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

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Message-ID: <20250417090728.5325e724@kernel.org>
Date: Thu, 17 Apr 2025 09:07:28 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: Victor Nogueira <victor@...atatu.com>
Cc: netdev@...r.kernel.org, jhs@...atatu.com, xiyou.wangcong@...il.com,
 jiri@...nulli.us, davem@...emloft.net, edumazet@...gle.com,
 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, 16 Apr 2025 07:24:22 -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.

Sorry for asking this question a bit late, but reentrant enqueue seems
error prone. Is there a clear use case for netem as a child?
If so should we also add some sort of "capability" to avoid new qdiscs
falling into the same trap, without giving the problem any thought?