Subject: Re: [PATCH 5/5] netfilter: convert x_tables to use RCU

From: Eric Dumazet <[email-masked]>
Date: Fri, 30 Jan 2009 08:02:39 +0100

Eric Dumazet a écrit :
> Stephen Hemminger a écrit :
>> On Fri, 30 Jan 2009 00:04:16 +0100
>> Eric Dumazet <[email-masked]> wrote:
>>
>>> Stephen Hemminger a écrit :
>>>> Replace existing reader/writer lock with Read-Copy-Update to
>>>> elminate the overhead of a read lock on each incoming packet.
>>>> This should reduce the overhead of iptables especially on SMP
>>>> systems.
>>>>
>>>> The previous code used a reader-writer lock for two purposes.
>>>> The first was to ensure that the xt_table_info reference was not in
>>>> process of being changed. Since xt_table_info is only freed via one
>>>> routine, it was a direct conversion to RCU.
>>>>
>>>> The other use of the reader-writer lock was to to block changes
>>>> to counters while they were being read. This synchronization was
>>>> fixed by the previous patch.  But still need to make sure table info
>>>> isn't going away.
>>>>
>>>> Signed-off-by: Stephen Hemminger <[email-masked]>
>>>>
>>>>
>>>> ---
>>>>  include/linux/netfilter/x_tables.h |   10 ++++++-
>>>>  net/ipv4/netfilter/arp_tables.c    |   12 ++++-----
>>>>  net/ipv4/netfilter/ip_tables.c     |   12 ++++-----
>>>>  net/ipv6/netfilter/ip6_tables.c    |   12 ++++-----
>>>>  net/netfilter/x_tables.c           |   48 ++++++++++++++++++++++++++-----------
>>>>  5 files changed, 60 insertions(+), 34 deletions(-)
>>>>
>>>> --- a/include/linux/netfilter/x_tables.h	2009-01-28 22:04:39.316517913 -0800
>>>> +++ b/include/linux/netfilter/x_tables.h	2009-01-28 22:14:54.648490491 -0800
>>>> @@ -352,8 +352,8 @@ struct xt_table
>>>>  	/* What hooks you will enter on */
>>>>  	unsigned int valid_hooks;
>>>>  
>>>> -	/* Lock for the curtain */
>>>> -	rwlock_t lock;
>>>> +	/* Lock for curtain */
>>>> +	spinlock_t lock;
>>>>  
>>>>  	/* Man behind the curtain... */
>>>>  	struct xt_table_info *private;
>>>> @@ -386,6 +386,12 @@ struct xt_table_info
>>>>  	/* Secret compartment */
>>>>  	seqcount_t *seq;
>>>>  
>>>> +	/* For the dustman... */
>>>> +	union {
>>>> +		struct rcu_head rcu;
>>>> +		struct work_struct work;
>>>> +	};
>>>> +
>>>>  	/* ipt_entry tables: one per CPU */
>>>>  	/* Note : this field MUST be the last one, see XT_TABLE_INFO_SZ */
>>>>  	char *entries[1];
>>>> --- a/net/ipv4/netfilter/arp_tables.c	2009-01-28 22:13:16.423490077 -0800
>>>> +++ b/net/ipv4/netfilter/arp_tables.c	2009-01-28 22:14:54.648490491 -0800
>>>> @@ -238,8 +238,8 @@ unsigned int arpt_do_table(struct sk_buf
>>>>  	indev = in ? in->name : nulldevname;
>>>>  	outdev = out ? out->name : nulldevname;
>>>>  
>>>> -	read_lock_bh(&table->lock);
>>>> -	private = table->private;
>>>> +	rcu_read_lock_bh();
>>>> +	private = rcu_dereference(table->private);
>>>>  	table_base = (void *)private->entries[smp_processor_id()];
>>>>  	seq = per_cpu_ptr(private->seq, smp_processor_id());
>>>>  	e = get_entry(table_base, private->hook_entry[hook]);
>>>> @@ -315,7 +315,7 @@ unsigned int arpt_do_table(struct sk_buf
>>>>  			e = (void *)e + e->next_offset;
>>>>  		}
>>>>  	} while (!hotdrop);
>>>> -	read_unlock_bh(&table->lock);
>>>> +	rcu_read_unlock_bh();
>>>>  
>>>>  	if (hotdrop)
>>>>  		return NF_DROP;
>>>> @@ -1163,8 +1163,8 @@ static int do_add_counters(struct net *n
>>>>  		goto free;
>>>>  	}
>>>>  
>>>> -	write_lock_bh(&t->lock);
>>>> -	private = t->private;
>>>> +	rcu_read_lock_bh();
>>>> +	private = rcu_dereference(t->private);
>>>>  	if (private->number != num_counters) {
>>>>  		ret = -EINVAL;
>>>>  		goto unlock_up_free;
>>>> @@ -1179,7 +1179,7 @@ static int do_add_counters(struct net *n
>>>>  			   paddc,
>>>>  			   &i);
>>>>   unlock_up_free:
>>>> -	write_unlock_bh(&t->lock);
>>>> +	rcu_read_unlock_bh();
>>>>  	xt_table_unlock(t);
>>>>  	module_put(t->me);
>>>>   free:
>>>> --- a/net/ipv4/netfilter/ip_tables.c	2009-01-28 22:06:10.596739805 -0800
>>>> +++ b/net/ipv4/netfilter/ip_tables.c	2009-01-28 22:14:54.648490491 -0800
>>>> @@ -348,9 +348,9 @@ ipt_do_table(struct sk_buff *skb,
>>>>  	mtpar.family  = tgpar.family = NFPROTO_IPV4;
>>>>  	tgpar.hooknum = hook;
>>>>  
>>>> -	read_lock_bh(&table->lock);
>>>> +	rcu_read_lock_bh();
>>>>  	IP_NF_ASSERT(table->valid_hooks & (1 << hook));
>>>> -	private = table->private;
>>>> +	private = rcu_dereference(table->private);
>>>>  	table_base = (void *)private->entries[smp_processor_id()];
>>>>  	seq = per_cpu_ptr(private->seq, smp_processor_id());
>>>>  	e = get_entry(table_base, private->hook_entry[hook]);
>>>> @@ -449,7 +449,7 @@ ipt_do_table(struct sk_buff *skb,
>>>>  		}
>>>>  	} while (!hotdrop);
>>>>  
>>>> -	read_unlock_bh(&table->lock);
>>>> +	rcu_read_unlock_bh();
>>>>  
>>>>  #ifdef DEBUG_ALLOW_ALL
>>>>  	return NF_ACCEPT;
>>>> @@ -1408,8 +1408,8 @@ do_add_counters(struct net *net, void __
>>>>  		goto free;
>>>>  	}
>>>>  
>>>> -	write_lock_bh(&t->lock);
>>>> -	private = t->private;
>>>> +	rcu_read_lock_bh();
>>>> +	private = rcu_dereference(t->private);
>>> I feel litle bit nervous seeing a write_lock_bh() changed to a rcu_read_lock()
>> Facts, it is only updating entries on current cpu
> 
> Yes, like done in ipt_do_table() ;)
> 
> Fact is we need to tell other threads, running on other cpus, that an update
>  of our entries is running.
> 
> Let me check if your v4 and xt_counters abstraction already solved this problem.

Hum, I just checked and indeed there is a problem...

#define SUM_COUNTER(s,c)  do { (s).bcnt += (c).bcnt; (s).pcnt += (c).pcnt; } while(0)

need to be changed to use 

#define SUM_COUNTER(s, c)  do { xt_incr_counter(s, (c).cnt, (c).pcnt);} while (0)