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

["Zhou, Yun" <yun.zhou@windriver.com>]

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.
> 

Yes, ip6erspan has the same problem. I will fix it in v3.

Thanks,
Yun