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

[Zhou, Yun]

On 7/13/26 16:53, Ido Schimmel wrote:
> 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.

Thanks a lot for your review. IPv6 indeed has the same issue. I will fix 
it in v2 as you suggested.

BR,
Yun