Subject: Re: [PATCH 2/5] netfilter: nft_rbtree: no need for spinlock from set destroy path

From: Pablo Neira Ayuso <[email-masked]>
Date: Tue, 23 Sep 2014 13:01:59 +0200

On Tue, Sep 23, 2014 at 02:52:37AM -0700, Eric Dumazet wrote:
> On Tue, 2014-09-23 at 11:24 +0200, Pablo Neira Ayuso wrote:
> > The sets are released from the rcu callback, after the rule is removed
> > from the chain list, which implies that nfnetlink cannot update the
> > rbtree and no packets are walking on the set anymore. Thus, we can get
> > rid of the spinlock in the set destroy path there.
> > 
> > Signed-off-by: Pablo Neira Ayuso <[email-masked]>
> > Reviewied-by: Thomas Graf <[email-masked]>
> > ---
> >  net/netfilter/nft_rbtree.c |    2 --
> >  1 file changed, 2 deletions(-)
> > 
> > diff --git a/net/netfilter/nft_rbtree.c b/net/netfilter/nft_rbtree.c
> > index e1836ff..46214f2 100644
> > --- a/net/netfilter/nft_rbtree.c
> > +++ b/net/netfilter/nft_rbtree.c
> > @@ -234,13 +234,11 @@ static void nft_rbtree_destroy(const struct nft_set *set)
> >  	struct nft_rbtree_elem *rbe;
> >  	struct rb_node *node;
> >  
> > -	spin_lock_bh(&nft_rbtree_lock);
> >  	while ((node = priv->root.rb_node) != NULL) {
> >  		rb_erase(node, &priv->root);
> >  		rbe = rb_entry(node, struct nft_rbtree_elem, node);
> >  		nft_rbtree_elem_destroy(set, rbe);
> >  	}
> > -	spin_unlock_bh(&nft_rbtree_lock);
> >  }
> >  
> >  static bool nft_rbtree_estimate(const struct nft_set_desc *desc, u32 features,
> 
> BTW, do you know if destroying an rbtree is faster this way, or using
> rb_first() ?
> 
> Most cases I see in the kernel use a rb_first instead of taking the
> root.
> 
> Examples : (its not an exhaustive list)
> 
> net/netfilter/xt_connlimit.c:402
> net/sched/sch_netem.c:380
> net/sched/sch_fq.c:519
> drivers/infiniband/hw/mlx4/cm.c:439
> drivers/iommu/iova.c:324
> drivers/md/dm-thin.c:1491
> drivers/mtd/mtdswap.c:625
> drivers/mtd/ubi/attach.c:636
> 
> This might be better for large trees, to get better cache locality,
> but I have no experimental data.

I'll send a follow up patch for nf-next to use rb_first() in that
patch. Thanks Eric.