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

[Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>]

On Sun, Apr 12, 2026 at 02:15:27PM -0400, Xin Long wrote:
> udp_tunnel_xmit_skb() / udp_tunnel6_xmit_skb() are expected to run with
> BH disabled.  After commit 6f1a9140ecda ("add xmit recursion limit to
> tunnel xmit functions"), on the path:
> 
>   udp(6)_tunnel_xmit_skb() -> ip(6)tunnel_xmit()
> 
> dev_xmit_recursion_inc()/dec() must stay balanced on the same CPU.
> 
> Without local_bh_disable(), the context may move between CPUs, which can
> break the inc/dec pairing. This may lead to incorrect recursion level
> detection and cause packets to be dropped in ip(6)_tunnel_xmit() or
> __dev_queue_xmit().
> 
> Fix it by disabling BH around both IPv4 and IPv6 SCTP UDP xmit paths.
> 
> In my testing, after enabling the SCTP over UDP:
> 
>   # ip net exec ha sysctl -w net.sctp.udp_port=9899
>   # ip net exec ha sysctl -w net.sctp.encap_port=9899
>   # ip net exec hb sysctl -w net.sctp.udp_port=9899
>   # ip net exec hb sysctl -w net.sctp.encap_port=9899
> 
>   # ip net exec ha iperf3 -s
> 
> - without this patch:
> 
>   # ip net exec hb iperf3 -c 192.168.0.1 --sctp
>   [  5]   0.00-10.00  sec  37.2 MBytes  31.2 Mbits/sec  sender
>   [  5]   0.00-10.00  sec  37.1 MBytes  31.1 Mbits/sec  receiver
> 
> - with this patch:
> 
>   # ip net exec hb iperf3 -c 192.168.0.1 --sctp
>   [  5]   0.00-10.00  sec  3.14 GBytes  2.69 Gbits/sec  sender
>   [  5]   0.00-10.00  sec  3.14 GBytes  2.69 Gbits/sec  receiver
> 
> Fixes: 6f1a9140ecda ("add xmit recursion limit to tunnel xmit functions")
> Fixes: 046c052b475e ("sctp: enable udp tunneling socks")
> Signed-off-by: Xin Long <lucien.xin@gmail.com>

Nice catch!

Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>