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

From: Eric Dumazet <[email-masked]>
Date: Tue, 21 Apr 2009 05:56:55 +0200

Lai Jiangshan a écrit :
> Stephen Hemminger wrote:
>> +/**
>> + * xt_table_info_rdlock_bh - recursive read lock for xt table info
>> + *
>> + * Table processing calls this to hold off any changes to table
>> + * (on current CPU). Always leaves with bottom half disabled.
>> + * If called recursively, then assumes bh/preempt already disabled.
>> + */
>> +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))
> 
> Maybe I missed something. I think softirq may be still enabled here.
> So what happen when xt_info_rdlock_bh() called recursively here?

well, first time its called, you are right softirqs are enabled until
the point we call spin_lock_bh(), right after this line :

> 
>> +		spin_lock_bh(&lock->lock);
>> +	preempt_enable_no_resched();

After this line, both softirqs and preempt are disabled.

Future calls to this function temporarly raise preemptcount and decrease it.
(Null effect)

>> +}
>> +EXPORT_SYMBOL_GPL(xt_info_rdlock_bh);
>> +
> 
> Is this OK for you:
> 
> void xt_info_rdlock_bh(void)
> {
> 	struct xt_info_lock *lock;
> 
> 	local_bh_disable();

well, Stephen was trying to not change preempt count for the 2nd, 3rd, 4th?... invocation of this function.
This is how I understood the code.

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

Thanks for reviewing Lai