netdev - Re: [patch net-next 6/8] ipv4: fib: Add an API to request a FIB dump

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Message-ID: <dcbbcefa-3ca7-c0ed-09a7-1d0e6f96e256@stressinduktion.org>
Date:   Wed, 16 Nov 2016 18:35:45 +0100
From:   Hannes Frederic Sowa <hannes@...essinduktion.org>
To:     Ido Schimmel <idosch@...sch.org>
Cc:     Jiri Pirko <jiri@...nulli.us>, netdev@...r.kernel.org,
        davem@...emloft.net, idosch@...lanox.com, eladr@...lanox.com,
        yotamg@...lanox.com, nogahf@...lanox.com, arkadis@...lanox.com,
        ogerlitz@...lanox.com, roopa@...ulusnetworks.com,
        dsa@...ulusnetworks.com, nikolay@...ulusnetworks.com,
        andy@...yhouse.net, vivien.didelot@...oirfairelinux.com,
        andrew@...n.ch, f.fainelli@...il.com, alexander.h.duyck@...el.com,
        kuznet@....inr.ac.ru, jmorris@...ei.org, yoshfuji@...ux-ipv6.org,
        kaber@...sh.net
Subject: Re: [patch net-next 6/8] ipv4: fib: Add an API to request a FIB dump

On 16.11.2016 16:18, Ido Schimmel wrote:
> Hi Hannes,
> 
> On Wed, Nov 16, 2016 at 03:51:01PM +0100, Hannes Frederic Sowa wrote:
>> On 16.11.2016 15:09, Jiri Pirko wrote:
>>> From: Ido Schimmel <idosch@...lanox.com>
>>>
>>> Commit b90eb7549499 ("fib: introduce FIB notification infrastructure")
>>> introduced a new notification chain to notify listeners (f.e., switchdev
>>> drivers) about addition and deletion of routes.
>>>
>>> However, upon registration to the chain the FIB tables can already be
>>> populated, which means potential listeners will have an incomplete view
>>> of the tables.
>>>
>>> Solve that by adding an API to request a FIB dump. The dump itself it
>>> done using RCU in order not to starve consumers that need RTNL to make
>>> progress.
>>>
>>> Signed-off-by: Ido Schimmel <idosch@...lanox.com>
>>> Signed-off-by: Jiri Pirko <jiri@...lanox.com>
>>
>> Have you looked at potential inconsistencies resulting of RCU walking
>> the table and having concurrent inserts?
> 
> Yes. I did try to think about situations in which this approach will
> fail, but I could only find problems with concurrent removals, which I
> addressed in 5/8. In case of concurrent insertions, even if you missed
> the node, you would still get the ENTRY_ADD event to your listener.

Theoretically a node could still be installed while the deletion event
fired before registering the notifier. E.g. a synchronize_net before
dumping could help here?

I don't know how you prepare the data structures for inserting in into
the hardware, but if ordering matters, the notifier for a delete event
can be called before the dump installed the fib entry? E.g. you also
don't check what events are already queued in the delayed work queue so
far. I hope I understand the patches correctly?

Thanks,
Hannes