gstreamer/gst/gstatomicqueue.c
Matthew Waters 3d887c7f07 gst: don't use volatile to mean atomic
volatile is not sufficient to provide atomic guarantees and real atomics
should be used instead.  GCC 11 has started warning about using volatile
with atomic operations.

https://gitlab.gnome.org/GNOME/glib/-/merge_requests/1719

Discovered in https://gitlab.freedesktop.org/gstreamer/gst-plugins-good/-/issues/868

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/775>
2021-03-19 04:15:19 +00:00

423 lines
11 KiB
C

/* GStreamer
* Copyright (C) 2009 Edward Hervey <bilboed@bilboed.com>
* 2011 Wim Taymans <wim.taymans@gmail.com>
*
* gstatomicqueue.c:
*
* This library is free software; you can redistribute it and/or
* modify it under the terms of the GNU Library General Public
* License as published by the Free Software Foundation; either
* version 2 of the License, or (at your option) any later version.
*
* This library is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
* Library General Public License for more details.
*
* You should have received a copy of the GNU Library General Public
* License along with this library; if not, write to the
* Free Software Foundation, Inc., 51 Franklin St, Fifth Floor,
* Boston, MA 02110-1301, USA.
*/
#include "gst_private.h"
#include <string.h>
#include <gst/gst.h>
#include "gstatomicqueue.h"
#include "glib-compat-private.h"
/**
* SECTION:gstatomicqueue
* @title: GstAtomicQueue
* @short_description: An atomic queue implementation
*
* The #GstAtomicQueue object implements a queue that can be used from multiple
* threads without performing any blocking operations.
*/
G_DEFINE_BOXED_TYPE (GstAtomicQueue, gst_atomic_queue,
(GBoxedCopyFunc) gst_atomic_queue_ref,
(GBoxedFreeFunc) gst_atomic_queue_unref);
/* By default the queue uses 2 * sizeof(gpointer) * clp2 (max_items) of
* memory. clp2(x) is the next power of two >= than x.
*
* The queue can operate in low memory mode, in which it consumes almost
* half the memory at the expense of extra overhead in the readers. This
* is disabled by default because even without LOW_MEM mode, the memory
* consumption is still lower than a plain GList.
*/
#undef LOW_MEM
typedef struct _GstAQueueMem GstAQueueMem;
struct _GstAQueueMem
{
gint size;
gpointer *array;
gint head;
gint tail_write;
gint tail_read;
GstAQueueMem *next;
GstAQueueMem *free;
};
static guint
clp2 (guint n)
{
guint res = 1;
while (res < n)
res <<= 1;
return res;
}
static GstAQueueMem *
new_queue_mem (guint size, gint pos)
{
GstAQueueMem *mem;
mem = g_new (GstAQueueMem, 1);
/* we keep the size as a mask for performance */
mem->size = clp2 (MAX (size, 16)) - 1;
mem->array = g_new0 (gpointer, mem->size + 1);
mem->head = pos;
mem->tail_write = pos;
mem->tail_read = pos;
mem->next = NULL;
mem->free = NULL;
return mem;
}
static void
free_queue_mem (GstAQueueMem * mem)
{
g_free (mem->array);
g_free (mem);
}
struct _GstAtomicQueue
{
gint refcount;
#ifdef LOW_MEM
gint num_readers;
#endif
GstAQueueMem *head_mem;
GstAQueueMem *tail_mem;
GstAQueueMem *free_list;
};
static void
add_to_free_list (GstAtomicQueue * queue, GstAQueueMem * mem)
{
do {
mem->free = g_atomic_pointer_get (&queue->free_list);
} while (!g_atomic_pointer_compare_and_exchange (&queue->free_list,
mem->free, mem));
}
static void
clear_free_list (GstAtomicQueue * queue)
{
GstAQueueMem *free_list;
/* take the free list and replace with NULL */
do {
free_list = g_atomic_pointer_get (&queue->free_list);
if (free_list == NULL)
