ipvs: fix the connection sync failed in some cases

The sync_thread_backup only checks sk_receive_queue is empty or not,
there is a situation which cannot sync the connection entries when
sk_receive_queue is empty and sk_rmem_alloc is larger than sk_rcvbuf,
the sync packets are dropped in __udp_enqueue_schedule_skb, this is
because the packets in reader_queue is not read, so the rmem is
not reclaimed.

Here I add the check of whether the reader_queue of the udp sock is
empty or not to solve this problem.

Fixes: 2276f58a ("udp: use a separate rx queue for packet reception")
Reported-by: zhouxudong <zhouxudong8@huawei.com>
Signed-off-by: guodeqing <geffrey.guo@huawei.com>
Acked-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/ipvs/ip_vs_sync.c

@@ -1717,6 +1717,8 @@ static int sync_thread_backup(void *data)
 {
 	struct ip_vs_sync_thread_data *tinfo = data;
 	struct netns_ipvs *ipvs = tinfo->ipvs;
+	struct sock *sk = tinfo->sock->sk;
+	struct udp_sock *up = udp_sk(sk);
 	int len;
 
 	pr_info("sync thread started: state = BACKUP, mcast_ifn = %s, "
@@ -1724,12 +1726,14 @@ static int sync_thread_backup(void *data)
 		ipvs->bcfg.mcast_ifn, ipvs->bcfg.syncid, tinfo->id);
 
 	while (!kthread_should_stop()) {
-		wait_event_interruptible(*sk_sleep(tinfo->sock->sk),
-			 !skb_queue_empty(&tinfo->sock->sk->sk_receive_queue)
-			 || kthread_should_stop());
+		wait_event_interruptible(*sk_sleep(sk),
+					 !skb_queue_empty_lockless(&sk->sk_receive_queue) ||
+					 !skb_queue_empty_lockless(&up->reader_queue) ||
+					 kthread_should_stop());
 
 		/* do we have data now? */
-		while (!skb_queue_empty(&(tinfo->sock->sk->sk_receive_queue))) {
+		while (!skb_queue_empty_lockless(&sk->sk_receive_queue) ||
+		       !skb_queue_empty_lockless(&up->reader_queue)) {
 			len = ip_vs_receive(tinfo->sock, tinfo->buf,
 					ipvs->bcfg.sync_maxlen);
 			if (len <= 0) {