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

From: Evgeniy Polyakov <[email-masked]>
Date: Tue, 21 Apr 2009 12:16:49 +0400

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?

-- 
	Evgeniy Polyakov