Subject: Re: [RFC] tcf_bind_filter failure handling

From: Thomas Graf <[email-masked]>
Date: Mon, 13 Dec 2004 19:52:03 +0100

* Patrick McHardy <[email-masked]> 2004-12-13 19:11
> Thomas Graf wrote:
> 
> >The handling of a failure in tcf_bind_filter is inconsistent.
> >
> >u32: ignore
> >fw: ignore
> >route: ignore
> >rsvp: ignore
> >tcindex: error
> >
> >It might be a good idea to make this consistent. So in order to validate
> >the classid before making any changes we could simply lock it via get
> >(see patch below), return an error if it fails  and put it back in case
> >of an error further in the path or after binding the filter.
> >
> >Bindings not only locks the class from removal while a filter is
> >pointing to it. It speeds up classyfing by saving a lookup for every
> >tc_classify call. It's not really a problem if the class is not locked,
> >the qdisc will look it up and falls back to a default class if it
> >doesn't exists so it's rather a cosmetic/policy thing.
> >
> You should just fix tcindex not to care about errors in tcf_bind_filter.
> bind_tcf already locks the class. Some qdiscs (like prio) map bind_filter
> to get, but others (HTB, HFSC, CBQ) use a seperate counter because it is
> legal to end up with a refcnt > 0 after delete. When a class with filters
> pointing to it is tried to destroy they return -EBUSY, which can't be done
> by looking at the refcnt.

Little misunderstanding here. I'm not aiming at replacing tcf_bind_filter
with get.  My question is rather whether to regard tcf_bind_filter not setting
tcf_result->class as an error or ignore it. 

I'm all for ignoring it in tcindex, it requires some changes because
it checks tcf_result.class field to see if hash bucket is non-empty if
perfect hash is used but is not a problem at all.

The tcf_class_get/put would be required to ensure proper locking during
validation of parameters if validating the classid being last before
changing things doesn't make sense due to the need to undo expensive
operations required before binding.

I will fix tcindex, since you also agree on simply ignoring it and regard
the binding as an ptional locking and performance increase possibility
given to userspace.