Subject: Re: [PATCH net-next 02/19] netfilter: nft_set_rbtree: prefer sync gc to async worker

From: Simon Horman <[email-masked]>
Date: Fri, 3 Nov 2023 17:34:42 +0000

On Wed, Oct 25, 2023 at 11:25:38PM +0200, Pablo Neira Ayuso wrote:
> From: Florian Westphal <[email-masked]>
> 
> There is no need for asynchronous garbage collection, rbtree inserts
> can only happen from the netlink control plane.
> 
> We already perform on-demand gc on insertion, in the area of the
> tree where the insertion takes place, but we don't do a full tree
> walk there for performance reasons.
> 
> Do a full gc walk at the end of the transaction instead and
> remove the async worker.
> 
> Signed-off-by: Florian Westphal <[email-masked]>
> Signed-off-by: Pablo Neira Ayuso <[email-masked]>

...

> @@ -515,11 +523,7 @@ static void nft_rbtree_remove(const struct net *net,
>  	struct nft_rbtree *priv = nft_set_priv(set);
>  	struct nft_rbtree_elem *rbe = elem->priv;
>  
> -	write_lock_bh(&priv->lock);
> -	write_seqcount_begin(&priv->count);
> -	rb_erase(&rbe->node, &priv->root);
> -	write_seqcount_end(&priv->count);
> -	write_unlock_bh(&priv->lock);
> +	nft_rbtree_erase(priv, rbe);
>  }
>  
>  static void nft_rbtree_activate(const struct net *net,
> @@ -613,45 +617,40 @@ static void nft_rbtree_walk(const struct nft_ctx *ctx,
>  	read_unlock_bh(&priv->lock);
>  }
>  
> -static void nft_rbtree_gc(struct work_struct *work)
> +static void nft_rbtree_gc_remove(struct net *net, struct nft_set *set,
> +				 struct nft_rbtree *priv,
> +				 struct nft_rbtree_elem *rbe)
>  {
> +	struct nft_set_elem elem = {
> +		.priv	= rbe,
> +	};
> +
> +	nft_setelem_data_deactivate(net, set, &elem);
> +	nft_rbtree_erase(priv, rbe);
> +}
> +
> +static void nft_rbtree_gc(struct nft_set *set)
> +{
> +	struct nft_rbtree *priv = nft_set_priv(set);
>  	struct nft_rbtree_elem *rbe, *rbe_end = NULL;
>  	struct nftables_pernet *nft_net;

Hi Florian and Pablo,

I understand that this patch has been accepted upstream,
and that by implication this feedback is rather slow,
but I noticed that with this patch nft_net is now
set but otherwise unused in this function.

As flagged by clang-16 and gcc-13 W=1 builds.

> -	struct nft_rbtree *priv;
> +	struct rb_node *node, *next;
>  	struct nft_trans_gc *gc;
> -	struct rb_node *node;
> -	struct nft_set *set;
> -	unsigned int gc_seq;
>  	struct net *net;
>  
> -	priv = container_of(work, struct nft_rbtree, gc_work.work);
>  	set  = nft_set_container_of(priv);
>  	net  = read_pnet(&set->net);
>  	nft_net = nft_pernet(net);
> -	gc_seq  = READ_ONCE(nft_net->gc_seq);
>  
> -	if (nft_set_gc_is_pending(set))
> -		goto done;
> -
> -	gc = nft_trans_gc_alloc(set, gc_seq, GFP_KERNEL);
> +	gc = nft_trans_gc_alloc(set, 0, GFP_KERNEL);
>  	if (!gc)
> -		goto done;
> -
> -	read_lock_bh(&priv->lock);
> -	for (node = rb_first(&priv->root); node != NULL; node = rb_next(node)) {
> +		return;
>  
> -		/* Ruleset has been updated, try later. */
> -		if (READ_ONCE(nft_net->gc_seq) != gc_seq) {
> -			nft_trans_gc_destroy(gc);
> -			gc = NULL;
> -			goto try_later;
> -		}
> +	for (node = rb_first(&priv->root); node ; node = next) {
> +		next = rb_next(node);
>  
>  		rbe = rb_entry(node, struct nft_rbtree_elem, node);
>  
> -		if (nft_set_elem_is_dead(&rbe->ext))
> -			goto dead_elem;
> -
>  		/* elements are reversed in the rbtree for historical reasons,
>  		 * from highest to lowest value, that is why end element is
>  		 * always visited before the start element.

...