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

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Date:	Tue, 1 Sep 2015 17:42:00 -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 05:31:44PM -0700, Martin KaFai Lau wrote:
> On Tue, Sep 01, 2015 at 03:38:36PM -0700, Eric Dumazet wrote:
> > On Tue, 2015-09-01 at 15:25 -0700, Martin KaFai Lau wrote:
> > > 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()
> > >
> >
> > Yes, this is protected by normal rcu rules.
> >
> > dst wont be freed until all cpus exit their rcu read sections.
> For DST_NOCACHE, like:
>
> rcu_read_lock()
>
> ip6_tnl_dst_get()
>  			dst_release()	/* refcnt is 0 */
> 			=>call_rcu(dst_destroy)
> skb_dst_set_noref()
> __dev_queue_xmit()
> =>skb_dst_force()
> =>__dev_xmit_skb()
> =>q->enqueue()
>
> rcu_read_unlock()
> /* Here, I am holding a dst refcnt but
>  * the dst is already in the next
>  * rcu destroy cycle?
>  */
I look a closer look at dst_rcu_free() and your commit pointers.  I can see your point
for DST_NOCACHE.

However, dst_free() for not DST_NOCACHE is still an issue, I think.
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