Subject: Re: [PATCH net] sch_htb: Avoid grafting on htb_destroy_class_offload when destroying htb

From: Rahul Rameshbabu <[email-masked]>
Date: Wed, 11 Jan 2023 11:56:52 -0800

Jiri Pirko <[email-masked]> writes:

> Tue, Jan 10, 2023 at 09:20:04PM CET, [email-masked] wrote:
>>Peek at old qdisc and graft only when deleting leaf class in the htb. When
>>destroying the htb, the caller may already have grafted a new qdisc that is
>>not part of the htb structure being destroyed. htb_destroy_class_offload
>>should not peek at the qdisc of the netdev queue since that will either be
>
> You are not telling the codebase what to do. Do it in order to make it
> obvious what the patch is doing. That makes the patch description easier
> to understand.
>

I do think this description does describe what the patch is trying to
logically achieve rather than what the codebase itself is doing. The
goal of this patch is to correct htb_destroy_class_offload, so that it
will not attempt to graft a qdisc that does not belong to the htb (the
new qdisc), when destroying the htb entirely (the htb is destroyed when
replaced with another qdisc or explicitly destroyed). When deleting a
leaf, it makes sense to graft the leaf back if it failed to be deleted
by the device.

>
>>the new qdisc in the case of replacing the htb or simply a noop_qdisc is
>>the case of destroying the htb without a replacement qdisc.
>
>
> Looks to me like 2 fixes, shouldn't this be 2 patches instead?

This change is one logical fix in terms of refactoring the code to only
peek and graft the old qdisc when deleting a leaf of the htb. It
resolves issues in two use cases though.

  tc qdisc delete dev eth2 root handle 1: htb default 1 # deleting the htb

  tc qdisc replace dev eth2 root pfifo # replace htb with another qdisc

>
>
>>
>>Fixes: d03b195b5aa0 ("sch_htb: Hierarchical QoS hardware offload")
>>Signed-off-by: Rahul Rameshbabu <[email-masked]>
>>Cc: Eric Dumazet <[email-masked]>
>>Cc: Maxim Mikityanskiy <[email-masked]>
>>---
>> net/sched/sch_htb.c | 23 +++++++++++++----------
>> 1 file changed, 13 insertions(+), 10 deletions(-)
>>
>>diff --git a/net/sched/sch_htb.c b/net/sched/sch_htb.c
>>index 2238edece1a4..360ce8616fd2 100644
>>--- a/net/sched/sch_htb.c
>>+++ b/net/sched/sch_htb.c
>>@@ -1557,14 +1557,13 @@ static int htb_destroy_class_offload(struct Qdisc *sch, struct htb_class *cl,
>> 
>> 	WARN_ON(!q);
>> 	dev_queue = htb_offload_get_queue(cl);
>>-	old = htb_graft_helper(dev_queue, NULL);
>>-	if (destroying)
>>-		/* Before HTB is destroyed, the kernel grafts noop_qdisc to
>>-		 * all queues.
>>+	if (!destroying) {
>>+		old = htb_graft_helper(dev_queue, NULL);
>>+		/* Last qdisc grafted should be the same as cl->leaf.q when
>>+		 * calling htb_destroy
>> 		 */
>>-		WARN_ON(!(old->flags & TCQ_F_BUILTIN));
>>-	else
>> 		WARN_ON(old != q);
>>+	}
>> 
>> 	if (cl->parent) {
>> 		_bstats_update(&cl->parent->bstats_bias,
>>@@ -1581,10 +1580,14 @@ static int htb_destroy_class_offload(struct Qdisc *sch, struct htb_class *cl,
>> 	};
>> 	err = htb_offload(qdisc_dev(sch), &offload_opt);
>> 
>>-	if (!err || destroying)
>>-		qdisc_put(old);
>>-	else
>>-		htb_graft_helper(dev_queue, old);
>>+	/* htb_offload related errors when destroying cannot be handled */
>>+	WARN_ON(err && destroying);
>>+	if (!destroying) {
>>+		if (!err)
>>+			qdisc_put(old);
>>+		else
>>+			htb_graft_helper(dev_queue, old);
>>+	}
>> 
>> 	if (last_child)
>> 		return err;
>>-- 
>>2.36.2
>>