virtio-net: fix a race on 32bit arches

commit 3fa2a1df909 (virtio-net: per cpu 64 bit stats (v2)) added a race
on 32bit arches.

We must use separate syncp for rx and tx path as they can be run at the
same time on different cpus. Thus one sequence increment can be lost and
readers spin forever.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Stephen Hemminger <shemminger@vyatta.com>
Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Jason Wang <jasowang@redhat.com>
Acked-by: Rusty Russell <rusty@rustcorp.com.au>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/virtio_net.c

@@ -42,7 +42,8 @@ module_param(gso, bool, 0444);
 #define VIRTNET_DRIVER_VERSION "1.0.0"
 
 struct virtnet_stats {
-	struct u64_stats_sync syncp;
+	struct u64_stats_sync tx_syncp;
+	struct u64_stats_sync rx_syncp;
 	u64 tx_bytes;
 	u64 tx_packets;
 
@@ -300,10 +301,10 @@ static void receive_buf(struct net_device *dev, void *buf, unsigned int len)
 
 	hdr = skb_vnet_hdr(skb);
 
-	u64_stats_update_begin(&stats->syncp);
+	u64_stats_update_begin(&stats->rx_syncp);
 	stats->rx_bytes += skb->len;
 	stats->rx_packets++;
-	u64_stats_update_end(&stats->syncp);
+	u64_stats_update_end(&stats->rx_syncp);
 
 	if (hdr->hdr.flags & VIRTIO_NET_HDR_F_NEEDS_CSUM) {
 		pr_debug("Needs csum!\n");
@@ -565,10 +566,10 @@ static unsigned int free_old_xmit_skbs(struct virtnet_info *vi)
 	while ((skb = virtqueue_get_buf(vi->svq, &len)) != NULL) {
 		pr_debug("Sent skb %p\n", skb);
 
-		u64_stats_update_begin(&stats->syncp);
+		u64_stats_update_begin(&stats->tx_syncp);
 		stats->tx_bytes += skb->len;
 		stats->tx_packets++;
-		u64_stats_update_end(&stats->syncp);
+		u64_stats_update_end(&stats->tx_syncp);
 
 		tot_sgs += skb_vnet_hdr(skb)->num_sg;
 		dev_kfree_skb_any(skb);
@@ -703,12 +704,16 @@ static struct rtnl_link_stats64 *virtnet_stats(struct net_device *dev,
 		u64 tpackets, tbytes, rpackets, rbytes;
 
 		do {
-			start = u64_stats_fetch_begin(&stats->syncp);
+			start = u64_stats_fetch_begin(&stats->tx_syncp);
 			tpackets = stats->tx_packets;
 			tbytes   = stats->tx_bytes;
+		} while (u64_stats_fetch_retry(&stats->tx_syncp, start));
+
+		do {
+			start = u64_stats_fetch_begin(&stats->rx_syncp);
 			rpackets = stats->rx_packets;
 			rbytes   = stats->rx_bytes;
-		} while (u64_stats_fetch_retry(&stats->syncp, start));
+		} while (u64_stats_fetch_retry(&stats->rx_syncp, start));
 
 		tot->rx_packets += rpackets;
 		tot->tx_packets += tpackets;