net: openvswitch: fix data race in ovs_vport_get_upcall_stats

In ovs_vport_get_upcall_stats(), some statistics protected by
u64_stats_sync, are read and accumulated in ignorance of possible
u64_stats_fetch_retry() events. These statistics are already accumulated
by u64_stats_inc(). Fix this by reading them into temporary variables
first.

Fixes: 1933ea365aa7 ("net: openvswitch: Add support to count upcall packets")
Signed-off-by: David Yang <[e-mail]>
Acked-by: Ilya Maximets <[e-mail]>
Reviewed-by: Eric Dumazet <[e-mail]>
Reviewed-by: Aaron Conole <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Paolo Abeni <[e-mail]>

net/openvswitch/vport.c

···
  */
 int ovs_vport_get_upcall_stats(struct vport *vport, struct sk_buff *skb)
 {
+	u64 tx_success = 0, tx_fail = 0;
 	struct nlattr *nla;
 	int i;
 
-	__u64 tx_success = 0;
-	__u64 tx_fail = 0;
-
 	for_each_possible_cpu(i) {
 		const struct vport_upcall_stats_percpu *stats;
+		u64 n_success, n_fail;
 		unsigned int start;
 
 		stats = per_cpu_ptr(vport->upcall_stats, i);
 		do {
 			start = u64_stats_fetch_begin(&stats->syncp);
-			tx_success += u64_stats_read(&stats->n_success);
-			tx_fail += u64_stats_read(&stats->n_fail);
+			n_success = u64_stats_read(&stats->n_success);
+			n_fail = u64_stats_read(&stats->n_fail);
 		} while (u64_stats_fetch_retry(&stats->syncp, start));
+		tx_success += n_success;
+		tx_fail += n_fail;
 	}
 
 	nla = nla_nest_start_noflag(skb, OVS_VPORT_ATTR_UPCALL_STATS);