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Documentation / circular-buffers.txt




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Based on kernel version 3.16. Page generated on 2014-08-06 21:36 EST.

1				       ================
2				       CIRCULAR BUFFERS
3				       ================
4	
5	By: David Howells <dhowells@redhat.com>
6	    Paul E. McKenney <paulmck@linux.vnet.ibm.com>
7	
8	
9	Linux provides a number of features that can be used to implement circular
10	buffering.  There are two sets of such features:
11	
12	 (1) Convenience functions for determining information about power-of-2 sized
13	     buffers.
14	
15	 (2) Memory barriers for when the producer and the consumer of objects in the
16	     buffer don't want to share a lock.
17	
18	To use these facilities, as discussed below, there needs to be just one
19	producer and just one consumer.  It is possible to handle multiple producers by
20	serialising them, and to handle multiple consumers by serialising them.
21	
22	
23	Contents:
24	
25	 (*) What is a circular buffer?
26	
27	 (*) Measuring power-of-2 buffers.
28	
29	 (*) Using memory barriers with circular buffers.
30	     - The producer.
31	     - The consumer.
32	
33	
34	==========================
35	WHAT IS A CIRCULAR BUFFER?
36	==========================
37	
38	First of all, what is a circular buffer?  A circular buffer is a buffer of
39	fixed, finite size into which there are two indices:
40	
41	 (1) A 'head' index - the point at which the producer inserts items into the
42	     buffer.
43	
44	 (2) A 'tail' index - the point at which the consumer finds the next item in
45	     the buffer.
46	
47	Typically when the tail pointer is equal to the head pointer, the buffer is
48	empty; and the buffer is full when the head pointer is one less than the tail
49	pointer.
50	
51	The head index is incremented when items are added, and the tail index when
52	items are removed.  The tail index should never jump the head index, and both
53	indices should be wrapped to 0 when they reach the end of the buffer, thus
54	allowing an infinite amount of data to flow through the buffer.
55	
56	Typically, items will all be of the same unit size, but this isn't strictly
57	required to use the techniques below.  The indices can be increased by more
58	than 1 if multiple items or variable-sized items are to be included in the
59	buffer, provided that neither index overtakes the other.  The implementer must
60	be careful, however, as a region more than one unit in size may wrap the end of
61	the buffer and be broken into two segments.
62	
63	
64	============================
65	MEASURING POWER-OF-2 BUFFERS
66	============================
67	
68	Calculation of the occupancy or the remaining capacity of an arbitrarily sized
69	circular buffer would normally be a slow operation, requiring the use of a
70	modulus (divide) instruction.  However, if the buffer is of a power-of-2 size,
71	then a much quicker bitwise-AND instruction can be used instead.
72	
73	Linux provides a set of macros for handling power-of-2 circular buffers.  These
74	can be made use of by:
75	
76		#include <linux/circ_buf.h>
77	
78	The macros are:
79	
80	 (*) Measure the remaining capacity of a buffer:
81	
82		CIRC_SPACE(head_index, tail_index, buffer_size);
83	
84	     This returns the amount of space left in the buffer[1] into which items
85	     can be inserted.
86	
87	
88	 (*) Measure the maximum consecutive immediate space in a buffer:
89	
90		CIRC_SPACE_TO_END(head_index, tail_index, buffer_size);
91	
92	     This returns the amount of consecutive space left in the buffer[1] into
93	     which items can be immediately inserted without having to wrap back to the
94	     beginning of the buffer.
95	
96	
97	 (*) Measure the occupancy of a buffer:
98	
99		CIRC_CNT(head_index, tail_index, buffer_size);
100	
101	     This returns the number of items currently occupying a buffer[2].
102	
103	
104	 (*) Measure the non-wrapping occupancy of a buffer:
105	
106		CIRC_CNT_TO_END(head_index, tail_index, buffer_size);
107	
108	     This returns the number of consecutive items[2] that can be extracted from
109	     the buffer without having to wrap back to the beginning of the buffer.
110	
111	
112	Each of these macros will nominally return a value between 0 and buffer_size-1,
113	however:
114	
115	 [1] CIRC_SPACE*() are intended to be used in the producer.  To the producer
116	     they will return a lower bound as the producer controls the head index,
117	     but the consumer may still be depleting the buffer on another CPU and
118	     moving the tail index.
119	
120	     To the consumer it will show an upper bound as the producer may be busy
121	     depleting the space.
