Re: [PATCH nf] x_tables: Properly close read section with read_seqcount_retry

[Florian Westphal <fw@strlen.de>]

subashab@codeaurora.org <subashab@codeaurora.org> wrote:
> I've updated the patch with your comments.
> Do you expect a performance impact either in datapath or perhaps more in
> the rule installation with the rcu changes.

Rule installation.  synchronize_rcu() can take several seconds on busy
systems.

> diff --git a/net/ipv4/netfilter/arp_tables.c
> b/net/ipv4/netfilter/arp_tables.c
> index d1e04d2..dda5d8f 100644
> --- a/net/ipv4/netfilter/arp_tables.c
> +++ b/net/ipv4/netfilter/arp_tables.c
> @@ -203,7 +203,7 @@ unsigned int arpt_do_table(struct sk_buff *skb,
> 
>  	local_bh_disable();
>  	addend = xt_write_recseq_begin();
> -	private = READ_ONCE(table->private); /* Address dependency. */
> +	private = xt_table_get_private_protected(table);

Err, no, this needs to be plain rcu_dereference(table->private).
Same in the other _do_table() versions.

We do not hold the table mutex here.