Subject: Re: [PATCH] netfilter: use per-cpu recursive lock (v10)

From: Eric Dumazet <[email-masked]>
Date: Mon, 20 Apr 2009 20:25:14 +0200

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.
> 
> Signed-off-by: Stephen Hemminger <[email-masked]
> 
> ---
> Changes from earlier patches.
>   - function name changes
>   - disable bottom half in info_rdlock

OK, but we still have a problem on machines with >= 250 cpus,
because calling 250 times spin_lock() is going to overflow preempt_count,
as each spin_lock() increases preempt_count by one.

PREEMPT_MASK: 0x000000ff

add_preempt_count() should warn us about this overflow if CONFIG_DEBUG_PREEMPT is set

#ifdef CONFIG_DEBUG_PREEMPT
        /*
         * Spinlock count overflowing soon?
         */
        DEBUG_LOCKS_WARN_ON((preempt_count() & PREEMPT_MASK) >=
                                PREEMPT_MASK - 10);
#endif

My suggestion (in a previous mail) was to call preempt_disable() after each spin_lock(),
and of course doing the reverse on unlock path.

> +/**
> + * xt_info_wrlock_bh - lock xt table info for update
> + *
> + * Locks out all readers, and blocks bottom half
> + */
> +void xt_info_wrlock_bh(void)
> +{
> +	int i;
> +
> +	local_bh_disable();
 
/* at this point , preemption is disabled... */

> +	for_each_possible_cpu(i) {
> +		struct xt_info_lock *lock = &per_cpu(xt_info_locks, i);
> +		spin_lock(&lock->lock);
	
		preempt_enable(); /* avoid preempt count overflow */
		
> +		BUG_ON(lock->depth != -1);
> +	}
> +}
> +EXPORT_SYMBOL_GPL(xt_info_wrlock_bh);
> +
> +/**
> + * xt_info_wrunlock_bh - lock xt table info for update
> + *
> + * Unlocks all readers, and unblocks bottom half
> + */
> +void xt_info_wrunlock_bh(void) __releases(&lock->lock)
> +{
> +	int i;
> +
> +	for_each_possible_cpu(i) {
> +		struct xt_info_lock *lock = &per_cpu(xt_info_locks, i);
> +		BUG_ON(lock->depth != -1);

		preempt_disable() /* restore preempt count lowered in xt_info_wrlock_bh */

> +		spin_unlock(&lock->lock);
> +	}
> +	local_bh_enable();
> +}
> +EXPORT_SYMBOL_GPL(xt_info_wrunlock_bh);

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