[RFC] Question about tcp_sendmsg()

[Eric Dumazet <eric.dumazet@gmail.com>]

While investigating for various bug reports in tcp stack, I looked at
commit def87cf42069a (tcp: Slightly optimize tcp_sendmsg)

One question I have is that the 

sg = sk->sk_route_caps & NETIF_F_SG;

is now done at the beginning of tcp_sendmsg(), and kept in sg variable
for the whole tcp_sendmsg() duration, even if task has to wait for
space.

Previously sk->sk_route_caps & NETIF_F_SG was done in select_size()
itself.

I am wondering if this can have a side effect, if SG capability changes
while a thread has to wait in sk_stream_wait_memory(), and socket route
changes (sk_route_caps flips NETIF_F_SG bit)

Thanks

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 9fce8a8..8a4d9bd 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -1093,6 +1093,7 @@ wait_for_memory:
 				goto do_error;
 
 			mss_now = tcp_send_mss(sk, &size_goal, flags);
+			sg = sk->sk_route_caps & NETIF_F_SG;
 		}
 	}