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    SubjectRe: [PATCH] netfilter: use per-cpu spinlock rather than RCU
    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 ?
    >
    >
    >
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