Re: [PATCH] netfilter: per-cpu spin-lock with recursion (v0.8)

Eric Dumazet
2009-04-17T06:04:38Z

Stephen Hemminger a écrit :
> This version of x_tables (ip/ip6/arp) locking uses a per-cpu
> recursive lock that can be nested. It is sort of like existing kernel_lock,
> rwlock_t and even old 2.4 brlock.
>
> "Reader" is ip/arp/ip6 tables rule processing which runs per-cpu.
> It needs to ensure that the rules are not being changed while packet
> is being processed.
>
> "Writer" is used in two cases: first is replacing rules in which case
> all packets in flight have to be processed before rules are swapped,
> then counters are read from the old (stale) info. Second case is where
> counters need to be read on the fly, in this case all CPU's are blocked
> from further rule processing until values are aggregated.
>
> The idea for this came from an earlier version done by Eric Dumazet.
> Locking is done per-cpu, the fast path locks on the current cpu
> and updates counters. This reduces the contention of a
> single reader lock (in 2.6.29) without the delay of synchronize_net()
> (in 2.6.30-rc2).
>
>
> 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.
>
> Future optimizations possible:
> - Lockdep doesn't really handle this well
> - hot plug CPU case, if kernel is built with large # of CPU's, skip
> the inactive ones; migrate values when CPU is removed.
> - reading counters could be incremental by CPU.
>
> Signed-off-by: Stephen Hemminger <shemminger@vyatta.com
>

I like this version 8 of the patch, as it mixes all ideas we had,
but have two questions.

Previous netfilter code (and 2.6.30-rc2 one too) disable BH, not only preemption.

I see xt_table_info_lock_all(void) does block BH, so this one is safe.

I let Patrick or other tell us if its safe to run ipt_do_table()
with preemption disabled but BH enabled, I really dont know.

Also, please dont call this a 'recursive lock', since it is not a general
recursive lock, as pointed by Linus and Paul.

Second question is about MAX_LOCK_DEPTH

Why dont use this kind of construct to get rid of this limit ?

+void xt_table_info_lock_all(void)
> +{
> + int i;
> +
> + local_bh_disable();
> + for_each_possible_cpu(i) {
> + struct xt_lock *lock = &per_cpu(xt_info_locks, i);
> + spin_lock(&lock->lock);
> + preempt_enable_no_resched();
> + }
> +}
> +EXPORT_SYMBOL_GPL(xt_table_info_lock_all);