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

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Date:	Tue, 21 Apr 2009 13:22:27 +0800
From:	Lai Jiangshan <laijs@...fujitsu.com>
To:	Eric Dumazet <dada1@...mosbay.com>
CC:	Stephen Hemminger <shemminger@...tta.com>,
	Paul Mackerras <paulus@...ba.org>, paulmck@...ux.vnet.ibm.com,
	Evgeniy Polyakov <zbr@...emap.net>,
	David Miller <davem@...emloft.net>, kaber@...sh.net,
	torvalds@...ux-foundation.org, jeff.chua.linux@...il.com,
	mingo@...e.hu, jengelh@...ozas.de, r000n@...0n.net,
	linux-kernel@...r.kernel.org, netfilter-devel@...r.kernel.org,
	netdev@...r.kernel.org, benh@...nel.crashing.org,
	mathieu.desnoyers@...ymtl.ca
Subject: Re: [PATCH] netfilter: use per-cpu recursive lock (v11)

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



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