Subject: Re: [PATCH net v2] sch_htb: Avoid grafting on htb_destroy_class_offload when destroying htb

From: Rahul Rameshbabu <[email-masked]>
Date: Thu, 12 Jan 2023 14:10:01 -0800

Maxim Mikityanskiy <[email-masked]> writes:

> On Wed, Jan 11, 2023 at 12:37:33PM -0800, Rahul Rameshbabu wrote:
>> 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.
>> Peek at old qdisc and graft only when deleting a leaf class in the htb,
>> rather than when deleting the htb itself.
>> 
>> This fix resolves two use cases.
>> 
>>   1. Using tc to destroy the htb.
>>   2. Using tc to replace the htb with another qdisc (which also leads to
>>      the htb being destroyed).
>
> Please elaborate in the commit message what exactly was broken in these
> cases, i.e. premature dev_activate in both cases, and also accidental
> overwriting of the qdisc in case 2.

Ack.

>
>> 
>> 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
>
> Did you mean "when calling htb_delete"?
>
> Worth also commenting that on destroying, graft is done by qdisc_graft,
> and the latter also qdisc_puts the old one. Just to explain why we skip
> steps on destroying.
>

Yes, I did mean htb_delete. Ack.

>>  		 */
>> -		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);
>
> Not sure whether we want to WARN on this error...
>
> On destroying, we call htb_offload with TC_HTB_LEAF_DEL_LAST_FORCE,
> which makes the mlx5e driver proceed with deleting the node even if it
> failed to create a replacement node. Normally it cancels the deletion to
> keep the integrity of hardware structures, but on htb_destroy it doesn't
> matter, because everything is going to be torn down anyway. An error is
> still returned by the driver, but it's safe to ignore it, not worth a
> WARN at all.

I see. This makes sense to me.

>
> Another error flow, when the firmware command to delete a node fails for
> some reason, doesn't even lead to returning an error, because the worst
> that happens is a leak of hardware resources, and we can't do anything
> meaningful about it at that stage.

This was what I was trying to catch with the WARN_ON, with the hope that
at worst it wouldn't have any false positives. However, if there are
errors due to certain operation modes like TC_HTB_LEAF_DEL_LAST_FORCE
where the htb is still destroyed, this WARN_ON seems to be problematic
more than helpful. Will remove in my next revision.

>
> So, I don't think this WARN_ON is helpful, unless you also want to
> change the way mlx5e returns errors.
>
>> +	if (!destroying) {
>> +		if (!err)
>> +			qdisc_put(old);
>> +		else
>> +			htb_graft_helper(dev_queue, old);
>> +	}
>
> Looks good. I also suggest removing NULL-initialization of old to make
> sure one will get a compiler warning about an uninitialized variable if
> one changes the code in the future and accidentally uses old in the
> destroying flow.

Ack.

>
>>  
>>  	if (last_child)
>>  		return err;
>> -- 
>> 2.36.2
>> 
>> Previous related discussions
>> 
>> [1] https://lore.kernel.org/netdev/[email-masked]/
>> [2] https://lore.kernel.org/netdev/[email-masked]/
>> [3] https://lore.kernel.org/all/[email-masked]/