netdev - Re: [PATCH] netfilter: use per-cpu spinlock rather than RCU (v3)

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Message-ID: <20090418141455.GA7082@linux.vnet.ibm.com>
Date:	Sat, 18 Apr 2009 07:14:55 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Evgeniy Polyakov <zbr@...emap.net>
Cc:	David Miller <davem@...emloft.net>, kaber@...sh.net,
	torvalds@...ux-foundation.org, shemminger@...tta.com,
	dada1@...mosbay.com, jeff.chua.linux@...il.com, paulus@...ba.org,
	mingo@...e.hu, laijs@...fujitsu.com, 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 spinlock rather than RCU (v3)

On Sat, Apr 18, 2009 at 01:40:01PM +0400, Evgeniy Polyakov wrote:
> Hi.
> 
> On Thu, Apr 16, 2009 at 06:28:12PM -0700, Paul E. McKenney (paulmck@...ux.vnet.ibm.com) wrote:
> > +/* Single bit for grace-period index, low-order bits are nesting counter. */
> > +#define RCU_FGP_COUNT		1UL
> > +#define RCU_FGP_PARITY		(1UL << (sizeof(long) << 2))
> > +#define RCU_FGP_NEST_MASK	(RCU_FGP_PARITY - 1)
> > +
> > +extern long rcu_fgp_ctr;
> > +DECLARE_PER_CPU(long, rcu_fgp_active_readers);
> > +
> > +static inline void rcu_read_lock_fgp(void)
> > +{
> > +	long tmp;
> > +	long *uarp;
> > +
> > +	preempt_disable();
> > +	uarp = &__get_cpu_var(rcu_fgp_active_readers);
> > +	tmp = *uarp;
> > +	if (likely(!(tmp & RCU_FGP_NEST_MASK)))
> > +		*uarp = rcu_fgp_ctr;  /* Outermost rcu_read_lock(). */
> > +	else
> > +		*uarp = tmp + RCU_FGP_COUNT;  /* Nested rcu_read_lock(). */
> > +	barrier();
> > +}
> > +
> > +static inline void rcu_read_unlock_fgp(void)
> > +{
> > +	barrier();
> > +	__get_cpu_var(rcu_fgp_active_readers)--;
> 
> Shouldn't it be rcu_fgp_active_readers - RCU_FGP_COUNT?
> Although it is 1 by definition, it is more clear when understanding
> what's going on here.

Excellent point, fixed!

						Thanx, Paul
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