Re: [PATCH] netfilter: use per-cpu spinlock rather than RCU

Stephen Hemminger
2009-04-13T18:11:28Z

On Mon, 13 Apr 2009 19:40:24 +0200
Eric Dumazet <dada1@cosmosbay.com> wrote:

> Stephen Hemminger a écrit :
> > This is an alternative version of ip/ip6/arp tables locking using
> > per-cpu locks. This avoids the overhead of synchronize_net() during
> > update but still removes the expensive rwlock in earlier versions.
> >
> > The idea for this came from an earlier version done by Eric Duzamet.
> > Locking is done per-cpu, the fast path locks on the current cpu
> > and updates counters. The slow case involves acquiring the locks on
> > all cpu's.
> >
> > The mutex that was added for 2.6.30 in xt_table is unnecessary since
> > there already is a mutex for xt[af].mutex that is held.
> >
> > Tested basic functionality (add/remove/list), but don't have test cases
> > for stress, ip6tables or arptables.
> >
> > Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
>
> Patch seems good to me, but apparently xt_replace_table()
> misses the "acquiring the locks on all cpus" you mentioned in ChangeLog ?

It happens in get_counters already.

> I am still off-computers until tomorrow so cannot provide a patch for this, sorry.
>
> Some form of
>
> local_bh_disable();
> for_each_possible_cpu(cpu)
> spin_lock(&per_cpu(ip_tables_lock, cpu));
>
> oldinfo = private;
> /* do the substitution */
> table->private = newinfo;
> newinfo->initial_entries = oldinfo->initial_entries;
>
> for_each_possible_cpu(cpu)
> spin_unlock(&per_cpu(ip_tables_lock, cpu));
> local_bh_enable();
>
>
> But I wonder if this could hit a limit of max spinlocks held by this cpu, say on a 4096 cpu machine ?
>
>
>