Fix memcached networking issue that was caused by unsecure, parallel device...

Fix memcached networking issue that was caused by unsecure, parallel device invocation. I wasn't aware that the a device lock has to be held on the tx callback. Will look into it deeper tomorrow. The patch solves the issue

drivers/virtio-net.cc

@@ -98,16 +98,17 @@ namespace virtio {
         virtio_net_d("%s_start (transmit)", __FUNCTION__);
 
         /* Process packets */
+        vnet->_tx_ring_lock.lock();
         IF_DEQUEUE(&ifp->if_snd, m_head);
         while (m_head != NULL) {
             virtio_net_d("*** processing packet! ***");
 
             vnet->tx(m_head, false);
-
             IF_DEQUEUE(&ifp->if_snd, m_head);
         }
 
         vnet->kick(1);
+        vnet->_tx_ring_lock.unlock();
     }
 
     static void virtio_if_init(void* xsc)

drivers/virtio-net.hh

@@ -234,6 +234,9 @@ namespace virtio {
         void tx_gc();
         static hw_driver* probe(hw_device* dev);
 
+        // tx ring lock protects this ring for multiple access
+        mutex _tx_ring_lock;
+
     private:
 
         struct virtio_net_req {