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

From: Lai Jiangshan <[email-masked]>
Date: Tue, 21 Apr 2009 17:34:36 +0800

Eric Dumazet wrote:
> 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);
> }

This two functions is OK. But...

> 
> 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();
> }
> 
> 

David said:
Netfilter itself, is nesting.

When using bridging netfilter, iptables can be entered twice
in the same call chain.

And Stephen said:
In this version, I was trying to use/preserve the optimizations that
are done in spin_unlock_bh().

So:

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))
 		spin_lock_bh(&lock->lock);
	/* softirq is disabled now */
	++lock->depth;
	preempt_enable_no_resched();
}

xt_info_rdunlock_bh() is the same as v11.

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