linux/Documentation/circular-buffers.txt
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   1                               ================
   2                               CIRCULAR BUFFERS
   3                               ================
   4
   5By: David Howells <dhowells@redhat.com>
   6    Paul E. McKenney <paulmck@linux.vnet.ibm.com>
   7
   8
   9Linux provides a number of features that can be used to implement circular
  10buffering.  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
  18To use these facilities, as discussed below, there needs to be just one
  19producer and just one consumer.  It is possible to handle multiple producers by
  20serialising them, and to handle multiple consumers by serialising them.
  21
  22
  23Contents:
  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==========================
  35WHAT IS A CIRCULAR BUFFER?
  36==========================
  37
  38First of all, what is a circular buffer?  A circular buffer is a buffer of
  39fixed, 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
  47Typically when the tail pointer is equal to the head pointer, the buffer is
  48empty; and the buffer is full when the head pointer is one less than the tail
  49pointer.
  50
  51The head index is incremented when items are added, and the tail index when
  52items are removed.  The tail index should never jump the head index, and both
  53indices should be wrapped to 0 when they reach the end of the buffer, thus
  54allowing an infinite amount of data to flow through the buffer.
  55
  56Typically, items will all be of the same unit size, but this isn't strictly
  57required to use the techniques below.  The indices can be increased by more
  58than 1 if multiple items or variable-sized items are to be included in the
  59buffer, provided that neither index overtakes the other.  The implementer must
  60be careful, however, as a region more than one unit in size may wrap the end of
  61the buffer and be broken into two segments.
  62
  63
  64============================
  65MEASURING POWER-OF-2 BUFFERS
  66============================
  67
  68Calculation of the occupancy or the remaining capacity of an arbitrarily sized
  69circular buffer would normally be a slow operation, requiring the use of a
  70modulus (divide) instruction.  However, if the buffer is of a power-of-2 size,
  71then a much quicker bitwise-AND instruction can be used instead.
  72
  73Linux provides a set of macros for handling power-of-2 circular buffers.  These
  74can be made use of by:
  75
  76        #include <linux/circ_buf.h>
  77
  78The 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
 112Each of these macros will nominally return a value between 0 and buffer_size-1,
 113however:
 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===========================================
 138USING MEMORY BARRIERS WITH CIRCULAR BUFFERS
 139===========================================
 140
 141By using memory barriers in conjunction with circular buffers, you can avoid
 142the 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
 149There are two sides to this: the producer that fills the buffer, and the
 150consumer that empties it.  Only one thing should be filling a buffer at any one
 151time, and only one thing should be emptying a buffer at any one time, but the
 152two sides can operate simultaneously.
 153
 154
 155THE PRODUCER
 156------------
 157
 158The 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
 182This will instruct the CPU that the contents of the new item must be written
 183before the head index makes it available to the consumer and then instructs the
 184CPU that the revised head index must be written before the consumer is woken.
 185
 186Note that wake_up() does not guarantee any sort of barrier unless something
 187is actually awakened.  We therefore cannot rely on it for ordering.  However,
 188there is always one element of the array left empty.  Therefore, the
 189producer must produce two elements before it could possibly corrupt the
 190element currently being read by the consumer.  Therefore, the unlock-lock
 191pair between consecutive invocations of the consumer provides the necessary
 192ordering between the read of the index indicating that the consumer has
 193vacated a given element and the write by the producer to that same element.
 194
 195
 196THE CONSUMER
 197------------
 198
 199The 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
 221This will instruct the CPU to make sure the index is up to date before reading
 222the new item, and then it shall make sure the CPU has finished reading the item
 223before it writes the new tail pointer, which will erase the item.
 224
 225Note the use of ACCESS_ONCE() and smp_load_acquire() to read the
 226opposition index.  This prevents the compiler from discarding and
 227reloading its cached value - which some compilers will do across
 228smp_read_barrier_depends().  This isn't strictly needed if you can
 229be sure that the opposition index will _only_ be used the once.
 230The smp_load_acquire() additionally forces the CPU to order against
 231subsequent memory references.  Similarly, smp_store_release() is used
 232in both algorithms to write the thread's index.  This documents the
 233fact that we are writing to something that can be read concurrently,
 234prevents the compiler from tearing the store, and enforces ordering
 235against previous accesses.
 236
 237
 238===============
 239FURTHER READING
 240===============
 241
 242See also Documentation/memory-barriers.txt for a description of Linux's memory
 243barrier facilities.
 244
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