Re: [Patch v4 net 0/6] netem: Fix skb duplication logic and prevent infinite loops

[Stephen Hemminger <stephen@networkplumber.org>]

On Wed, 30 Jul 2025 12:17:12 -0700
Cong Wang <xiyou.wangcong@gmail.com> wrote:

> On Mon, Jul 21, 2025 at 10:00:30AM -0400, Jamal Hadi Salim wrote:
> > On Sat, Jul 19, 2025 at 6:04 PM Cong Wang <xiyou.wangcong@gmail.com> wrote:  
> > >
> > > This patchset fixes the infinite loops due to duplication in netem, the
> > > real root cause of this problem is enqueuing to the root qdisc, which is
> > > now changed to enqueuing to the same qdisc. This is more reasonable,
> > > more predictable from users' perspective, less error-proone and more elegant.
> > >
> > > Please see more details in patch 1/6 which contains two pages of detailed
> > > explanation including why it is safe and better.
> > >
> > > This replaces the patches from William, with much less code and without
> > > any workaround. More importantly, this does not break any use case.
> > >  
> > 
> > Cong, you are changing user expected behavior.
> > So instead of sending to the root qdisc, you are looping on the same
> > qdisc. I dont recall what the history is for the decision to go back
> > to the root qdisc - but one reason that sounds sensible is we want to
> > iterate through the tree hierarchy again. Stephen may remember.
> > The fact that the qfq issue is hit indicates the change has
> > consequences - and given the check a few lines above, more than likely
> > you are affecting the qlen by what you did.  
> 
> Please refer the changelog of patch 1/6, let me quote it here for you:
> 
>     The new netem duplication behavior does not break the documented
>     semantics of "creates a copy of the packet before queuing." The man page
>     description remains true since duplication occurs before the queuing
>     process, creating both original and duplicate packets that are then
>     enqueued. The documentation does not specify which qdisc should receive
>     the duplicates, only that copying happens before queuing. The implementation
>     choice to enqueue duplicates to the same qdisc (rather than root) is an
>     internal detail that maintains the documented behavior while preventing
>     infinite loops in hierarchical configurations.
> 
> I think it is reasonable to use man page as our agreement with users. I
> am open to other alternative agreements, if you have one. I hope using
> man page is not of my own preference here.
> 
> Thanks.

There is a chance that cases that mix duplication, corruption and drop
parameters would see different results after this patch.