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

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Message-Id: <20071030.011036.43329108.davem@davemloft.net>
Date:	Tue, 30 Oct 2007 01:10:36 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	paulmck@...ux.vnet.ibm.com
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

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.

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