linux-kernel - [PATCH] Do not use TSO/GSO when there is urgent data

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Date:	Fri, 21 Nov 2008 13:19:45 +0100
From:	Petr Tesarik <ptesarik@...e.cz>
To:	netdev@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org,
	"David S. Miller" <davem@...emloft.net>,
	Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>,
	Jan Šembera <jsembera@...e.cz>
Subject: [PATCH] Do not use TSO/GSO when there is urgent data

This patch fixes http://bugzilla.kernel.org/show_bug.cgi?id=12014

Since most (if not all) implementations of TSO and even the in-kernel
software GSO do not update the urgent pointer when splitting a large
segment, it is necessary to turn off TSO/GSO for all outgoing traffic
with the URG pointer set.

Looking at tcp_current_mss (and the preceding comment) I even think
this was the original intention. However, this approach is insufficient,
because TSO/GSO is turned off only for newly created frames, not for
frames which were already pending at the arrival of a message with
MSG_OOB set. These frames were created when TSO/GSO was enabled,
so they may be large, and they will have the urgent pointer set
in tcp_transmit_skb().

With this patch, such large packets will be fragmented again before
going to the transmit routine.

As a side note, at least the following NICs are known to screw up
the urgent pointer in the TCP header when doing TSO:

	Intel 82566MM (PCI ID 8086:1049)
	Intel 82566DC (PCI ID 8086:104b)
	Intel 82541GI (PCI ID 8086:1076)
	Broadcom NetXtreme II BCM5708 (PCI ID 14e4:164c)

Signed-off-by: Petr Tesarik <ptesarik@...e.cz>
CC: Jan Sembera <jsembera@...e.cz>
CC: Ilpo Jarvinen <ilpo.jarvinen@...sinki.fi>

--
 tcp_output.c |    7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -722,7 +722,8 @@ static void tcp_queue_skb(struct sock *sk, struct sk_buff 
*skb)
 static void tcp_set_skb_tso_segs(struct sock *sk, struct sk_buff *skb,
 				 unsigned int mss_now)
 {
-	if (skb->len <= mss_now || !sk_can_gso(sk)) {
+	if (skb->len <= mss_now || !sk_can_gso(sk) ||
+	    tcp_urg_mode(tcp_sk(sk))) {
 		/* Avoid the costly divide in the normal
 		 * non-TSO case.
 		 */
@@ -1163,7 +1164,9 @@ static int tcp_init_tso_segs(struct sock *sk, struct 
sk_buff *skb,
 {
 	int tso_segs = tcp_skb_pcount(skb);

-	if (!tso_segs || (tso_segs > 1 && tcp_skb_mss(skb) != mss_now)) {
+	if (!tso_segs ||
+	    (tso_segs > 1 && (tcp_skb_mss(skb) != mss_now ||
+			      tcp_urg_mode(tcp_sk(sk))))) {
 		tcp_set_skb_tso_segs(sk, skb, mss_now);
 		tso_segs = tcp_skb_pcount(skb);
 	}
--
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