Subject: Re: [RFT 4/4] netfilter: convert x_tables to use RCU

From: "Paul E. McKenney" <[email-masked]>
Date: Tue, 27 Jan 2009 23:37:27 -0800

On Tue, Jan 27, 2009 at 03:53:14PM -0800, Stephen Hemminger wrote:
> 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. There are a couple of harmless races in reading
> the counters, but we always give a valid values they just aren't
> exactly frozen in time across all cpu's.

Looks good from an RCU perspective!

Reviewed-by: Paul E. McKenney <[email-masked]>

> 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           |   44 ++++++++++++++++++++++++++-----------
>  5 files changed, 58 insertions(+), 32 deletions(-)
> 
> --- a/include/linux/netfilter/x_tables.h	2009-01-27 15:33:10.791377313 -0800
> +++ b/include/linux/netfilter/x_tables.h	2009-01-27 15:46:26.024128742 -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;
> @@ -383,6 +383,12 @@ struct xt_table_info
>  	unsigned int hook_entry[NF_INET_NUMHOOKS];
>  	unsigned int underflow[NF_INET_NUMHOOKS];
> 
> +	/* 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 */
>  	void *entries[1];
> --- a/net/ipv4/netfilter/arp_tables.c	2009-01-27 15:45:34.566650540 -0800
> +++ b/net/ipv4/netfilter/arp_tables.c	2009-01-27 15:46:26.024128742 -0800
> @@ -237,8 +237,8 @@ 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()];
>  	e = get_entry(table_base, private->hook_entry[hook]);
>  	back = get_entry(table_base, private->underflow[hook]);
> @@ -313,7 +313,7 @@ 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;
> @@ -1154,8 +1154,8 @@ 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;
> @@ -1170,7 +1170,7 @@ 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	2009-01-27 15:45:05.766673246 -0800
> +++ b/net/ipv4/netfilter/ip_tables.c	2009-01-27 15:46:26.024128742 -0800
> @@ -347,9 +347,9 @@ 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()];
>  	e = get_entry(table_base, private->hook_entry[hook]);
> 
> @@ -447,7 +447,7 @@ 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;
> @@ -1399,8 +1399,8 @@ 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);
>  	if (private->number != num_counters) {
>  		ret = -EINVAL;
>  		goto unlock_up_free;
> @@ -1415,7 +1415,7 @@ do_add_counters(struct net *net, void __
>  			  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/ipv6/netfilter/ip6_tables.c	2009-01-27 15:45:22.262639173 -0800
> +++ b/net/ipv6/netfilter/ip6_tables.c	2009-01-27 15:46:26.028128353 -0800
> @@ -373,9 +373,9 @@ ip6t_do_table(struct sk_buff *skb,
>  	mtpar.family  = tgpar.family = NFPROTO_IPV6;
>  	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()];
>  	e = get_entry(table_base, private->hook_entry[hook]);
> 
> @@ -476,7 +476,7 @@ ip6t_do_table(struct sk_buff *skb,
>  #ifdef CONFIG_NETFILTER_DEBUG
>  	((struct ip6t_entry *)table_base)->comefrom = NETFILTER_LINK_POISON;
>  #endif
> -	read_unlock_bh(&table->lock);
> +	rcu_read_unlock_bh();
> 
>  #ifdef DEBUG_ALLOW_ALL
>  	return NF_ACCEPT;
> @@ -1430,8 +1430,8 @@ 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);
>  	if (private->number != num_counters) {
>  		ret = -EINVAL;
>  		goto unlock_up_free;
> @@ -1446,7 +1446,7 @@ do_add_counters(struct net *net, void __
>  			  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/netfilter/x_tables.c	2009-01-27 15:14:06.004743434 -0800
> +++ b/net/netfilter/x_tables.c	2009-01-27 15:48:07.194667206 -0800
> @@ -611,17 +611,37 @@ struct xt_table_info *xt_alloc_table_inf
>  }
>  EXPORT_SYMBOL(xt_alloc_table_info);
> 
> -void xt_free_table_info(struct xt_table_info *info)
> +/* callback to do free for vmalloc'd case */
> +static void xt_free_table_info_work(struct work_struct *arg)
> +{
> +	struct xt_table_info *info = container_of(arg, struct xt_table_info, work);
> + 	unsigned int cpu;
> +
> +	for_each_possible_cpu(cpu)
> +		vfree(info->entries[cpu]);
> +	kfree(info);
> +}
> +
> +/* callback to do free after all cpu's are done */
> +static void xt_free_table_info_rcu(struct rcu_head *arg)
>  {
> -	int cpu;
> +	struct xt_table_info *info = container_of(arg, struct xt_table_info, rcu);
> 
> -	for_each_possible_cpu(cpu) {
> -		if (info->size <= PAGE_SIZE)
> +	if (info->size <= PAGE_SIZE) {
> +		unsigned int cpu;
> +		for_each_possible_cpu(cpu)
>  			kfree(info->entries[cpu]);
> -		else
> -			vfree(info->entries[cpu]);
> +		kfree(info);
> +	} else {
> +		/* can't safely call vfree in current context */
> +		INIT_WORK(&info->work, xt_free_table_info_work);
> +		schedule_work(&info->work);
>  	}
> -	kfree(info);
> +}
> +
> +void xt_free_table_info(struct xt_table_info *info)
> +{
> +	call_rcu(&info->rcu, xt_free_table_info_rcu);
>  }
>  EXPORT_SYMBOL(xt_free_table_info);
> 
> @@ -671,20 +691,20 @@ xt_replace_table(struct xt_table *table,
>  	struct xt_table_info *oldinfo, *private;
> 
>  	/* Do the substitution. */
> -	write_lock_bh(&table->lock);
> +	spin_lock_bh(&table->lock);
>  	private = table->private;
>  	/* Check inside lock: is the old number correct? */
>  	if (num_counters != private->number) {
>  		duprintf("num_counters != table->private->number (%u/%u)\n",
>  			 num_counters, private->number);
> -		write_unlock_bh(&table->lock);
> +		spin_unlock_bh(&table->lock);
>  		*error = -EAGAIN;
>  		return NULL;
>  	}
>  	oldinfo = private;
> -	table->private = newinfo;
> +	rcu_assign_pointer(table->private, newinfo);
>  	newinfo->initial_entries = oldinfo->initial_entries;
> -	write_unlock_bh(&table->lock);
> +	spin_unlock_bh(&table->lock);
> 
>  	return oldinfo;
>  }
> @@ -719,7 +739,7 @@ struct xt_table *xt_register_table(struc
> 
>  	/* Simplifies replace_table code. */
>  	table->private = bootstrap;
> -	rwlock_init(&table->lock);
> +	spin_lock_init(&table->lock);
> 
>  	if (!xt_replace_table(table, 0, newinfo, &ret))
>  		goto unlock;
> 
> -- 
> 
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