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+ ================
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+ CIRCULAR BUFFERS
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+ ================
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+
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+By: David Howells <dhowells@redhat.com>
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+ Paul E. McKenney <paulmck@linux.vnet.ibm.com>
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+
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+
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+Linux provides a number of features that can be used to implement circular
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+buffering. There are two sets of such features:
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+
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+ (1) Convenience functions for determining information about power-of-2 sized
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+ buffers.
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+
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+ (2) Memory barriers for when the producer and the consumer of objects in the
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+ buffer don't want to share a lock.
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+
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+To use these facilities, as discussed below, there needs to be just one
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+producer and just one consumer. It is possible to handle multiple producers by
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+serialising them, and to handle multiple consumers by serialising them.
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+
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+
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+Contents:
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+
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+ (*) What is a circular buffer?
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+
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+ (*) Measuring power-of-2 buffers.
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+
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+ (*) Using memory barriers with circular buffers.
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+ - The producer.
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+ - The consumer.
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+
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+
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+==========================
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+WHAT IS A CIRCULAR BUFFER?
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+==========================
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+
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+First of all, what is a circular buffer? A circular buffer is a buffer of
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+fixed, finite size into which there are two indices:
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+
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+ (1) A 'head' index - the point at which the producer inserts items into the
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+ buffer.
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+
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+ (2) A 'tail' index - the point at which the consumer finds the next item in
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+ the buffer.
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+
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+Typically when the tail pointer is equal to the head pointer, the buffer is
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+empty; and the buffer is full when the head pointer is one less than the tail
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+pointer.
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+
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+The head index is incremented when items are added, and the tail index when
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+items are removed. The tail index should never jump the head index, and both
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+indices should be wrapped to 0 when they reach the end of the buffer, thus
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+allowing an infinite amount of data to flow through the buffer.
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+
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+Typically, items will all be of the same unit size, but this isn't strictly
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+required to use the techniques below. The indices can be increased by more
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+than 1 if multiple items or variable-sized items are to be included in the
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+buffer, provided that neither index overtakes the other. The implementer must
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+be careful, however, as a region more than one unit in size may wrap the end of
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+the buffer and be broken into two segments.
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+
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+
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+============================
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+MEASURING POWER-OF-2 BUFFERS
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+============================
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+
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+Calculation of the occupancy or the remaining capacity of an arbitrarily sized
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+circular buffer would normally be a slow operation, requiring the use of a
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+modulus (divide) instruction. However, if the buffer is of a power-of-2 size,
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+then a much quicker bitwise-AND instruction can be used instead.
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+
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+Linux provides a set of macros for handling power-of-2 circular buffers. These
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+can be made use of by:
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+
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+ #include <linux/circ_buf.h>
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+
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+The macros are:
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+
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+ (*) Measure the remaining capacity of a buffer:
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+
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+ CIRC_SPACE(head_index, tail_index, buffer_size);
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+
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+ This returns the amount of space left in the buffer[1] into which items
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+ can be inserted.
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+
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+
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+ (*) Measure the maximum consecutive immediate space in a buffer:
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+
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+ CIRC_SPACE_TO_END(head_index, tail_index, buffer_size);
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+
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+ This returns the amount of consecutive space left in the buffer[1] into
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+ which items can be immediately inserted without having to wrap back to the
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+ beginning of the buffer.
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+
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+
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+ (*) Measure the occupancy of a buffer:
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+
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+ CIRC_CNT(head_index, tail_index, buffer_size);
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+
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+ This returns the number of items currently occupying a buffer[2].
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+
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+
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+ (*) Measure the non-wrapping occupancy of a buffer:
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+
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+ CIRC_CNT_TO_END(head_index, tail_index, buffer_size);
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+
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+ This returns the number of consecutive items[2] that can be extracted from
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+ the buffer without having to wrap back to the beginning of the buffer.
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+
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+
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+Each of these macros will nominally return a value between 0 and buffer_size-1,
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+however:
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+
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+ [1] CIRC_SPACE*() are intended to be used in the producer. To the producer
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+ they will return a lower bound as the producer controls the head index,
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+ but the consumer may still be depleting the buffer on another CPU and
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+ moving the tail index.
