[PATCH] net: gre: fix lltx regression for GRE tunnels with SEQ/CSUM

[Yun Zhou <yun.zhou@windriver.com>]

Before commit 00d066a4d4ed ("netdev_features: convert NETIF_F_LLTX to
dev->lltx"), NETIF_F_LLTX was set unconditionally in __gre_tunnel_init()
alongside GRE_FEATURES:

    dev->features |= GRE_FEATURES | NETIF_F_LLTX;

When that commit converted NETIF_F_LLTX to the dev->lltx flag, it
placed 'dev->lltx = true' after the SEQ/CSUM early returns instead
of before them. This causes GRE/GRETAP tunnels with SEQ or CSUM+encap
to lose lockless TX, reintroducing _xmit_lock acquisition around their
ndo_start_xmit. Since GRE xmit re-enters the stack via
ip_tunnel_xmit(), holding _xmit_lock risks ABBA deadlock with the
underlay device.

  CPU0                        CPU1
  ----                        ----
  lock(&qdisc_xmit_lock_key#6);
                              lock(&qdisc_xmit_lock_key#3);
                              lock(&qdisc_xmit_lock_key#6);
  lock(&qdisc_xmit_lock_key#3);

Fix by moving dev->lltx = true before the early returns, restoring
the original unconditional behavior.

Fixes: 00d066a4d4ed ("netdev_features: convert NETIF_F_LLTX to dev->lltx")
Signed-off-by: Yun Zhou <yun.zhou@windriver.com>
---
 net/ipv4/ip_gre.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/ip_gre.c b/net/ipv4/ip_gre.c
index 9fbff16cda1d..8c5ad8ec8d09 100644
--- a/net/ipv4/ip_gre.c
+++ b/net/ipv4/ip_gre.c
@@ -1018,6 +1018,8 @@ static void __gre_tunnel_init(struct net_device *dev)
 	dev->features		|= GRE_FEATURES;
 	dev->hw_features	|= GRE_FEATURES;
 
+	dev->lltx = true;
+
 	/* TCP offload with GRE SEQ is not supported, nor can we support 2
 	 * levels of outer headers requiring an update.
 	 */
@@ -1029,8 +1031,6 @@ static void __gre_tunnel_init(struct net_device *dev)
 
 	dev->features |= NETIF_F_GSO_SOFTWARE;
 	dev->hw_features |= NETIF_F_GSO_SOFTWARE;
-
-	dev->lltx = true;
 }
 
 static int ipgre_tunnel_init(struct net_device *dev)
-- 
2.43.0