netdev - Re: [PATCH net-next v11 08/12] virtio_net: Use existing classifier if possible

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Message-ID: <20251119013423-mutt-send-email-mst@kernel.org>
Date: Wed, 19 Nov 2025 01:35:21 -0500
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Dan Jurgens <danielj@...dia.com>
Cc: netdev@...r.kernel.org, jasowang@...hat.com, pabeni@...hat.com,
	virtualization@...ts.linux.dev, parav@...dia.com,
	shshitrit@...dia.com, yohadt@...dia.com, xuanzhuo@...ux.alibaba.com,
	eperezma@...hat.com, jgg@...pe.ca, kevin.tian@...el.com,
	kuba@...nel.org, andrew+netdev@...n.ch, edumazet@...gle.com
Subject: Re: [PATCH net-next v11 08/12] virtio_net: Use existing classifier
 if possible

On Wed, Nov 19, 2025 at 12:26:23AM -0600, Dan Jurgens wrote:
> On 11/18/25 3:55 PM, Michael S. Tsirkin wrote:
> > On Tue, Nov 18, 2025 at 08:38:58AM -0600, Daniel Jurgens wrote:
> >> Classifiers can be used by more than one rule. If there is an existing
> >> classifier, use it instead of creating a new one.
> 
> >> +	struct virtnet_classifier *tmp;
> >> +	unsigned long i;
> >>  	int err;
> >>  
> >> -	err = xa_alloc(&ff->classifiers, &c->id, c,
> >> +	xa_for_each(&ff->classifiers, i, tmp) {
> >> +		if ((*c)->size == tmp->size &&
> >> +		    !memcmp(&tmp->classifier, &(*c)->classifier, tmp->size)) {
> > 
> > note that classifier has padding bytes.
> > comparing these with memcmp is not safe, is it?
> 
> The reserved bytes are set to 0, this is fine.

I mean the compiler padding.  set to 0 where?

> > 
> > 
> >> +			refcount_inc(&tmp->refcount);
> >> +			kfree(*c);
> >> +			*c = tmp;
> >> +			goto out;
> >> +		}
> >> +	}
> >> +
> >> +	err = xa_alloc(&ff->classifiers, &(*c)->id, *c,
> >>  		       XA_LIMIT(0, le32_to_cpu(ff->ff_caps->classifiers_limit) - 1),
> >>  		       GFP_KERNEL);
> >>  	if (err)
> > 
> > what kind of locking prevents two threads racing in this code?
> 
> The ethtool calls happen under rtnl_lock.
> 
> > 
> > 
> >> @@ -6932,29 +6945,30 @@ static int setup_classifier(struct virtnet_ff *ff, struct virtnet_classifier *c)
> >>  		      (*c)->size);
> >>  	if (err)
> >>  		goto err_xarray;
> >>  
> >> +	refcount_set(&(*c)->refcount, 1);
> > 
> > 
> > so you insert uninitialized refcount? can't another thread find it
> > meanwhile?
> 
> Again, rtnl_lock.
> 
> 
> >>  
> >>  	err = insert_rule(ff, eth_rule, c->id, key, key_size);
> >>  	if (err) {
> >>  		/* destroy_classifier will free the classifier */
> > 
> > will free is no longer correct, is it?
> 
> Clarified the comment.
> 
> > 
> >> -		destroy_classifier(ff, c->id);
> >> +		try_destroy_classifier(ff, c->id);
> >>  		goto err_key;
> >>  	}
> >>  
> >> -- 
> >> 2.50.1
> >