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+
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+ To the consumer it will show an upper bound as the producer may be busy
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+ depleting the space.
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+
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+ [2] CIRC_CNT*() are intended to be used in the consumer. To the consumer they
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+ will return a lower bound as the consumer controls the tail index, but the
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+ producer may still be filling the buffer on another CPU and moving the
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+ head index.
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+
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+ To the producer it will show an upper bound as the consumer may be busy
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+ emptying the buffer.
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+
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+ [3] To a third party, the order in which the writes to the indices by the
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+ producer and consumer become visible cannot be guaranteed as they are
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+ independent and may be made on different CPUs - so the result in such a
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+ situation will merely be a guess, and may even be negative.
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+
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+
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+===========================================
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+USING MEMORY BARRIERS WITH CIRCULAR BUFFERS
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+===========================================
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+
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+By using memory barriers in conjunction with circular buffers, you can avoid
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+the need to:
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+
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+ (1) use a single lock to govern access to both ends of the buffer, thus
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+ allowing the buffer to be filled and emptied at the same time; and
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+
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+ (2) use atomic counter operations.
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+
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+There are two sides to this: the producer that fills the buffer, and the
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+consumer that empties it. Only one thing should be filling a buffer at any one
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+time, and only one thing should be emptying a buffer at any one time, but the
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+two sides can operate simultaneously.
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+
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+
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+THE PRODUCER
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+------------
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+
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+The producer will look something like this:
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+
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+ spin_lock(&producer_lock);
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+
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+ unsigned long head = buffer->head;
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+ unsigned long tail = ACCESS_ONCE(buffer->tail);
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+
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+ if (CIRC_SPACE(head, tail, buffer->size) >= 1) {
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+ /* insert one item into the buffer */
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+ struct item *item = buffer[head];
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+
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+ produce_item(item);
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+
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+ smp_wmb(); /* commit the item before incrementing the head */
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+
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+ buffer->head = (head + 1) & (buffer->size - 1);
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+
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+ /* wake_up() will make sure that the head is committed before
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+ * waking anyone up */
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+ wake_up(consumer);
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+ }
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+
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+ spin_unlock(&producer_lock);
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+
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+This will instruct the CPU that the contents of the new item must be written
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+before the head index makes it available to the consumer and then instructs the
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+CPU that the revised head index must be written before the consumer is woken.
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+
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+Note that wake_up() doesn't have to be the exact mechanism used, but whatever
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+is used must guarantee a (write) memory barrier between the update of the head
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+index and the change of state of the consumer, if a change of state occurs.
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+
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+
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+THE CONSUMER
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+------------
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+
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+The consumer will look something like this:
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+
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+ spin_lock(&consumer_lock);
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+
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+ unsigned long head = ACCESS_ONCE(buffer->head);
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+ unsigned long tail = buffer->tail;
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+
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+ if (CIRC_CNT(head, tail, buffer->size) >= 1) {
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+ /* read index before reading contents at that index */
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+ smp_read_barrier_depends();
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+
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+ /* extract one item from the buffer */
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+ struct item *item = buffer[tail];
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+
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+ consume_item(item);
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+
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+ smp_mb(); /* finish reading descriptor before incrementing tail */
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+
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+ buffer->tail = (tail + 1) & (buffer->size - 1);
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+ }
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+
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+ spin_unlock(&consumer_lock);
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+
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+This will instruct the CPU to make sure the index is up to date before reading
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+the new item, and then it shall make sure the CPU has finished reading the item
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+before it writes the new tail pointer, which will erase the item.
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+
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+
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+Note the use of ACCESS_ONCE() in both algorithms to read the opposition index.
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+This prevents the compiler from discarding and reloading its cached value -
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+which some compilers will do across smp_read_barrier_depends(). This isn't
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+strictly needed if you can be sure that the opposition index will _only_ be
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+used the once.
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+
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+
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+===============
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+FURTHER READING
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+===============
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+
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+See also Documentation/memory-barriers.txt for a description of Linux's memory
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+barrier facilities.
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