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

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

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