
-out of order wrt other memory writes by the owner CPU.
-
-
-* Implementation for a given architecture
-
-It can be done by slightly modifying the standard atomic operations : only
-their UP variant must be kept. It typically means removing LOCK prefix (on
-i386 and x86_64) and any SMP synchronization barrier. If the architecture does
-not have a different behavior between SMP and UP, including asm-generic/local.h
-in your architecture's local.h is sufficient.
-
-The local_t type is defined as an opaque signed long by embedding an
-atomic_long_t inside a structure. This is made so a cast from this type to a
-long fails. The definition looks like :
-
-typedef struct { atomic_long_t a; } local_t;
-
-
-* Rules to follow when using local atomic operations
-
-- Variables touched by local ops must be per cpu variables.
-- _Only_ the CPU owner of these variables must write to them.
-- This CPU can use local ops from any context (process, irq, softirq, nmi, ...)
-  to update its local_t variables.
-- Preemption (or interrupts) must be disabled when using local ops in
-  process context to   make sure the process won't be migrated to a
-  different CPU between getting the per-cpu variable and doing the
-  actual local op.
-- When using local ops in interrupt context, no special care must be
-  taken on a mainline kernel, since they will run on the local CPU with
-  preemption already disabled. I suggest, however, to explicitly
-  disable preemption anyway to make sure it will still work correctly on
-  -rt kernels.
-- Reading the local cpu variable will provide the current copy of the
-  variable.
-- Reads of these variables can be done from any CPU, because updates to
-  "long", aligned, variables are always atomic. Since no memory
-  synchronization is done by the writer CPU, an outdated copy of the
-  variable can be read when reading some _other_ cpu's variables.
-
-
-* How to use local atomic operations
-
-#include <linux/percpu.h>
-#include <asm/local.h>
-
-static DEFINE_PER_CPU(local_t, counters) = LOCAL_INIT(0);
-
-
-* Counting
-
-Counting is done on all the bits of a signed long.
-
-In preemptible context, use get_cpu_var() and put_cpu_var() around local atomic
-operations : it makes sure that preemption is disabled around write access to
-the per cpu variable. For instance :
-
-	local_inc(&get_cpu_var(counters));
-	put_cpu_var(counters);
-
-If you are already in a preemption-safe context, you can use
-this_cpu_ptr() instead.
-
-	local_inc(this_cpu_ptr(&counters));
-
-
-
-* Reading the counters
-
-Those local counters can be read from foreign CPUs to sum the count. Note that
-the data seen by local_read across CPUs must be considered to be out of order
-relatively to other memory writes happening on the CPU that owns the data.
-
-	long sum = 0;
-	for_each_online_cpu(cpu)
-		sum += local_read(&per_cpu(counters, cpu));
-
-If you want to use a remote local_read to synchronize access to a resource
-between CPUs, explicit smp_wmb() and smp_rmb() memory barriers must be used
-respectively on the writer and the reader CPUs. It would be the case if you use
-the local_t variable as a counter of bytes written in a buffer : there should
-be a smp_wmb() between the buffer write and the counter increment and also a
-smp_rmb() between the counter read and the buffer read.
-
-
-Here is a sample module which implements a basic per cpu counter using local.h.
-
---- BEGIN ---
-/* test-local.c
- *
- * Sample module for local.h usage.
- */
-
-
-#include <asm/local.h>
-#include <linux/module.h>
-#include <linux/timer.h>
-
-static DEFINE_PER_CPU(local_t, counters) = LOCAL_INIT(0);
-
-static struct timer_list test_timer;
-
-/* IPI called on each CPU. */
-static void test_each(void *info)
-{
-	/* Increment the counter from a non preemptible context */
-	printk("Increment on cpu %d\n", smp_processor_id());
-	local_inc(this_cpu_ptr(&counters));
-
-	/* This is what incrementing the variable would look like within a
-	 * preemptible context (it disables preemption) :
-	 *
-	 * local_inc(&get_cpu_var(counters));
-	 * put_cpu_var(counters);
-	 */
-}
-
-static void do_test_timer(unsigned long data)
-{
-	int cpu;
-
-	/* Increment the counters */
-	on_each_cpu(test_each, NULL, 1);
-	/* Read all the counters */
-	printk("Counters read from CPU %d\n", smp_processor_id());
-	for_each_online_cpu(cpu) {
-		printk("Read : CPU %d, count %ld\n", cpu,
-			local_read(&per_cpu(counters, cpu)));
-	}
-	del_timer(&test_timer);
-	test_timer.expires = jiffies + 1000;
-	add_timer(&test_timer);
-}
-
-static int __init test_init(void)
-{
-	/* initialize the timer that will increment the counter */
-	init_timer(&test_timer);
-	test_timer.function = do_test_timer;
-	test_timer.expires = jiffies + 1;
-	add_timer(&test_timer);
-
-	return 0;
-}
-
-static void __exit test_exit(void)
-{
-	del_timer_sync(&test_timer);
-}
-
-module_init(test_init);
-module_exit(test_exit);
-
-MODULE_LICENSE("GPL");
-MODULE_AUTHOR("Mathieu Desnoyers");
-MODULE_DESCRIPTION("Local Atomic Ops");
---- END ---

Documentation/process/volatile-considered-harmful.rst

···
+
+.. _volatile_considered_harmful:
+
 Why the "volatile" type class should not be used
 ------------------------------------------------