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Date: Fri, 30 Jan 2009 07:53:09 +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 :
> On Fri, 30 Jan 2009 00:04:16 +0100
> Eric Dumazet <dada1@...mosbay.com> 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 <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()
>
> 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.
>
>> 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.
> add_counter_to_entry is local to current CPU.
>
>
>> (With write_lock_bh(), this corruption could not occur)
>>
>>
> --
--
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