Subject: Re: [PATCH] (3/3) netem: adjust parent qlen when duplicating

From: Patrick McHardy <[email-masked]>
Date: Wed, 04 May 2005 01:35:29 +0200

Stephen Hemminger wrote:
> Fix qlen underrun when doing duplication with netem. If netem is used as 
> leaf discipline, then the parent needs to be tweaked when packets  are duplicated.

> +			/* Since one packet can generate two packets in the
> +			 * queue, the parent's qlen accounting gets confused,
> +			 * so fix it.
> +			 */
> +			qp = qdisc_lookup(sch->dev, TC_H_MAJ(sch->parent));
> +			if (qp)
> +				qp->q.qlen++;

This only works in a hierarchy with just one qdisc above netem, there
could be up to seven (check_loop_fn prevents more than that). It's also
not safe because it violates qdisc locking rules, when this code is
executed dev->queue_lock is already taken and qdisc_lookup() grabs
qdisc_tree_lock, but they can only be taken in the other order.

Regards
Patrick