mirror of
https://github.com/torvalds/linux.git
synced 2024-12-05 02:23:16 +00:00
f1d8b71c5f
Each text file under Documentation follows a different format. Some doesn't even have titles! Change its representation to follow the adopted standard, using ReST markups for it to be parseable by Sphinx: - Mark titles with ReST notation; - comment the contents table; - Use :Author: tag for authorship; - mark literal blocks as such; - use valid numbered list markups; - Don't capitalize titles. Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com> Signed-off-by: Jonathan Corbet <corbet@lwn.net>
239 lines
8.2 KiB
Plaintext
239 lines
8.2 KiB
Plaintext
================
|
|
Circular Buffers
|
|
================
|
|
|
|
:Author: David Howells <dhowells@redhat.com>
|
|
:Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
|
|
|
|
|
|
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
|
|
============================
|
|
|
|
Calculation of the occupancy or the remaining capacity of an arbitrarily sized
|
|
circular buffer would normally be a slow operation, requiring the use of a
|
|
modulus (divide) instruction. However, if the buffer is of a power-of-2 size,
|
|
then a much quicker bitwise-AND instruction can be used instead.
|
|
|
|
Linux provides a set of macros for handling power-of-2 circular buffers. These
|
|
can be made use of by::
|
|
|
|
#include <linux/circ_buf.h>
|
|
|
|
The macros 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[2].
|
|
|
|
|
|
(#) 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[2] 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 another 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;
|
|
/* The spin_unlock() and next spin_lock() provide needed ordering. */
|
|
unsigned long tail = READ_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_store_release(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() does not guarantee any sort of barrier unless something
|
|
is actually awakened. We therefore cannot rely on it for ordering. However,
|
|
there is always one element of the array left empty. Therefore, the
|
|
producer must produce two elements before it could possibly corrupt the
|
|
element currently being read by the consumer. Therefore, the unlock-lock
|
|
pair between consecutive invocations of the consumer provides the necessary
|
|
ordering between the read of the index indicating that the consumer has
|
|
vacated a given element and the write by the producer to that same element.
|
|
|
|
|
|
The Consumer
|
|
------------
|
|
|
|
The consumer will look something like this::
|
|
|
|
spin_lock(&consumer_lock);
|
|
|
|
/* Read index before reading contents at that index. */
|
|
unsigned long head = smp_load_acquire(buffer->head);
|
|
unsigned long tail = buffer->tail;
|
|
|
|
if (CIRC_CNT(head, tail, buffer->size) >= 1) {
|
|
|
|
/* extract one item from the buffer */
|
|
struct item *item = buffer[tail];
|
|
|
|
consume_item(item);
|
|
|
|
/* Finish reading descriptor before incrementing tail. */
|
|
smp_store_release(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 READ_ONCE() and smp_load_acquire() to read the
|
|
opposition index. This prevents the compiler from discarding and
|
|
reloading its cached value - which some compilers will do across
|
|
smp_read_barrier_depends(). This isn't strictly needed if you can
|
|
be sure that the opposition index will _only_ be used the once.
|
|
The smp_load_acquire() additionally forces the CPU to order against
|
|
subsequent memory references. Similarly, smp_store_release() is used
|
|
in both algorithms to write the thread's index. This documents the
|
|
fact that we are writing to something that can be read concurrently,
|
|
prevents the compiler from tearing the store, and enforces ordering
|
|
against previous accesses.
|
|
|
|
|
|
Further reading
|
|
===============
|
|
|
|
See also Documentation/memory-barriers.txt for a description of Linux's memory
|
|
barrier facilities.
|