Paolo's debug for #444

net/mptcp/protocol.c

@@ -1309,7 +1309,10 @@ static int mptcp_sendmsg_frag(struct sock *sk, struct sock *ssk,
 		/* all mptcp-level data is acked, no skbs should be present into the
 		 * ssk write queue
 		 */
-		WARN_ON_ONCE(reuse_skb);
+		if (WARN_ON_ONCE(reuse_skb))
+			pr_warn("acked bytes %lld sent bytes %lld snd_una %lld skb seq %lld skb len %d data seq %lld subflows %d",
+				msk->bytes_acked, msk->bytes_sent, snd_una, mpext ? mpext->data_seq : -1,
+				skb->len, dfrag->data_seq + info->sent, msk->pm.subflows);
 	}
 
 	copy = min_t(size_t, copy, info->limit - info->sent);