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

From: Stephen Hemminger <[email-masked]>
Date: Fri, 24 Apr 2009 09:18:39 -0700

On Fri, 24 Apr 2009 06:58:39 +0200
Eric Dumazet <[email-masked]> 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 <[email-masked]>
> 
> 
> 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.

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