Subject: Re: [PATCH 4/5] netfilter: use sequence number synchronization for counters

From: Eric Dumazet <[email-masked]>
Date: Thu, 29 Jan 2009 09:47:53 +0100

Stephen Hemminger a écrit :
> Change how synchronization is done on the iptables counters. Use seqcount
> wrapper instead of depending on reader/writer lock.
> 
> Signed-off-by: Stephen Hemminger <[email-masked]>
> 
> 
> ---
>  include/linux/netfilter/x_tables.h |    3 +++
>  net/ipv4/netfilter/arp_tables.c    |   24 +++++++++++++++++++-----
>  net/ipv4/netfilter/ip_tables.c     |   24 +++++++++++++++++++-----
>  net/ipv6/netfilter/ip6_tables.c    |   32 +++++++++++++++++++++++---------
>  net/netfilter/x_tables.c           |   11 +++++++++++
>  5 files changed, 75 insertions(+), 19 deletions(-)
> 4
> --- a/net/ipv4/netfilter/arp_tables.c	2009-01-28 21:24:39.223991934 -0800
> +++ b/net/ipv4/netfilter/arp_tables.c	2009-01-28 22:13:16.423490077 -0800
> @@ -230,6 +230,7 @@ unsigned int arpt_do_table(struct sk_buf
>  	void *table_base;
>  	const struct xt_table_info *private;
>  	struct xt_target_param tgpar;
> +	seqcount_t *seq;
>  
>  	if (!pskb_may_pull(skb, arp_hdr_len(skb->dev)))
>  		return NF_DROP;
> @@ -240,6 +241,7 @@ unsigned int arpt_do_table(struct sk_buf
>  	read_lock_bh(&table->lock);
>  	private = table->private;
>  	table_base = (void *)private->entries[smp_processor_id()];
> +	seq = per_cpu_ptr(private->seq, smp_processor_id());

But, why not using a global seqcount_t, shared by all tables, no matter they
are arp_tables, ip_tables, ip6_tables ?

A global PER_CPU variable, not dynamically allocated, so that its
access can be faster (no indirection), and uses exactly 4 bytes per
possible cpu.

DEFINE_PER_CPU(seqcount_t,  nf_seqcount);