Re: [PATCH] netfilter: use per-CPU recursive lock {XIV}

Jarek Poplawski
2009-04-24T20:44:28Z

Stephen Hemminger wrote, On 04/24/2009 06:18 PM:

> On Fri, 24 Apr 2009 06:58:39 +0200
> Eric Dumazet <dada1@cosmosbay.com> wrote:
>
>> Stephen Hemminger a écrit :
>>> In days of old in 2.6.29, netfilter did locketh using a
>>> lock of the reader kind when doing its table business, and do
>>> a writer when with pen in hand like a overworked accountant
>>> did replace the tables. This sucketh and caused the single
>>> lock to fly back and forth like a poor errant boy.
>>>
>>> But then netfilter was blessed with RCU and the performance
>>> was divine, but alas there were those that suffered for
>>> trying to replace their many rules one at a time.
>>>
>>> So now RCU must be vanquished from the scene, and better
>>> chastity belts be placed upon this valuable asset most dear.
>>> The locks that were but one are now replaced by one per suitor.
>>>
>>> The repair was made after much discussion involving
>>> Eric the wise, and Linus the foul. With flowers springing
>>> up amid the thorns some peace has finally prevailed and
>>> all is soothed. This patch and purple prose was penned by
>>> in honor of "Talk like Shakespeare" day.
>>>
>>> Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
>>
>> Philip Davis of the university’s School of English said :
>>
>> "Shakespeare surprises the brain and catches it off guard in
>> a manner that produces a sudden burst of activity - a sense
>> of drama created out of the simplest of things."
>>
>> http://www.physorg.com/news85664210.html
>>
>>> ---
>>> What hath changed over the last two setting suns:
>>> * more words, mostly correct...
>>>
>>> * no need to locketh for writeh on current cpu tis
>>> always so
>>>
>>> * the locking of all cpu's on replace is always done as
>>> part of the get_counters cycle, so the sychronize swip
>>> in replace tables is gone with only a comment remaing
>>>
>>> include/linux/netfilter/x_tables.h | 55 ++++++++++++++--
>>> net/ipv4/netfilter/arp_tables.c | 125 ++++++++++--------------------------
>>> net/ipv4/netfilter/ip_tables.c | 126 ++++++++++---------------------------
>>> net/ipv6/netfilter/ip6_tables.c | 123 ++++++++++--------------------------
>>> net/netfilter/x_tables.c | 55 ++++++++--------
>>> 5 files changed, 188 insertions(+), 296 deletions(-)
>>>
>>
>>>
>>> static int __init xt_init(void)
>>> {
>>> - int i, rv;
>>> + unsigned int i;
>>> + int rv;
>>> + static struct lock_class_key xt_lock_key[NR_CPUS];
>> Could we avoid this [NR_CPUS] thing ?
>>
>>> +
>>> + for_each_possible_cpu(i) {
>>> + rwlock_t *lock = &per_cpu(xt_info_locks, i);
>>> +
>>> + rwlock_init(lock);
>>> + lockdep_set_class(lock, xt_lock_key+i);
>>> + }
>>
>> Did you tried :
>>
>> static DECLARE_PER_CPU(struct lock_class_key, xt_locks_key);
>>
>
> The lock keys are really only used by lock dep, and I thought per cpu
> space was more scarce on some arch.
>

Maybe I'm wrong but after this change: "- only acquire one cpu write
lock at a time" lockdep_set_class() might be unnecessary. Alas I'm
not able to test it.

Jarek P.