[Cake] Re: [PATCH net v8 1/2] net/sched: sch_cake: Fix incorrect qlen reduction in cake_drop

[Jakub Kicinski <kuba@kernel.org>]

On Fri, 28 Nov 2025 10:15:53 +0100 Toke Høiland-Jørgensen wrote:
> Xiang Mei <xmei5@asu.edu> writes:
> 
> > In cake_drop(), qdisc_tree_reduce_backlog() is used to update the qlen
> > and backlog of the qdisc hierarchy. Its caller, cake_enqueue(), assumes
> > that the parent qdisc will enqueue the current packet. However, this
> > assumption breaks when cake_enqueue() returns NET_XMIT_CN: the parent
> > qdisc stops enqueuing current packet, leaving the tree qlen/backlog
> > accounting inconsistent. This mismatch can lead to a NULL dereference
> > (e.g., when the parent Qdisc is qfq_qdisc).
> >
> > This patch computes the qlen/backlog delta in a more robust way by
> > observing the difference before and after the series of cake_drop()
> > calls, and then compensates the qdisc tree accounting if cake_enqueue()
> > returns NET_XMIT_CN.
> >
> > To ensure correct compensation when ACK thinning is enabled, a new
> > variable is introduced to keep qlen unchanged.
> >
> > Fixes: 15de71d06a40 ("net/sched: Make cake_enqueue return NET_XMIT_CN when past buffer_limit")
> > Signed-off-by: Xiang Mei <xmei5@asu.edu>  
> 
> Please retain tags when reposting...
> 
> Reviewed-by: Toke Høiland-Jørgensen <toke@toke.dk>

AI code review asks:

When ACK thinning occurs, the incoming packet contributes len -
ack_pkt_len bytes to sch->qstats.backlog, but this compensation uses
the full len value. Should this be prev_backlog - (len - ack_pkt_len)
to match what was actually added to the backlog?