tcp: plug skb_still_in_host_queue() to TSQ

Jakub and Neil reported an increase of RTO timers whenever
TX completions are delayed a bit more (by increasing
NIC TX coalescing parameters)

Main issue is that TCP stack has a logic preventing a packet
being retransmit if the prior clone has not yet been
orphaned or freed.

This logic came with commit 1f3279a ("tcp: avoid
retransmits of TCP packets hanging in host queues")

Thankfully, in the case skb_still_in_host_queue() detects
the initial clone is still in flight, it can use TSQ logic
that will eventually retry later, at the moment the clone
is freed or orphaned.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Neil Spring <ntspring@fb.com>
Reported-by: Jakub Kicinski <kuba@kernel.org>
Cc: Neal Cardwell <ncardwell@google.com>
Cc: Yuchung Cheng <ycheng@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Author: edumazet

include/linux/skbuff.h

@@ -1140,7 +1140,7 @@ static inline bool skb_fclone_busy(const struct sock *sk,
 
 	return skb->fclone == SKB_FCLONE_ORIG &&
 	       refcount_read(&fclones->fclone_ref) > 1 &&
-	       fclones->skb2.sk == sk;
+	       READ_ONCE(fclones->skb2.sk) == sk;
 }
 
 /**

net/ipv4/tcp_output.c

@@ -2775,13 +2775,17 @@ bool tcp_schedule_loss_probe(struct sock *sk, bool advancing_rto)
  * a packet is still in a qdisc or driver queue.
  * In this case, there is very little point doing a retransmit !
  */
-static bool skb_still_in_host_queue(const struct sock *sk,
+static bool skb_still_in_host_queue(struct sock *sk,
 				    const struct sk_buff *skb)
 {
 	if (unlikely(skb_fclone_busy(sk, skb))) {
-		NET_INC_STATS(sock_net(sk),
-			      LINUX_MIB_TCPSPURIOUS_RTX_HOSTQUEUES);
-		return true;
+		set_bit(TSQ_THROTTLED, &sk->sk_tsq_flags);
+		smp_mb__after_atomic();
+		if (skb_fclone_busy(sk, skb)) {
+			NET_INC_STATS(sock_net(sk),
+				      LINUX_MIB_TCPSPURIOUS_RTX_HOSTQUEUES);
+			return true;
+		}
 	}
 	return false;
 }