netdev - Re: [PATCH net-next v4 3/6] Convert neigh_* seq

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Message-ID: <20241016205437.12812-1-kuniyu@amazon.com>
Date: Wed, 16 Oct 2024 13:54:37 -0700
From: Kuniyuki Iwashima <kuniyu@...zon.com>
To: <gnaaman@...venets.com>
CC: <davem@...emloft.net>, <edumazet@...gle.com>, <kuba@...nel.org>,
	<kuniyu@...zon.com>, <netdev@...r.kernel.org>, <pabeni@...hat.com>
Subject: Re: [PATCH net-next v4 3/6] Convert neigh_* seq_file functions to use hlist

From: Gilad Naaman <gnaaman@...venets.com>
Date: Wed, 16 Oct 2024 09:11:52 +0000
> > > -		if (++state->bucket >= (1 << nht->hash_shift))
> > > -			break;
> > 
> > Let's keep this,
> > 
> > 
> > > +		while (!n && ++state->bucket < (1 << nht->hash_shift))
> > > +			n = neigh_first_rcu(&nht->hash_heads[state->bucket]);
> > >  
> > > -		n = rcu_dereference(nht->hash_buckets[state->bucket]);
> > 
> > and simply fetch neigh_first_entry().
> > 
> > Then, we can let the next loop handle the termination condition
> > and keep the code simple.
> 
> Unfortunately `hlist_for_each_entry_continue_rcu` dereferences `n`
> first thing using `hlist_next_rcu`, before checking for NULL:

Right, and I noticed we even can't use neigh_first_entry() after
checking against NULL because the first entry will be skipped by
hlist_for_each_entry_continue_rcu().

> 
>     #define hlist_for_each_entry_continue_rcu(pos, member)			\
>     	for (pos = hlist_entry_safe(rcu_dereference_raw(hlist_next_rcu( \
>     			&(pos)->member)), typeof(*(pos)), member);	\
>     	     pos;							\
>     	     pos = hlist_entry_safe(rcu_dereference_raw(hlist_next_rcu(	\
>     			&(pos)->member)), typeof(*(pos)), member))
> 
> If I'm using it, I have to add a null-check after calling `neigh_first_entry`.
> 
> Another alternative is to use `hlist_for_each_entry_from_rcu`,
> but it'll require calling `next` one time before the loop.
> 
> Is this preferable?

How about factorising the operations inside loops and use
hlist_for_each_entry_continue() and call neigh_get_first()
in neigh_get_next() ?

completely not tested:

---8<---
static struct neighbour *neigh_get_valid(struct seq_file *seq,
					 struct neighbour *n,
					 loff_t *pos)
{
	struct net *net = seq_file_net(seq);

	if (!net_eq(dev_net(n->dev), net))
		return NULL;

	if (state->neigh_sub_iter) {
		loff_t fakep = 0;
		void *v;

		v = state->neigh_sub_iter(state, n, pos ? pos : &fakep);
		if (!v)
			return NULL;
		if (pos)
			return v;
	}

	if (!(state->flags & NEIGH_SEQ_SKIP_NOARP))
		return n;

	if (READ_ONCE(n->nud_state) & ~NUD_NOARP)
		return n;

	return NULL;
}

static struct neighbour *neigh_get_first(struct seq_file *seq)
{
	struct neigh_seq_state *state = seq->private;
	struct neigh_hash_table *nht = state->nht;
	struct net *net = seq_file_net(seq);
	struct neighbour *n, *tmp;

	state->flags &= ~NEIGH_SEQ_IS_PNEIGH;

	while (++state->bucket < (1 << nht->hash_shift)) {
		neigh_for_each(n, &nht->hash_heads[state->bucket]) {
			tmp = neigh_get_valid(seq, n, pos);
			if (tmp)
				return tmp;
		}
	}

	return NULL;
}

static struct neighbour *neigh_get_next(struct seq_file *seq,
					struct neighbour *n,
					loff_t *pos)
{
	struct neigh_seq_state *state = seq->private;
	struct neigh_hash_table *nht = state->nht;
	struct net *net = seq_file_net(seq);
	struct neighbour *tmp;

	if (state->neigh_sub_iter) {
		void *v = state->neigh_sub_iter(state, n, pos);

		if (v)
			return n;
	}

	hlist_for_each_entry_continue(n, hash) {
		tmp = neigh_get_valid(seq, n, pos);
		if (tmp) {
			n = tmp;
			goto out;
		}
	}

	n = neigh_get_first(seq);
out:
	if (n && pos)
		--(*pos);

	return n;
}
---8<---