tcp: guarantee forward progress in tcp_sendmsg()

Under high rx pressure, it is possible tcp_sendmsg() never has a
chance to allocate an skb and loop forever as sk_flush_backlog()
would always return true.

Fix this by calling sk_flush_backlog() only if one skb had been
allocated and filled before last backlog check.

Fixes: d41a69f1d390 ("tcp: make tcp_sendmsg() aware of socket backlog")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Soheil Hassas Yeganeh <soheil@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp.c

@@ -1084,6 +1084,7 @@ int tcp_sendmsg(struct sock *sk, struct msghdr *msg, size_t size)
 	struct sockcm_cookie sockc;
 	int flags, err, copied = 0;
 	int mss_now = 0, size_goal, copied_syn = 0;
+	bool process_backlog = false;
 	bool sg;
 	long timeo;
 
@@ -1167,9 +1168,10 @@ new_segment:
 			if (!sk_stream_memory_free(sk))
 				goto wait_for_sndbuf;
 
-			if (sk_flush_backlog(sk))
+			if (process_backlog && sk_flush_backlog(sk)) {
+				process_backlog = false;
 				goto restart;
-
+			}
 			skb = sk_stream_alloc_skb(sk,
 						  select_size(sk, sg),
 						  sk->sk_allocation,
@@ -1177,6 +1179,7 @@ new_segment:
 			if (!skb)
 				goto wait_for_memory;
 
+			process_backlog = true;
 			/*
 			 * Check whether we can use HW checksum.
 			 */