return;
} while (!g_atomic_pointer_compare_and_exchange (&queue->free_list, free_list,
NULL));
while (free_list) {
GstAQueueMem *next = free_list->free;
free_queue_mem (free_list);
free_list = next;
}
}
/**
* gst_atomic_queue_new:
* @initial_size: initial queue size
*
* Create a new atomic queue instance. @initial_size will be rounded up to the
* nearest power of 2 and used as the initial size of the queue.
*
* Returns: a new #GstAtomicQueue
*/
GstAtomicQueue *
gst_atomic_queue_new (guint initial_size)
{
GstAtomicQueue *queue;
queue = g_new (GstAtomicQueue, 1);
queue->refcount = 1;
#ifdef LOW_MEM
queue->num_readers = 0;
#endif
queue->head_mem = queue->tail_mem = new_queue_mem (initial_size, 0);
queue->free_list = NULL;
return queue;
}
/**
* gst_atomic_queue_ref:
* @queue: a #GstAtomicQueue
*
* Increase the refcount of @queue.
*/
void
gst_atomic_queue_ref (GstAtomicQueue * queue)
{
g_return_if_fail (queue != NULL);
g_atomic_int_inc (&queue->refcount);
}
static void
gst_atomic_queue_free (GstAtomicQueue * queue)
{
free_queue_mem (queue->head_mem);
if (queue->head_mem != queue->tail_mem)
free_queue_mem (queue->tail_mem);
clear_free_list (queue);
g_free (queue);
}
/**
* gst_atomic_queue_unref:
* @queue: a #GstAtomicQueue
*
* Unref @queue and free the memory when the refcount reaches 0.
*/
void
gst_atomic_queue_unref (GstAtomicQueue * queue)
{
g_return_if_fail (queue != NULL);
if (g_atomic_int_dec_and_test (&queue->refcount))
gst_atomic_queue_free (queue);
}
/**
* gst_atomic_queue_peek:
* @queue: a #GstAtomicQueue
*
* Peek the head element of the queue without removing it from the queue.
*
* Returns: (transfer none) (nullable): the head element of @queue or
* %NULL when the queue is empty.
*/
gpointer
gst_atomic_queue_peek (GstAtomicQueue * queue)
{
GstAQueueMem *head_mem;
gint head, tail, size;
g_return_val_if_fail (queue != NULL, NULL);
while (TRUE) {
GstAQueueMem *next;
head_mem = g_atomic_pointer_get (&queue->head_mem);
head = g_atomic_int_get (&head_mem->head);
tail = g_atomic_int_get (&head_mem->tail_read);
size = head_mem->size;
/* when we are not empty, we can continue */
if (G_LIKELY (head != tail))
break;
/* else array empty, try to take next */
next = g_atomic_pointer_get (&head_mem->next);
if (next == NULL)
return NULL;
/* now we try to move the next array as the head memory. If we fail to do that,
* some other reader managed to do it first and we retry */
if (!g_atomic_pointer_compare_and_exchange (&queue->head_mem, head_mem,
next))
continue;
/* when we managed to swing the head pointer the old head is now
* useless and we add it to the freelist. We can't free the memory yet
* because we first need to make sure no reader is accessing it anymore. */
add_to_free_list (queue, head_mem);
}
return head_mem->array[head & size];
}
/**
* gst_atomic_queue_pop:
* @queue: a #GstAtomicQueue
*
* Get the head element of the queue.
*
* Returns: (transfer full): the head element of @queue or %NULL when
* the queue is empty.
*/
gpointer
gst_atomic_queue_pop (GstAtomicQueue * queue)
{
gpointer ret;
GstAQueueMem *head_mem;
gint head, tail, size;
g_return_val_if_fail (queue != NULL, NULL);
#ifdef LOW_MEM
g_atomic_int_inc (&queue->num_readers);
#endif
do {
while (TRUE) {
GstAQueueMem *next;
head_mem = g_atomic_pointer_get (&queue->head_mem);
head = g_atomic_int_get (&head_mem->head);
tail = g_atomic_int_get (&head_mem->tail_read);
size = head_mem->size;
/* when we are not empty, we can continue */
if G_LIKELY
