[PATCH] Document Linux's circular buffering capabilities [ver #2]

[David Howells]

Document the circular buffering capabilities available in Linux.

Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
---

 Documentation/circular-buffers.txt |  231 ++++++++++++++++++++++++++++++++++++
 Documentation/memory-barriers.txt  |   20 +++
 include/linux/circ_buf.h           |    4 +
 3 files changed, 255 insertions(+), 0 deletions(-)
 create mode 100644 Documentation/circular-buffers.txt


diff --git a/Documentation/circular-buffers.txt b/Documentation/circular-buffers.txt
new file mode 100644
index 0000000..cde764c
--- /dev/null
+++ b/Documentation/circular-buffers.txt
@@ -0,0 +1,231 @@
+			       ================
+			       CIRCULAR BUFFERS
+			       ================
+
+By: David Howells <dhowells@xxxxxxxxxx>
+    Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
+
+
+Linux provides a number of features that can be used to implement circular
+buffering.  There are two sets of such features:
+
+ (1) Convenience functions for determining information about power-of-2 sized
+     buffers.
+
+ (2) Memory barriers for when the producer and the consumer of objects in the
+     buffer don't want to share a lock.
+
+To use these facilities, as discussed below, there needs to be just one
+producer and just one consumer.  It is possible to handle multiple producers by
+serialising them, and to handle multiple consumers by serialising them.
+
+
+Contents:
+
+ (*) What is a circular buffer?
+
+ (*) Measuring power-of-2 buffers.
+
+ (*) Using memory barriers with circular buffers.
+     - The producer.
+     - The consumer.
+
+
+==========================
+WHAT IS A CIRCULAR BUFFER?
+==========================
+
+First of all, what is a circular buffer?  A circular buffer is a buffer of
+fixed, finite size into which there are two indices:
+
+ (1) A 'head' index - the point at which the producer inserts items into the
+     buffer.
+
+ (2) A 'tail' index - the point at which the consumer finds the next item in
+     the buffer.
+
+Typically when the tail pointer is equal to the head pointer, the buffer is
+empty; and the buffer is full when the head pointer is one less than the tail
+pointer.
+
+The head index is incremented when items are added, and the tail index when
+items are removed.  The tail index should never jump the head index, and both
+indices should be wrapped to 0 when they reach the end of the buffer, thus
+allowing an infinite amount of data to flow through the buffer.
+
+Typically, items will all be of the same unit size, but this isn't strictly
+required to use the techniques below.  The indices can be increased by more
+than 1 if multiple items or variable-sized items are to be included in the
+buffer, provided that neither index overtakes the other.  The implementer must
+be careful, however, as a region more than one unit in size may wrap the end of
+the buffer and be broken into two segments.
+
+
+============================
+MEASURING POWER-OF-2 BUFFERS
+============================
+
+Circular buffers that are of a size that is an exact power of two can have
+their item count and buffer space assessed really quickly through the use of a
+bitwise-AND instruction.  Non-power-of-2 sized buffers must use a modulus
+(divide) instruction instead which is likely to be very slow.
+
+There are a set of macros to do this in Linux, that can be made use of by:
+
+	#include <linux/circ_buf.h>
+
+These are:
+
+ (*) Measure the remaining capacity of a buffer:
+
+	CIRC_SPACE(head_index, tail_index, buffer_size);
+
+     This returns the amount of space left in the buffer[1] into which items can
+     be inserted.
+
+
+ (*) Measure the maximum consecutive immediate space in a buffer:
+
+	CIRC_SPACE_TO_END(head_index, tail_index, buffer_size);
+
+     This returns the amount of consecutive space left in the buffer[1] into which
+     items can be immediately inserted without having to wrap back to the
+     beginning of the buffer.
+
+
+ (*) Measure the occupancy of a buffer:
+
+	CIRC_CNT(head_index, tail_index, buffer_size);
+
+     This returns the number of items currently occupying a buffer.
+
+
+ (*) Measure the non-wrapping occupancy of a buffer:
+
+	CIRC_CNT_TO_END(head_index, tail_index, buffer_size);
+
+     This returns the number of consecutive items that can be extracted from
+     the buffer without having to wrap back to the beginning of the buffer.
+
+
+Each of these macros will nominally return a value between 0 and buffer_size-1,
+however:
+
+ [1] CIRC_SPACE*() are intended to be used in the producer.  To the producer
+     they will return a lower bound as the producer controls the head index,
+     but the consumer may still be depleting the buffer on another CPU and
+     moving the tail index.
+
+     To the consumer it will show an upper bound as the producer may be busy
+     depleting the space.
+
