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

[Ido Schimmel]

Patch prefix should be [PATCH net]:

https://docs.kernel.org/process/maintainer-netdev.html

On Thu, Jul 09, 2026 at 06:52:43PM +0800, Yun Zhou wrote:
> 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")

Both sashiko instances indicate that this commit introduced the same bug
in ip6gre_tnl_init_features(). Please perform the same fix there and
fold it into v2.

> Signed-off-by: Yun Zhou <yun.zhou@xxxxxxxxxxxxx>
> ---
>  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
>