netdev - [RFC] Question about tcp

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Date:	Thu, 15 Jul 2010 15:14:48 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Miller <davem@...emloft.net>,
	Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>,
	Krishna Kumar <krkumar2@...ibm.com>
Cc:	netdev <netdev@...r.kernel.org>
Subject: [RFC] Question about tcp_sendmsg()

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;
 		}
 	}
 


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