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

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Date:   Tue, 26 Sep 2017 06:20:30 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     Wei Wang <weiwan@...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 10:52 PM, Wei Wang <weiwan@...gle.com> wrote:
> 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.

Oh well, for some reason I was seeing WARN_ON(1)  here, since this is
a construct I often add in my tests ...