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15.21.0223.015; Sun, 19 Jul 2026 12:40:57 +0000 Message-ID: <91ea5c5c-5476-4420-b852-45c6677d6134@windriver.com> Date: Sun, 19 Jul 2026 20:40:49 +0800 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH net v2] net: erspan: set lltx to avoid sch_direct_xmit deadlock To: idosch@nvidia.com Cc: netdev@vger.kernel.org, linux-kernel@vger.kernel.org, dsahern@kernel.org, davem@davemloft.net, edumazet@google.com, kuba@kernel.org, pabeni@redhat.com, horms@kernel.org, yun.zhou@windriver.com References: <20260713151435.1815104-1-yun.zhou@windriver.com> Content-Language: en-US From: "Zhou, Yun" In-Reply-To: <20260713151435.1815104-1-yun.zhou@windriver.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-ClientProxiedBy: TYCP301CA0089.JPNP301.PROD.OUTLOOK.COM (2603:1096:405:7b::11) To CH0PR11MB5332.namprd11.prod.outlook.com (2603:10b6:610:bf::17) Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 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definitions=main-2607190138 Hi Ido, Could you please help review this patch when you have a moment? Thanks, Yun On 7/13/2026 11:14 PM, Yun Zhou wrote: > erspan_xmit() re-enters the network stack via ip_tunnel_xmit(), causing > nested acquisition of _xmit_lock on the underlay device while already > holding the ERSPAN device's _xmit_lock. Both are ARPHRD_ETHER and share > the same lockdep class, creating an ABBA deadlock: > > sch_direct_xmit [lock erspan] -> erspan_xmit -> ip_tunnel_xmit -> > ip_output -> __dev_queue_xmit -> sch_direct_xmit [lock underlay] > > Set dev->lltx = true so HARD_TX_LOCK() skips the spinlock for ERSPAN. > This is safe as erspan_xmit() has no shared mutable state: o_seqno is > atomic, stats use atomic_long_inc, and dst_cache is per-CPU. GRETAP, > the sibling device with identical xmit structure, already sets lltx. > > Closes: https://syzkaller.appspot.com/bug?extid=9bda1b9fbb7fbdf9b62b > Reported-by: syzbot+9bda1b9fbb7fbdf9b62b@syzkaller.appspotmail.com > Fixes: 84e54fe0a5ea ("gre: introduce native tunnel support for ERSPAN") > Signed-off-by: Yun Zhou > --- > v2: > - change subject prefix to [PATCH net] > > net/ipv4/ip_gre.c | 2 ++ > 1 file changed, 2 insertions(+) > > diff --git a/net/ipv4/ip_gre.c b/net/ipv4/ip_gre.c > index 3efdfb4ffa21..9fbff16cda1d 100644 > --- a/net/ipv4/ip_gre.c > +++ b/net/ipv4/ip_gre.c > @@ -1363,6 +1363,8 @@ 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);