netdev - Re: [patch net-next v2 09/11] ipv4: fib: Add an API to request a FIB dump

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Date:   Wed, 23 Nov 2016 21:53:28 +0200
From:   Ido Schimmel <idosch@...sch.org>
To:     Hannes Frederic Sowa <hannes@...essinduktion.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,
        kaber@...sh.net
Subject: Re: [patch net-next v2 09/11] ipv4: fib: Add an API to request a FIB
 dump

On Wed, Nov 23, 2016 at 06:47:03PM +0100, Hannes Frederic Sowa wrote:
> Hmm, I think you need to read the sequence counter under rtnl_lock to
> have an ordering with the rest of the updates to the RCU trie. Otherwise
> you don't know if the fib trie has the correct view regarding to the
> incoming notifications as a whole. This is also necessary during restarts.

I spent quite a lot of time thinking about this specific issue, but I
couldn't convince myself that the read should be done under RTNL and I'm
not sure I understand your reasoning. Can you please elaborate?

If, before each notification sent, we call atomic_inc() and then call
atomic_read() at the end, then how can we be tricked?

Thanks for looking into this!