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Sun, 20 Sep 2026 02:48:51 +0000 Message-ID: Date: Sun, 20 Sep 2026 10:48:42 +0800 User-Agent: Mozilla Thunderbird Beta Subject: Re: [PATCH net v4] net: erspan: set lltx to avoid sch_direct_xmit deadlock To: Ido Schimmel Cc: dsahern@kernel.org, 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> <20260916082028.GA879236@shredder> Content-Language: en-US From: "Zhou, Yun" In-Reply-To: <20260916082028.GA879236@shredder> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-ClientProxiedBy: SI1PR02CA0002.apcprd02.prod.outlook.com (2603:1096:4:1f7::16) To CH0PR11MB5332.namprd11.prod.outlook.com (2603:10b6:610:bf::17) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: 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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, 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, 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 >> ("gre: fix ERSPAN o_flags race/corruption in xmit and fill_info"). >> GRETAP, the sibling device with identical xmit structure, already sets >> lltx. > > In v3 I asked that the commit message: > > 1. State that the overlay and underlay devices should be of the same > type (both erspan or both ip6erspan) for the splat to happen. > The devices do not need to be the same type. The actual syzbot report [1] is between IPv4 erspan and vti6 - two completely different tunnel types: -> #1: erspan_xmit (net/ipv4/ip_gre.c) holding &qdisc_xmit_lock_key#2 -> #0: vti6_tnl_xmit (net/ipv6/ip6_vti.c) holding &qdisc_xmit_lock_key#3 I can reproduce this reliably with erspan <-> vti6 cross-routed in a netns (no same-type stacking involved). Additionally, I think it makes sense to set lltx=true for vti6 as well. [1] https://syzkaller.appspot.com/text?tag=CrashReport&x=15b29287180000 Thanks, Yun