On Mon, 26 Nov 2012 15:56:52 +0800
Jason Wang [off-list ref] wrote:

Some deivces do not free the old tx skbs immediately after it has been sent
(usually in tx interrupt). One such example is virtio-net which optimizes for
virt and only free the possible old tx skbs during the next packet sending. This
would lead the pktgen to wait forever in the refcount of the skb if no other
pakcet will be sent afterwards.

Solving this issue by introducing a new flag IFF_TX_SKB_FREE_DELAY which could
notify the pktgen that the device does not free skb immediately after it has
been sent and let it not to wait for the refcount to be one.

Signed-off-by: Jason Wang <jasowang@redhat.com>

Another alternative would be using skb_orphan() and skb->destructor.
There are other cases where skb's are not freed right away.