linux-kernel - Re: dn_route.c momentarily exiting RCU read-side critical section

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Message-ID: <20071105161203.GB9479@linux.vnet.ibm.com>
Date:	Mon, 5 Nov 2007 08:12:03 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	davem@...emloft.net, linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org, SteveW@....org, dipankar@...ibm.com
Subject: Re: dn_route.c momentarily exiting RCU read-side critical section

On Mon, Nov 05, 2007 at 07:53:04PM +0800, Herbert Xu wrote:
> Paul E. McKenney <paulmck@...ux.vnet.ibm.com> wrote:
> >> 
> >> > net/decnet/dn_route.c in dn_rt_cache_get_next() is as follows:
> >> > 
> >> > static struct dn_route *dn_rt_cache_get_next(struct seq_file *seq, struct dn_route *rt)
> >> > {
> >> >     struct dn_rt_cache_iter_state *s = rcu_dereference(seq->private);
> >> > 
> >> >     rt = rt->u.dst.dn_next;
> >> >     while(!rt) {
> >> >             rcu_read_unlock_bh();
> >> >             if (--s->bucket < 0)
> >> >                     break;
> >
> > OK, for my next stupid question: why is the rcu_dereference(seq->private)
> > required, as opposed to simply seq->private?
> 
> It was put there by someone who went through the code converting
> all occurances of smp_read_barrier_depends to rcu_dereference.
> In this instance the rcu_dereference conversion doesn't make much
> sense so we should probably just revert it.

Thank you for the info!  Stupid question #3: what sequence of events
would the smp_read_barrier_depends() be defending against?

							Thanx, Paul
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