Subject: Re: [PATCH] netfilter: use per-cpu recursive lock (v11)

From: Eric Dumazet <[email-masked]>
Date: Tue, 21 Apr 2009 10:55:59 +0200

Evgeniy Polyakov a écrit :
> Hi.
> 
> On Tue, Apr 21, 2009 at 02:52:30PM +0800, Lai Jiangshan ([email-masked]) wrote:
>>> +void xt_info_rdlock_bh(void)
>>>> +{
>>>> +	struct xt_info_lock *lock;
>>>> +
>>>> +	preempt_disable();
>>>> +	lock = &__get_cpu_var(xt_info_locks);
>>>> +	if (likely(++lock->depth == 0))
>> So what happen when xt_info_rdlock_bh() called recursively here?
>>
>>>> +		spin_lock_bh(&lock->lock);
>>>> +	preempt_enable_no_resched();
>>>> +}
>>>> +EXPORT_SYMBOL_GPL(xt_info_rdlock_bh);
>>>> +
>> ----------
>> Is this OK? (Now I suppose we can enter the read-side critical region
>> in irq context)
>>
>> void xt_info_rdlock_bh(void)
>> {
>> 	unsigned long flags;
>> 	struct xt_info_lock *lock;
>>
>> 	local_irq_save(flags);
>> 	lock = &__get_cpu_var(xt_info_locks);
>> 	if (likely(++lock->depth == 0))
>> 		spin_lock_bh(&lock->lock);
>> 	local_irq_restore(flags);
>> }
> 
> Netfilter as long as other generic network pathes are never accessed
> from interrupt context, but your analysis looks right for the softirq
> case.
> 
> Stephen, should preempt_disable() be replaced with local_bh_disable() to
> prevent softirq to race on the same cpu for the lock's depth field? Or
> can it be made atomic?
> 

Maybe just dont care about calling several time local_bh_disable()
(since we were doing this in previous kernels anyway, we used to call read_lock_bh())

This shortens fastpath, is faster than local_irq_save()/local_irq_restore(),
and looks better.

void xt_info_rdlock_bh(void)
{
	struct xt_info_lock *lock;

	local_bh_disable();
 	lock = &__get_cpu_var(xt_info_locks);
 	if (likely(++lock->depth == 0))
 		spin_lock(&lock->lock);
}

void xt_info_rdunlock_bh(void)
{
	struct xt_info_lock *lock = &__get_cpu_var(xt_info_locks);

	BUG_ON(lock->depth < 0);
	if (likely(--lock->depth < 0))
		 spin_unlock(&lock->lock);
	local_bh_enable();
}