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([2a02:a03f:a75e:9a00:98a0:d751:7986:3f62]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-488fc177dafsm378108875e9.4.2026.04.20.12.02.56 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 20 Apr 2026 12:02:56 -0700 (PDT) Message-ID: Date: Mon, 20 Apr 2026 21:02:55 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] gtp: disable BH before calling udp_tunnel_xmit_skb() To: David Carlier , Pablo Neira Ayuso , Harald Welte , Andrew Lunn , Eric Dumazet , Jakub Kicinski , Paolo Abeni Cc: Weiming Shi , osmocom-net-gprs@lists.osmocom.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org References: <20260417055408.4667-1-devnexen@gmail.com> Content-Language: en-US From: Justin Iurman In-Reply-To: <20260417055408.4667-1-devnexen@gmail.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit 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.