[Cake] Re: [PATCH net v7 1/2] net/sched: sch_cake: Fix incorrect qlen reduction in cake_drop

[Cong Wang <xiyou.wangcong@gmail.com>]

On Wed, Nov 26, 2025 at 12:45:12PM -0700, Xiang Mei wrote:
> 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>

Acked-by: Cong Wang <cwang@multikernel.io>

Thanks for your patience!