dataplane: do not free VirtQueueElement in vring_push()

VirtQueueElement is allocated in vring_pop() so it seems to make sense
that vring_push() should free it.  Alas, virtio-blk frees
VirtQueueElement itself in virtio_blk_free_request().

This patch solves a double-free assertion in glib's g_slice_free().

Rename vring_free_element() to vring_unmap_element() since it no longer
frees the VirtQueueElement.
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>
Tested-by: NChristian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

hw/virtio/dataplane/vring.c

@@ -272,7 +272,7 @@ static int get_indirect(Vring *vring, VirtQueueElement *elem,
     return 0;
 }
 
-void vring_free_element(VirtQueueElement *elem)
+static void vring_unmap_element(VirtQueueElement *elem)
 {
     int i;
 
@@ -287,8 +287,6 @@ void vring_free_element(VirtQueueElement *elem)
     for (i = 0; i < elem->in_num; i++) {
         vring_unmap(elem->in_sg[i].iov_base, true);
     }
-
-    g_slice_free(VirtQueueElement, elem);
 }
 
 /* This looks in the virtqueue and for the first available buffer, and converts
@@ -402,7 +400,8 @@ out:
         vring->broken = true;
     }
     if (elem) {
-        vring_free_element(elem);
+        vring_unmap_element(elem);
+        g_slice_free(VirtQueueElement, elem);
     }
     *p_elem = NULL;
     return ret;
@@ -418,7 +417,7 @@ void vring_push(Vring *vring, VirtQueueElement *elem, int len)
     unsigned int head = elem->index;
     uint16_t new;
 
-    vring_free_element(elem);
+    vring_unmap_element(elem);
 
     /* Don't touch vring if a fatal error occurred */
     if (vring->broken) {

include/hw/virtio/dataplane/vring.h

@@ -55,6 +55,5 @@ bool vring_enable_notification(VirtIODevice *vdev, Vring *vring);
 bool vring_should_notify(VirtIODevice *vdev, Vring *vring);
 int vring_pop(VirtIODevice *vdev, Vring *vring, VirtQueueElement **elem);
 void vring_push(Vring *vring, VirtQueueElement *elem, int len);
-void vring_free_element(VirtQueueElement *elem);
 
 #endif /* VRING_H */