Re: [PATCH] gtp: disable BH before calling udp_tunnel_xmit_skb()

Justin Iurman
2026-04-21T17:59:01Z

On 4/20/26 21:58, Jakub Kicinski wrote:
> On Mon, 20 Apr 2026 21:02:55 +0200 Justin Iurman wrote:
>> On 4/17/26 07:54, David Carlier wrote:
>>> gtp_genl_send_echo_req() runs as a generic netlink doit handler in
>>> process context with BH not disabled. It calls udp_tunnel_xmit_skb(),
>>> which eventually invokes iptunnel_xmit() — that uses __this_cpu_inc/dec
>>> on softnet_data.xmit.recursion to track the tunnel xmit recursion level.
>>>
>>> Without local_bh_disable(), the task may migrate between
>>> dev_xmit_recursion_inc() and dev_xmit_recursion_dec(), breaking the
>>> per-CPU counter pairing. The result is stale or negative recursion
>>> levels that can later produce false-positive
>>> SKB_DROP_REASON_RECURSION_LIMIT drops on either CPU.
>>>
>>> The other udp_tunnel_xmit_skb() call sites in gtp.c are unaffected:
>>> the data path runs under ndo_start_xmit and the echo response handlers
>>> run from the UDP encap rx softirq, both with BH already disabled.
>>>
>>> Fix it by disabling BH around the udp_tunnel_xmit_skb() call, mirroring
>>> commit 2cd7e6971fc2 ("sctp: disable BH before calling
>>> udp_tunnel_xmit_skb()").
>>
>> Why not fix iptunnel_xmit() directly, rather than fixing all possible
>> callers? Basically, jut like we did for lwtunnel_{output|xmit}(). The
>> advantage would be that we no longer have to worry about BHs in the
>> callers, and BHs would only be disabled when necessary.
>
> Oops, I pushed this already. The bot hasn't caught up yet.
> Let's revisit this if we find another caller in process context?

No worries, works for me!