netdev - [PATCH net-next] tcp: guarantee forward progress in tcp

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Date:	Mon, 02 May 2016 21:49:25 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Eric Dumazet <edumazet@...gle.com>,
	David Miller <davem@...emloft.net>
Cc:	netdev <netdev@...r.kernel.org>,
	Soheil Hassas Yeganeh <soheil@...gle.com>,
	Alexei Starovoitov <ast@...com>,
	Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
Subject: [PATCH net-next] tcp: guarantee forward progress in tcp_sendmsg()

From: Eric Dumazet <edumazet@...gle.com>

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@...gle.com>
---
 net/ipv4/tcp.c |    7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index b945c2b046c5ead5503505f250c3c67761b284ae..5c7ed147449c1b7ba029b12e033ad779a631460a 100644
--- a/net/ipv4/tcp.c
+++ b/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.
 			 */