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

Eric Dumazet
2009-04-21T08:58:23Z

Evgeniy Polyakov a écrit :
> Hi.
>
> On Tue, Apr 21, 2009 at 02:52:30PM +0800, Lai Jiangshan (laijs@cn.fujitsu.com) 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();
}