Re: [PATCH net repost 1/2] net/sched: codel: bound the dropping loop per dequeue call

["Toke Høiland-Jørgensen" <toke@toke.dk>]

Jamal Hadi Salim <jhs@mojatatu.com> writes:

> The CoDel control law schedules the next drop one interval/sqrt(count)
> after the previous drop, using the configured interval
> (codel_params.interval). For very small intervals the scheduled step
> rounds down to zero, so the dropping loop in codel_dequeue() never
> advances and drains the entire backlog under the qdisc lock in one
> call - an unprivileged user can trigger a soft lockup this way.
>
> Fix in the shared codel code used by both codel and fq_codel:
>
> 1. Make the control-law step at least 1 tick so the dropping loop
>    always moves forward.
>
> 2. Cap the dropping loop at CODEL_MAX_DROPS_PER_DEQUEUE (256) drops
>    per codel_dequeue() call, resyncing drop_next to now when the cap
>    is hit: the catch-up owed to the loop grows with the idle gap and
>    the backlog, which no interval threshold can bound. This is a
>    deliberate behaviour change after long idle gaps.

Both of these seem reasonable!

Reviewed-by: Toke Høiland-Jørgensen <toke@toke.dk>