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

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Message-ID: <20071030151254.GA8285@linux.vnet.ibm.com>
Date:	Tue, 30 Oct 2007 08:12:54 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	David Miller <davem@...emloft.net>
Cc:	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 Tue, Oct 30, 2007 at 01:10:36AM -0700, David Miller wrote:
> From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
> Date: Mon, 29 Oct 2007 14:15:40 -0700
> 
> > 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;
> > 
> > ...  But what happens if seq->private is freed up right here?
> > ...  Or what prevents this from happening?
>  ...
> > Similar code is in rt_cache_get_next().
> > 
> > So, what am I missing here?
> 
> seq->private is allocated on file open (here via seq_open_private()),
> and freed up on file close (via seq_release_private).
> 
> So it cannot be freed up in the middle of an iteration.

Thank you for the info!!!

OK, for my next stupid question: why is the rcu_dereference(seq->private)
required, as opposed to simply seq->private?

							Thanx, Paul
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