Re: [PATCH net v2] net: erspan: set lltx to avoid sch_direct_xmit deadlock

[Zhou, Yun]

On 7/20/26 15:53, Ido Schimmel wrote:
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> On Mon, Jul 13, 2026 at 11:14:35PM +0800, 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.
>
> erspan_xmit() (unlike gre_tap_xmit()) is performing non-atomic
> __clear_bit() on shared tunnel flags and KCSAN will probably flag it.
>
> Eric had a patch [1] that changes erspan_xmit() to use a private copy of
> these flags. I think it's better to wait for Eric's patch to be merged
> before setting lltx.
>
> Eric, can you please submit v2 of your patch to net?
>
> Also, doesn't ip6erspan suffer from the same problem? Please try to
> reproduce and fix.

The following command sequence can reproduce the problem:

ip link set lo up

ip addr add fd00:a::1/128 dev lo
ip addr add fd00:b::1/128 dev lo

ip link add ip6erspan_dev type ip6erspan \
    local fd00:a::1 remote fd00:a::2 \
    seq key 1 erspan_ver 1 erspan 1
ip link set ip6erspan_dev up

ip link add ip6gretap_dev type ip6gretap \
    local fd00:b::1 remote fd00:b::2 seq
ip link set ip6gretap_dev up

ip -6 route add fd00:a::2/128 dev ip6gretap_dev
ip -6 route add fd00:b::2/128 dev ip6erspan_dev

ip -6 neigh add fd00:a::2 lladdr 00:11:22:33:44:55 dev ip6gretap_dev
ip -6 neigh add fd00:b::2 lladdr 00:11:22:33:44:66 dev ip6erspan_dev