Re: [PATCH net v2] net: gre: fix lltx regression for GRE tunnels with SEQ/CSUM

[Ido Schimmel]

On Mon, Jul 13, 2026 at 11:09:45PM +0800, Yun Zhou wrote:
> Before commit 00d066a4d4ed ("netdev_features: convert NETIF_F_LLTX to
> dev->lltx"), NETIF_F_LLTX was set unconditionally in both
> __gre_tunnel_init() and ip6gre_tnl_init_features() 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/ip6gre 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 in both
> functions, restoring the original unconditional behavior.
> 
> Fixes: 00d066a4d4ed ("netdev_features: convert NETIF_F_LLTX to dev->lltx")
> Signed-off-by: Yun Zhou <yun.zhou@xxxxxxxxxxxxx>

Reviewed-by: Ido Schimmel <idosch@xxxxxxxxxx>