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

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Message-ID: <20161116151829.h6kapbjes7xxxcyi@splinter.mtl.com>
Date:   Wed, 16 Nov 2016 17:18:29 +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,
        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

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.