1 ================
2 Circular Buffers
3 ================
4
5 :Author: David Howells <dhowells@redhat.com>
6 :Author: Paul E. McKenney <paulmck@linux.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 Measuring power-of-2 buffers
64 ============================
65
66 Calculation of the occupancy or the remaining capacity of an arbitrarily sized
67 circular buffer would normally be a slow operation, requiring the use of a
68 modulus (divide) instruction. However, if the buffer is of a power-of-2 size,
69 then a much quicker bitwise-AND instruction can be used instead.
70
71 Linux provides a set of macros for handling power-of-2 circular buffers. These
72 can be made use of by::
73
74 #include <linux/circ_buf.h>
75
76 The macros are:
77
78 (#) Measure the remaining capacity of a buffer::
79
80 CIRC_SPACE(head_index, tail_index, buffer_size);
81
82 This returns the amount of space left in the buffer[1] into which items
83 can be inserted.
84
85
86 (#) Measure the maximum consecutive immediate space in a buffer::
87
88 CIRC_SPACE_TO_END(head_index, tail_index, buffer_size);
89
90 This returns the amount of consecutive space left in the buffer[1] into
91 which items can be immediately inserted without having to wrap back to the
92 beginning of the buffer.
93
94
95 (#) Measure the occupancy of a buffer::
96
97 CIRC_CNT(head_index, tail_index, buffer_size);
98
99 This returns the number of items currently occupying a buffer[2].
100
101
102 (#) Measure the non-wrapping occupancy of a buffer::
103
104 CIRC_CNT_TO_END(head_index, tail_index, buffer_size);
105
106 This returns the number of consecutive items[2] that can be extracted from
107 the buffer without having to wrap back to the beginning of the buffer.
108
109
110 Each of these macros will nominally return a value between 0 and buffer_size-1,
111 however:
112
113 (1) CIRC_SPACE*() are intended to be used in the producer. To the producer
114 they will return a lower bound as the producer controls the head index,
115 but the consumer may still be depleting the buffer on another CPU and
116 moving the tail index.
117
118 To the consumer it will show an upper bound as the producer may be busy
119 depleting the space.
120
121 (2) CIRC_CNT*() are intended to be used in the consumer. To the consumer they
122 will return a lower bound as the consumer controls the tail index, but the
123 producer may still be filling the buffer on another CPU and moving the
124 head index.
125
126 To the producer it will show an upper bound as the consumer may be busy
127 emptying the buffer.
128
129 (3) To a third party, the order in which the writes to the indices by the
130 producer and consumer become visible cannot be guaranteed as they are
131 independent and may be made on different CPUs - so the result in such a
132 situation will merely be a guess, and may even be negative.
133
134 Using memory barriers with circular buffers
135 ===========================================
136
137 By using memory barriers in conjunction with circular buffers, you can avoid
138 the need to:
139
140 (1) use a single lock to govern access to both ends of the buffer, thus
141 allowing the buffer to be filled and emptied at the same time; and
142
143 (2) use atomic counter operations.
144
145 There are two sides to this: the producer that fills the buffer, and the
146 consumer that empties it. Only one thing should be filling a buffer at any one
147 time, and only one thing should be emptying a buffer at any one time, but the
148 two sides can operate simultaneously.
149
150
151 The producer
152 ------------
153
154 The producer will look something like this::
155
156 spin_lock(&producer_lock);
157
158 unsigned long head = buffer->head;
159 /* The spin_unlock() and next spin_lock() provide needed ordering. */
160 unsigned long tail = READ_ONCE(buffer->tail);
161
162 if (CIRC_SPACE(head, tail, buffer->size) >= 1) {
163 /* insert one item into the buffer */
164 struct item *item = buffer[head];
165
166 produce_item(item);
167
168 smp_store_release(buffer->head,
169 (head + 1) & (buffer->size - 1));
170
171 /* wake_up() will make sure that the head is committed before
172 * waking anyone up */
173 wake_up(consumer);
174 }
175
176 spin_unlock(&producer_lock);
177
178 This will instruct the CPU that the contents of the new item must be written
179 before the head index makes it available to the consumer and then instructs the
180 CPU that the revised head index must be written before the consumer is woken.
181
182 Note that wake_up() does not guarantee any sort of barrier unless something
183 is actually awakened. We therefore cannot rely on it for ordering. However,
184 there is always one element of the array left empty. Therefore, the
185 producer must produce two elements before it could possibly corrupt the
186 element currently being read by the consumer. Therefore, the unlock-lock
187 pair between consecutive invocations of the consumer provides the necessary
188 ordering between the read of the index indicating that the consumer has
189 vacated a given element and the write by the producer to that same element.
190
191
192 The Consumer
193 ------------
194
195 The consumer will look something like this::
196
197 spin_lock(&consumer_lock);
198
199 /* Read index before reading contents at that index. */
200 unsigned long head = smp_load_acquire(buffer->head);
201 unsigned long tail = buffer->tail;
202
203 if (CIRC_CNT(head, tail, buffer->size) >= 1) {
204
205 /* extract one item from the buffer */
206 struct item *item = buffer[tail];
207
208 consume_item(item);
209
210 /* Finish reading descriptor before incrementing tail. */
211 smp_store_release(buffer->tail,
212 (tail + 1) & (buffer->size - 1));
213 }
214
215 spin_unlock(&consumer_lock);
216
217 This will instruct the CPU to make sure the index is up to date before reading
218 the new item, and then it shall make sure the CPU has finished reading the item
219 before it writes the new tail pointer, which will erase the item.
220
221 Note the use of READ_ONCE() and smp_load_acquire() to read the
222 opposition index. This prevents the compiler from discarding and
223 reloading its cached value. This isn't strictly needed if you can
224 be sure that the opposition index will _only_ be used the once.
225 The smp_load_acquire() additionally forces the CPU to order against
226 subsequent memory references. Similarly, smp_store_release() is used
227 in both algorithms to write the thread's index. This documents the
228 fact that we are writing to something that can be read concurrently,
229 prevents the compiler from tearing the store, and enforces ordering
230 against previous accesses.
231
232
233 Further reading
234 ===============
235
236 See also Documentation/memory-barriers.txt for a description of Linux's memory
237 barrier facilities.
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