net: provide dev_lstats_read() helper

Many network drivers use hand-coded implementation of the same thing,
let's factorize things so that u64_stats_t adoption is done once.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/loopback.c

@@ -99,13 +99,13 @@ static netdev_tx_t loopback_xmit(struct sk_buff *skb,
 	return NETDEV_TX_OK;
 }
 
-static void loopback_get_stats64(struct net_device *dev,
-				 struct rtnl_link_stats64 *stats)
+void dev_lstats_read(struct net_device *dev, u64 *packets, u64 *bytes)
 {
-	u64 bytes = 0;
-	u64 packets = 0;
 	int i;
 
+	*packets = 0;
+	*bytes = 0;
+
 	for_each_possible_cpu(i) {
 		const struct pcpu_lstats *lb_stats;
 		u64 tbytes, tpackets;
@@ -114,12 +114,22 @@ static void loopback_get_stats64(struct net_device *dev,
 		lb_stats = per_cpu_ptr(dev->lstats, i);
 		do {
 			start = u64_stats_fetch_begin_irq(&lb_stats->syncp);
-			tbytes = lb_stats->bytes;
 			tpackets = lb_stats->packets;
+			tbytes = lb_stats->bytes;
 		} while (u64_stats_fetch_retry_irq(&lb_stats->syncp, start));
-		bytes   += tbytes;
-		packets += tpackets;
+		*bytes   += tbytes;
+		*packets += tpackets;
 	}
+}
+EXPORT_SYMBOL(dev_lstats_read);
+
+static void loopback_get_stats64(struct net_device *dev,
+				 struct rtnl_link_stats64 *stats)
+{
+	u64 packets, bytes;
+
+	dev_lstats_read(dev, &packets, &bytes);
+
 	stats->rx_packets = packets;
 	stats->tx_packets = packets;
 	stats->rx_bytes   = bytes;

include/linux/netdevice.h

@@ -2401,6 +2401,8 @@ struct pcpu_lstats {
 	struct u64_stats_sync syncp;
 } __aligned(2 * sizeof(u64));
 
+void dev_lstats_read(struct net_device *dev, u64 *packets, u64 *bytes);
+
 #define __netdev_alloc_pcpu_stats(type, gfp)				\
 ({									\
 	typeof(type) __percpu *pcpu_stats = alloc_percpu_gfp(type, gfp);\