(head != tail)
break;
/* else array empty, try to take next */
next = g_atomic_pointer_get (&head_mem->next);
if (next == NULL)
return NULL;
/* now we try to move the next array as the head memory. If we fail to do that,
* some other reader managed to do it first and we retry */
if G_UNLIKELY
(!g_atomic_pointer_compare_and_exchange (&queue->head_mem, head_mem,
next))
continue;
/* when we managed to swing the head pointer the old head is now
* useless and we add it to the freelist. We can't free the memory yet
* because we first need to make sure no reader is accessing it anymore. */
add_to_free_list (queue, head_mem);
}
ret = head_mem->array[head & size];
} while G_UNLIKELY
(!g_atomic_int_compare_and_exchange (&head_mem->head, head, head + 1));
#ifdef LOW_MEM
/* decrement number of readers, when we reach 0 readers we can be sure that
* none is accessing the memory in the free list and we can try to clean up */
if (g_atomic_int_dec_and_test (&queue->num_readers))
clear_free_list (queue);
#endif
return ret;
}
/**
* gst_atomic_queue_push:
* @queue: a #GstAtomicQueue
* @data: the data
*
* Append @data to the tail of the queue.
*/
void
gst_atomic_queue_push (GstAtomicQueue * queue, gpointer data)
{
GstAQueueMem *tail_mem;
gint head, tail, size;
g_return_if_fail (queue != NULL);
do {
while (TRUE) {
GstAQueueMem *mem;
tail_mem = g_atomic_pointer_get (&queue->tail_mem);
head = g_atomic_int_get (&tail_mem->head);
tail = g_atomic_int_get (&tail_mem->tail_write);
size = tail_mem->size;
/* we're not full, continue */
if G_LIKELY
(tail - head <= size)
break;
/* else we need to grow the array, we store a mask so we have to add 1 */
mem = new_queue_mem ((size << 1) + 1, tail);
/* try to make our new array visible to other writers */
if G_UNLIKELY
(!g_atomic_pointer_compare_and_exchange (&queue->tail_mem, tail_mem,
mem)) {
/* we tried to swap the new writer array but something changed. This is
* because some other writer beat us to it, we free our memory and try
* again */
free_queue_mem (mem);
continue;
}
/* make sure that readers can find our new array as well. The one who
* manages to swap the pointer is the only one who can set the next
* pointer to the new array */
g_atomic_pointer_set (&tail_mem->next, mem);
}
} while G_UNLIKELY
(!g_atomic_int_compare_and_exchange (&tail_mem->tail_write, tail, tail + 1));
tail_mem->array[tail & size] = data;
/* now wait until all writers have completed their write before we move the
* tail_read to this new item. It is possible that other writers are still
* updating the previous array slots and we don't want to reveal their changes
* before they are done. FIXME, it would be nice if we didn't have to busy
* wait here. */
while G_UNLIKELY
(!g_atomic_int_compare_and_exchange (&tail_mem->tail_read, tail, tail + 1));
}
/**
* gst_atomic_queue_length:
* @queue: a #GstAtomicQueue
*
* Get the amount of items in the queue.
*
* Returns: the number of elements in the queue.
*/
guint
gst_atomic_queue_length (GstAtomicQueue * queue)
{
GstAQueueMem *head_mem, *tail_mem;
gint head, tail;
g_return_val_if_fail (queue != NULL, 0);
#ifdef LOW_MEM
g_atomic_int_inc (&queue->num_readers);
#endif
head_mem = g_atomic_pointer_get (&queue->head_mem);
head = g_atomic_int_get (&head_mem->head);
tail_mem = g_atomic_pointer_get (&queue->tail_mem);
tail = g_atomic_int_get (&tail_mem->tail_read);
#ifdef LOW_MEM
if (g_atomic_int_dec_and_test (&queue->num_readers))
clear_free_list (queue);
#endif
return tail - head;
}