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Date: Tue, 21 Apr 2009 13:22:27 +0800
From: Lai Jiangshan <laijs@cn.fujitsu.com>
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To: Eric Dumazet <dada1@cosmosbay.com>
CC: Stephen Hemminger <shemminger@vyatta.com>,
       Paul Mackerras <paulus@samba.org>, paulmck@linux.vnet.ibm.com,
       Evgeniy Polyakov <zbr@ioremap.net>, David Miller <davem@davemloft.net>,
       kaber@trash.net, torvalds@linux-foundation.org,
       jeff.chua.linux@gmail.com, mingo@elte.hu, jengelh@medozas.de,
       r000n@r000n.net, linux-kernel@vger.kernel.org,
       netfilter-devel@vger.kernel.org, netdev@vger.kernel.org,
       benh@kernel.crashing.org, mathieu.desnoyers@polymtl.ca
Subject: Re: [PATCH] netfilter: use per-cpu recursive lock (v11)
References: <alpine.LFD.2.00.0904151705120.4042@localhost.localdomain>	<49E72E83.50702@trash.net>	<20090416.153354.170676392.davem@davemloft.net>	<20090416234955.GL6924@linux.vnet.ibm.com>	<20090417012812.GA25534@linux.vnet.ibm.com>	<20090418094001.GA2369@ioremap.net>	<20090418141455.GA7082@linux.vnet.ibm.com>	<20090420103414.1b4c490f@nehalam>	<49ECBE0A.7010303@cosmosbay.com>	<18924.59347.375292.102385@cargo.ozlabs.ibm.com>	<20090420215827.GK6822@linux.vnet.ibm.com>	<18924.64032.103954.171918@cargo.ozlabs.ibm.com> <20090420160121.268a8226@nehalam> <49ED406F.2040401@cn.fujitsu.com> <49ED4407.8010200@cosmosbay.com>
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Eric Dumazet wrote:
> 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 :

xt_info_rdlock_bh() called recursively here will enter the
critical region without &__get_cpu_var(xt_info_locks)->lock.

Because xt_info_rdlock_bh() called recursively here sees
lock->depth >= 0, and "++lock->depth == 0" is false.

> 
> 
>>> +		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);
>> }
>>

Sorry for it.
Is this OK:

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);
	else
		local_bh_enable();
}

I did not think things carefully enough, and I do know
nothing about ip/ip6/arp.

Lai