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atomicqueue: fix subtle race

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Fix a race where the reader would see the updated the tail pointer before the
write could write the data into the queue. Fix this by having a separate reader
tail pointer that is only incremented after the writer wrote the data.
This commit is contained in:
Wim Taymans 2012-02-24 12:51:24 +01:00
parent beea57dca7
commit 61e8fa0dc5
1 changed files with 8 additions and 3 deletions
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11
gst/gstatomicqueue.c
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@ -57,6 +57,7 @@ struct _GstAQueueMem
gpointer *array; gpointer *array;
volatile gint head; volatile gint head;
volatile gint tail; volatile gint tail;
volatile gint tail_read;
GstAQueueMem *next; GstAQueueMem *next;
GstAQueueMem *free; GstAQueueMem *free;
}; };
@ -84,6 +85,7 @@ new_queue_mem (guint size, gint pos)
mem->array = g_new0 (gpointer, mem->size + 1); mem->array = g_new0 (gpointer, mem->size + 1);
mem->head = pos; mem->head = pos;
mem->tail = pos; mem->tail = pos;
mem->tail_read = pos;
mem->next = NULL; mem->next = NULL;
mem->free = NULL; mem->free = NULL;
@ -234,7 +236,7 @@ gst_atomic_queue_peek (GstAtomicQueue * queue)
head_mem = g_atomic_pointer_get (&queue->head_mem); head_mem = g_atomic_pointer_get (&queue->head_mem);
head = g_atomic_int_get (&head_mem->head); head = g_atomic_int_get (&head_mem->head);
tail = g_atomic_int_get (&head_mem->tail); tail = g_atomic_int_get (&head_mem->tail_read);
size = head_mem->size; size = head_mem->size;
/* when we are not empty, we can continue */ /* when we are not empty, we can continue */
@ -291,7 +293,7 @@ gst_atomic_queue_pop (GstAtomicQueue * queue)
head_mem = g_atomic_pointer_get (&queue->head_mem); head_mem = g_atomic_pointer_get (&queue->head_mem);
head = g_atomic_int_get (&head_mem->head); head = g_atomic_int_get (&head_mem->head);
tail = g_atomic_int_get (&head_mem->tail); tail = g_atomic_int_get (&head_mem->tail_read);
size = head_mem->size; size = head_mem->size;
/* when we are not empty, we can continue */ /* when we are not empty, we can continue */
@ -380,6 +382,9 @@ gst_atomic_queue_push (GstAtomicQueue * queue, gpointer data)
tail + 1)); tail + 1));
tail_mem->array[tail & size] = data; tail_mem->array[tail & size] = data;
/* and now the readers can read */
g_atomic_int_inc (&tail_mem->tail_read);
} }
/** /**
@ -408,7 +413,7 @@ gst_atomic_queue_length (GstAtomicQueue * queue)
head = g_atomic_int_get (&head_mem->head); head = g_atomic_int_get (&head_mem->head);
tail_mem = g_atomic_pointer_get (&queue->tail_mem); tail_mem = g_atomic_pointer_get (&queue->tail_mem);
tail = g_atomic_int_get (&tail_mem->tail); tail = g_atomic_int_get (&tail_mem->tail_read);
#ifdef LOW_MEM #ifdef LOW_MEM
if (g_atomic_int_dec_and_test (&queue->num_readers)) if (g_atomic_int_dec_and_test (&queue->num_readers))