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15.21.0428.015; Mon, 21 Sep 2026 10:46:25 +0000 Message-ID: <3e23a695-d3dc-4a77-88b7-afc68869853d@windriver.com> Date: Mon, 21 Sep 2026 18:46:16 +0800 User-Agent: Mozilla Thunderbird Beta Subject: Re: [PATCH net v4] net: erspan: set lltx to avoid sch_direct_xmit deadlock To: netdev-bot+sashiko@kernel.org Cc: dsahern@kernel.org, idosch@nvidia.com, davem@davemloft.net, edumazet@google.com, kuba@kernel.org, pabeni@redhat.com, horms@kernel.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org References: <20260916061314.936440-1-yun.zhou@windriver.com> <178988609227.2160803.11846126417836059113@kernel.org> Content-Language: en-US From: "Zhou, Yun" In-Reply-To: <178988609227.2160803.11846126417836059113@kernel.org> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-ClientProxiedBy: TYCPR01CA0201.jpnprd01.prod.outlook.com (2603:1096:405:7a::18) To CH0PR11MB5332.namprd11.prod.outlook.com (2603:10b6:610:bf::17) Precedence: bulk X-Mailing-List: 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engine=8.22.0-2609040000 definitions=main-2609210153 On 9/20/26 14:34, netdev-bot+sashiko@kernel.org wrote: > Thank you for your contribution! Sashiko AI review found 1 potential > issue(s) to consider. > > Critical: 0 · High: 0 · Medium: 1 · Low: 0 > > If you believe none of the issues are legitimate, please reply to each > issue of Medium severity or higher and explain why the AI is wrong. > If any of them are legitimate and you plan to respin, please let patchwork > know by including "pw-bot: cr" as a separate line at the end of your reply > (one such reply per series is enough). > > - [Medium] The patch enables `dev->lltx = true` for both IPv4 ERSPAN > (`erspan_tunnel_init()`, net/ipv4/ip_gre.c:1371) and IPv6 ERSPAN… > > -- > > net: erspan: set lltx to avoid sch_direct_xmit deadlock > > This sets dev->lltx = true for the IPv4 and IPv6 ERSPAN tunnel devices so > that HARD_TX_LOCK() skips the txq _xmit_lock, avoiding the nested > _xmit_lock acquisition when erspan_xmit() re-enters the stack through the > underlay device. The change is tagged for stable with Fixes tags pointing > at 84e54fe0a5ea and 5a963eb61b7c. > > Closes: https://syzkaller.appspot.com/bug?extid=9bda1b9fbb7fbdf9b62b > >> diff --git a/net/ipv4/ip_gre.c b/net/ipv4/ip_gre.c >> index 82309efd417e0..0058cb20e36a1 100644 >> --- a/net/ipv4/ip_gre.c >> +++ b/net/ipv4/ip_gre.c >> @@ -1367,6 +1367,8 @@ static int erspan_tunnel_init(struct net_device *dev) >> dev->features |= GRE_FEATURES; >> dev->hw_features |= GRE_FEATURES; >> dev->priv_flags |= IFF_LIVE_ADDR_CHANGE; >> + /* Skip TX lock: xmit re-enters stack, risking ABBA with underlay */ >> + dev->lltx = true; >> netif_keep_dst(dev); >> >> return ip_tunnel_init(dev); > > [Severity: Medium] > The commit message justifies the change with: > > "This is safe as erspan_xmit() has no shared mutable state: o_seqno is > atomic, TX stats are per-CPU u64_stats, dst_cache is per-CPU, and > o_flags is no longer modified in the xmit path since commit 9958e69b9893" > > That statement holds in this tree, but the patch also carries Cc: > stable@vger.kernel.org with Fixes tags on 84e54fe0a5ea and 5a963eb61b7c. > Commit 9958e69b9893 ("gre: fix ERSPAN o_flags race/corruption in xmit and > fill_info") has neither a Fixes tag nor Cc: stable, so it is not queued for > any stable tree. > > Both hunks here only touch the *_init() functions, so they apply cleanly to > any tree that has the dev->lltx bitfield (6.12 and later) even when > 9958e69b9893 is absent. In such a tree erspan_xmit() still does the > read-modify-write on the shared per-tunnel word: > > net/ipv4/ip_gre.c:erspan_xmit() (pre-9958e69b9893) { > ... > __clear_bit(IP_TUNNEL_SEQ_BIT, tunnel->parms.o_flags); > ... > __clear_bit(IP_TUNNEL_KEY_BIT, tunnel->parms.o_flags); > __gre_xmit(skb, dev, &tunnel->parms.iph, proto); > ... > } > > Once lltx is set, is anything left serializing two concurrent > erspan_xmit() calls on the same single-queue device, given that the txq > _xmit_lock was the only thing keeping those __clear_bit() sequences from > overlapping? The same question applies to the race against > erspan_fill_info(), which in those trees sets IP_TUNNEL_KEY_BIT back into > tunnel->parms.o_flags under RTNL only. > > If a lost update resurrects IP_TUNNEL_KEY_BIT or IP_TUNNEL_SEQ_BIT, can > gre_build_header() then emit optional GRE fields that tunnel->tun_hlen > never accounted for, i.e. exactly the header corruption described in > 9958e69b9893? > > Would it make sense to name 9958e69b9893 as an explicit stable > prerequisite in a note below the --- line, or to fold the equivalent > o_flags localization into this series, so stable maintainers do not apply > the lltx change on its own? > Okay, I will add the prerequisite in v5.