[PATCH net-next] tcp: guarantee forward progress in tcp_sendmsg()

[Eric Dumazet]

From: Eric Dumazet <eduma...@google.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 <eduma...@google.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.
                         */