netdev - Re: [PATCH] netfilter: use per-cpu spinlock rather than RCU

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Date:	Mon, 13 Apr 2009 16:37:26 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
cc:	Stephen Hemminger <shemminger@...tta.com>,
	paulmck@...ux.vnet.ibm.com, davem@...emloft.net, paulus@...ba.org,
	mingo@...e.hu, laijs@...fujitsu.com, jeff.chua.linux@...il.com,
	dada1@...mosbay.com, jengelh@...ozas.de, kaber@...sh.net,
	r000n@...0n.net, linux-kernel@...r.kernel.org,
	netfilter-devel@...r.kernel.org, netdev@...r.kernel.org,
	benh@...nel.crashing.org
Subject: Re: [PATCH] netfilter: use per-cpu spinlock rather than RCU

On Mon, 13 Apr 2009, Andrew Morton wrote:
> > > >  
> > > > -	rcu_read_lock_bh();
> > > > -	private = rcu_dereference(table->private);
> > > > -	table_base = rcu_dereference(private->entries[smp_processor_id()]);
> > > > +	local_bh_disable();
> > > > +	spin_lock(&__get_cpu_var(ip_tables_lock));
> > > 
> > > spin_lock_bh()?
> > 
> > No. get_cpu_var implies smp_processor_id which is not safe
> > without preempt_disable (ie bh disable).
> 
> spin_lock_bh() will dtrt, but spelling it out seems a good idea.

No, spin_lock_bh() will _not_ do the right thing. 

On UP it will actually work for two reasons: it will work because (a) it's 
UP, so there are no issues with smp_processor_id() to beging with, but 
also because even if there _were_ issues, it would still work because it 
would all expand as a macro, and the preempt_disable() will actually 
happen before the argument is evaluated.

But on SMP, spin_lock_bh() expands to just _spin_lock_bh(), and is a real 
function - and the argument will be evaluated before the call (obviously), 
and thus before the preempt_disable().

So

	local_bh_disable();
	spin_lock(&__get_cpu_var(ip_tables_lock));

is correct, and 

	spin_lock_bh(&__get_cpu_var(ip_tables_lock));

is _not_ correct. The latter will do "&__get_cpu_var(ip_tables_lock)"
with no protection from the process being switched to another CPU.

			Linus
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