netdev - Re: [PATCH net v2 0/5] net_sched: Adapt qdiscs for reentrant enqueue cases

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Message-ID: <4295ec79-035c-4858-9ec4-eb639767d12b@redhat.com>
Date: Tue, 22 Apr 2025 13:21:22 +0200
From: Paolo Abeni <pabeni@...hat.com>
To: Cong Wang <xiyou.wangcong@...il.com>,
 Victor Nogueira <victor@...atatu.com>
Cc: netdev@...r.kernel.org, jhs@...atatu.com, jiri@...nulli.us,
 davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org, toke@...hat.com,
 gerrard.tai@...rlabs.sg, pctammela@...atatu.com
Subject: Re: [PATCH net v2 0/5] net_sched: Adapt qdiscs for reentrant enqueue
 cases

On 4/17/25 9:23 PM, Cong Wang wrote:
> On Wed, Apr 16, 2025 at 07:24:22AM -0300, Victor Nogueira wrote:
>> As described in Gerrard's report [1], there are cases where netem can
>> make the qdisc enqueue callback reentrant. Some qdiscs (drr, hfsc, ets,
>> qfq) break whenever the enqueue callback has reentrant behaviour.
>> This series addresses these issues by adding extra checks that cater for
>> these reentrant corner cases. This series has passed all relevant test
>> cases in the TDC suite.
>>
>> [1] https://lore.kernel.org/netdev/CAHcdcOm+03OD2j6R0=YHKqmy=VgJ8xEOKuP6c7mSgnp-TEJJbw@mail.gmail.com/
>>
> 
> I am wondering why we need to enqueue the duplicate skb before enqueuing
> the original skb in netem? IOW, why not just swap them?

It's not clear to me what you are suggesting, could you please rephrase
and/or expand the above?

When duplication packets, I think we will need to call root->enqueue()
no matter what, to ensure proper accounting, and that would cause the
re-entrancy issue. What I'm missing?

Thanks,

Paolo