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

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Message-ID: <20071105235117.GA18396@gondor.apana.org.au>
Date:	Tue, 6 Nov 2007 07:51:17 +0800
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
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 08:12:03AM -0800, Paul E. McKenney 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;
>
> Thank you for the info!  Stupid question #3: what sequence of events
> would the smp_read_barrier_depends() be defending against?

The reading of rt from the hash bucket and the dereferencing above.
We need to make sure that we see the initialised rt.

Cheers,
-- 
Visit Openswan at http://www.openswan.org/
Email: Herbert Xu ~{PmV>HI~} <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
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