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

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Date:	Mon, 20 Apr 2009 21:15:38 -0700
From:	Stephen Hemminger <shemminger@...tta.com>
To:	Eric Dumazet <dada1@...mosbay.com>
Cc:	Lai Jiangshan <laijs@...fujitsu.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)

On Tue, 21 Apr 2009 05:56:55 +0200
Eric Dumazet <dada1@...mosbay.com> 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 :
> 
> 
> > 
> >> +		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.
> > 

In this version, I was trying to use/preserve the optimizations that
are done in spin_unlock_bh().
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