netdev - Re: [PATCH net] ipv6: remove incorrect WARN_ON() in fib6

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Message-ID: <CAEA6p_Du641cbcvr5JVXbLMh0T9fAauWDcXqNmrNoJkddMVCBA@mail.gmail.com>
Date:   Mon, 25 Sep 2017 22:52:38 -0700
From:   Wei Wang <weiwan@...gle.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     Martin KaFai Lau <kafai@...com>,
        David Miller <davem@...emloft.net>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: [PATCH net] ipv6: remove incorrect WARN_ON() in fib6_del()

On Mon, Sep 25, 2017 at 7:23 PM, Eric Dumazet <edumazet@...gle.com> wrote:
> On Mon, Sep 25, 2017 at 7:07 PM, Martin KaFai Lau <kafai@...com> wrote:
>
>> I am probably still missing something.
>>
>> Considering the del operation should be under the writer lock,
>> if rt->rt6i_node should be NULL (for rt that has already been
>> removed from fib6), why this WARN_ON() is triggered?
>>
>> An example may help.
>>
>
> Look at the stack trace, you'll find the answers...
>
> ip6_link_failure() -> ip6_del_rt()
>
> Note that rt might have been deleted from the _tree_ already.

Had a brief talk with Martin.
He has a valid point.
The current WARN_ON() code is as follows:
#if RT6_DEBUG >= 2
       if (rt->dst.obsolete > 0) {
               WARN_ON(fn);
               return -ENOENT;
       }
#endif

The WARN_ON() only triggers when fn is not NULL. (I missed it before.)
In theory, fib6_del() calls fib6_del_route() which should set
rt->rt6i_node to NULL and rt->dst.obsolete to DST_OBSOLETE_DEAD within
the same write_lock session.
If those 2 values are inconsistent, it indicates something is wrong.
Will need more time to root cause the issue.

Please ignore this patch. Sorry about the confusion.