+ [2] CIRC_CNT*() are intended to be used in the consumer.  To the consumer they
+     will return a lower bound as the consumer controls the tail index, but the
+     producer may still be filling the buffer on andother CPU and moving the
+     head index.
+
+     To the producer it will show an upper bound as the consumer may be busy
+     emptying the buffer.
+
+ [3] To a third party, the order in which the writes to the indices by the
+     producer and consumer become visible cannot be guaranteed as they are
+     independent and may be made on different CPUs - so the result in such a
+     situation will merely be a guess, and may even be negative.
+
+
+===========================================
+USING MEMORY BARRIERS WITH CIRCULAR BUFFERS
+===========================================
+
+By using memory barriers in conjunction with circular buffers, you can avoid
+the need to:
+
+ (1) use a single lock to govern access to both ends of the buffer, thus
+     allowing the buffer to be filled and emptied at the same time; and
+
+ (2) use atomic counter operations.
+
+There are two sides to this: the producer that fills the buffer, and the
+consumer that empties it.  Only one thing should be filling a buffer at any one
+time, and only one thing should be emptying a buffer at any one time, but the
+two sides can operate simultaneously.
+
+
+THE PRODUCER
+------------
+
+The producer will look something like this:
+
+	spin_lock(&producer_lock);
+
+	unsigned long head = buffer->head;
+	unsigned long tail = ACCESS_ONCE(buffer->tail);
+
+	if (CIRC_SPACE(head, tail, buffer->size) >= 1) {
+		/* insert one item into the buffer */
+		struct item *item = buffer[head];
+
+		produce_item(item);
+
+		smp_wmb(); /* commit the item before incrementing the head */
+
+		buffer->head = (head + 1) & (buffer->size - 1);
+
+		/* wake_up() will make sure that the head is committed before
+		 * waking anyone up */
+		wake_up(consumer);
+	}
+
+	spin_unlock(&producer_lock);
+
+This will instruct the CPU that the contents of the new item must be written
+before the head index makes it available to the consumer and then instructs the
+CPU that the revised head index must be written before the consumer is woken.
+
+Note that wake_up() doesn't have to be the exact mechanism used, but whatever
+is used must guarantee a (write) memory barrier between the update of the head
+index and the change of state of the consumer, if a change of state occurs.
+
+
+THE CONSUMER
+------------
+
+The consumer will look something like this:
+
+	spin_lock(&consumer_lock);
+
+	unsigned long head = ACCESS_ONCE(buffer->head);
+	unsigned long tail = buffer->tail;
+
+	if (CIRC_CNT(head, tail, buffer->size) >= 1) {
+		/* read index before reading contents at that index */
+		smp_read_barrier_depends();
+
+		/* extract one item from the buffer */
+		struct item *item = buffer[tail];
+
+		consume_item(item);
+
+		smp_mb(); /* finish reading descriptor before incrementing tail */
+
+		buffer->tail = (tail + 1) & (buffer->size - 1);
+	}
+
+	spin_unlock(&consumer_lock);
+
+This will instruct the CPU to make sure the index is up to date before reading
+the new item, and then it shall make sure the CPU has finished reading the item
+before it writes the new tail pointer, which will erase the item.
+
+
+Note the use of ACCESS_ONCE() in both algorithms to read the opposition index.
+This prevents the compiler from discarding and reloading its cached value -
+which some compilers will do, even after an implied compiler barrier.
+
+
+===============
+FURTHER READING
+===============
+
+See also Documentation/memory-barriers.txt for a description of Linux's memory
+barrier facilities.
diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index 7f5809e..631ad2f 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -3,6 +3,7 @@
 			 ============================
 
 By: David Howells <dhowells@xxxxxxxxxx>
+    Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
 
 Contents:
 
@@ -60,6 +61,10 @@ Contents:
 
      - And then there's the Alpha.
 
+ (*) Example uses.
+
+     - Circular buffers.
+
  (*) References.
 
 
@@ -2226,6 +2231,21 @@ The Alpha defines the Linux kernel's memory barrier model.
 See the subsection on "Cache Coherency" above.
 
 
+============
+EXAMPLE USES
+============
+
+CIRCULAR BUFFERS
+----------------
+
+Memory barriers can be used to implement circular buffering without the need
+of a lock to serialise the producer with the consumer.  See:
+
+	Documentation/circular-buffers.txt
+
+for details.
+
+
 ==========
 REFERENCES
 ==========
diff --git a/include/linux/circ_buf.h b/include/linux/circ_buf.h
index a2ed059..90f2471 100644
--- a/include/linux/circ_buf.h
+++ b/include/linux/circ_buf.h
@@ -1,3 +1,7 @@
+/*
+ * See Documentation/circular-buffers.txt for more information.
+ */
+
 #ifndef _LINUX_CIRC_BUF_H
 #define _LINUX_CIRC_BUF_H 1
 

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