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Date: Fri, 30 Jan 2009 00:04:16 +0100
From: Eric Dumazet <dada1@...mosbay.com>
To: Stephen Hemminger <shemminger@...tta.com>
CC: David Miller <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [PATCH 5/5] netfilter: convert x_tables to use RCU
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 <shemminger@...tta.com>
>
>
> ---
> 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()
Also, add_counter_to_entry() is not using seqcount protection, so another thread
doing an iptables -L in parallel with this thread will possibly get corrupted counters.
(With write_lock_bh(), this corruption could not occur)
--
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