122	
123	 [2] CIRC_CNT*() are intended to be used in the consumer.  To the consumer they
124	     will return a lower bound as the consumer controls the tail index, but the
125	     producer may still be filling the buffer on another CPU and moving the
126	     head index.
127	
128	     To the producer it will show an upper bound as the consumer may be busy
129	     emptying the buffer.
130	
131	 [3] To a third party, the order in which the writes to the indices by the
132	     producer and consumer become visible cannot be guaranteed as they are
133	     independent and may be made on different CPUs - so the result in such a
134	     situation will merely be a guess, and may even be negative.
135	
136	
137	===========================================
138	USING MEMORY BARRIERS WITH CIRCULAR BUFFERS
139	===========================================
140	
141	By using memory barriers in conjunction with circular buffers, you can avoid
142	the need to:
143	
144	 (1) use a single lock to govern access to both ends of the buffer, thus
145	     allowing the buffer to be filled and emptied at the same time; and
146	
147	 (2) use atomic counter operations.
148	
149	There are two sides to this: the producer that fills the buffer, and the
150	consumer that empties it.  Only one thing should be filling a buffer at any one
151	time, and only one thing should be emptying a buffer at any one time, but the
152	two sides can operate simultaneously.
153	
154	
155	THE PRODUCER
156	------------
157	
158	The producer will look something like this:
159	
160		spin_lock(&producer_lock);
161	
162		unsigned long head = buffer->head;
163		/* The spin_unlock() and next spin_lock() provide needed ordering. */
164		unsigned long tail = ACCESS_ONCE(buffer->tail);
165	
166		if (CIRC_SPACE(head, tail, buffer->size) >= 1) {
167			/* insert one item into the buffer */
168			struct item *item = buffer[head];
169	
170			produce_item(item);
171	
172			smp_store_release(buffer->head,
173					  (head + 1) & (buffer->size - 1));
174	
175			/* wake_up() will make sure that the head is committed before
176			 * waking anyone up */
177			wake_up(consumer);
178		}
179	
180		spin_unlock(&producer_lock);
181	
182	This will instruct the CPU that the contents of the new item must be written
183	before the head index makes it available to the consumer and then instructs the
184	CPU that the revised head index must be written before the consumer is woken.
185	
186	Note that wake_up() does not guarantee any sort of barrier unless something
187	is actually awakened.  We therefore cannot rely on it for ordering.  However,
188	there is always one element of the array left empty.  Therefore, the
189	producer must produce two elements before it could possibly corrupt the
190	element currently being read by the consumer.  Therefore, the unlock-lock
191	pair between consecutive invocations of the consumer provides the necessary
192	ordering between the read of the index indicating that the consumer has
193	vacated a given element and the write by the producer to that same element.
194	
195	
196	THE CONSUMER
197	------------
198	
199	The consumer will look something like this:
200	
201		spin_lock(&consumer_lock);
202	
203		/* Read index before reading contents at that index. */
204		unsigned long head = smp_load_acquire(buffer->head);
205		unsigned long tail = buffer->tail;
206	
207		if (CIRC_CNT(head, tail, buffer->size) >= 1) {
208	
209			/* extract one item from the buffer */
210			struct item *item = buffer[tail];
211	
212			consume_item(item);
213	
214			/* Finish reading descriptor before incrementing tail. */
215			smp_store_release(buffer->tail,
216					  (tail + 1) & (buffer->size - 1));
217		}
218	
219		spin_unlock(&consumer_lock);
220	
221	This will instruct the CPU to make sure the index is up to date before reading
222	the new item, and then it shall make sure the CPU has finished reading the item
223	before it writes the new tail pointer, which will erase the item.
224	
225	Note the use of ACCESS_ONCE() and smp_load_acquire() to read the
226	opposition index.  This prevents the compiler from discarding and
227	reloading its cached value - which some compilers will do across
228	smp_read_barrier_depends().  This isn't strictly needed if you can
229	be sure that the opposition index will _only_ be used the once.
230	The smp_load_acquire() additionally forces the CPU to order against
231	subsequent memory references.  Similarly, smp_store_release() is used
232	in both algorithms to write the thread's index.  This documents the
233	fact that we are writing to something that can be read concurrently,
234	prevents the compiler from tearing the store, and enforces ordering
235	against previous accesses.
236	
237	
238	===============
239	FURTHER READING
240	===============
241	
242	See also Documentation/memory-barriers.txt for a description of Linux's memory
243	barrier facilities.
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