netdev - Re: [PATCH net 3/3] ipv6: Fix dst_entry refcnt bugs in ip6

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Date:	Tue, 1 Sep 2015 15:25:33 -0700
From:	Martin KaFai Lau <kafai@...com>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	netdev <netdev@...r.kernel.org>,
	"David S. Miller" <davem@...emloft.net>,
	Kernel Team <kernel-team@...com>
Subject: Re: [PATCH net 3/3] ipv6: Fix dst_entry refcnt bugs in ip6_tunnel

On Tue, Sep 01, 2015 at 02:26:58PM -0700, Eric Dumazet wrote:
> On Tue, 2015-09-01 at 13:55 -0700, Martin KaFai Lau wrote:
> > On Tue, Sep 01, 2015 at 01:14:20PM -0700, Eric Dumazet wrote:
> > > It should not be a problem. refcnt is taken when/if necessary (skb
> > > queued on a qdisc for example)
> > >
> > > We have other uses of skb_dst_set_noref()
> > >
> > > Please describe the problem ?
> > The current ip6_tnl_dst_get() does not take the dst refcnt.
> >
> > If the dst is released after ip6_tnl_dst_get() and before
> > skb_dst_set_noref(), would it cause an issue?
>
> We are under rcu here, and a dst in a cache is protected by RCU by
> definition.
>
> skb_dst_set_noref() has following debugging clause, does it trigger for
> you ?
>
> WARN_ON(!rcu_read_lock_held() && !rcu_read_lock_bh_held());
No. I did not see this.

I am probably missing something.  Do you mean the rcu can
protect the followings:


ip6_tnl_dst_get()
			dst_release()
						dst_free() /* refcnt is 0 */
skb_dst_set_noref()

--------

or for DST_NOCACHE

ip6_tnl_dst_get()
			dst_release() /* refcnt is 0 */
skb_dst_set_noref()
/* ip6tunnel_xmit()
 * or its callees
 * take a dst refcnt
 */

--------

or the above situation does not matter/will not happen?
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