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

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Message-ID: <aAFVHqypw/snAOwu@pop-os.localdomain>
Date: Thu, 17 Apr 2025 12:23:10 -0700
From: Cong Wang <xiyou.wangcong@...il.com>
To: 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,
	pabeni@...hat.com, 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 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?

Thanks