netdev - Re: [patch net-next 11/17] ipv6: fib: Allow non-FIB users to take reference on route

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Date:   Wed, 19 Jul 2017 09:49:37 -0600
From:   David Ahern <dsahern@...il.com>
To:     Jiri Pirko <jiri@...nulli.us>, netdev@...r.kernel.org
Cc:     davem@...emloft.net, idosch@...lanox.com, mlxsw@...lanox.com,
        roopa@...ulusnetworks.com, nikolay@...ulusnetworks.com,
        kafai@...com, hannes@...essinduktion.org, yoshfuji@...ux-ipv6.org,
        edumazet@...gle.com, yanhaishuang@...s.chinamobile.com
Subject: Re: [patch net-next 11/17] ipv6: fib: Allow non-FIB users to take
 reference on route

On 7/19/17 1:02 AM, Jiri Pirko wrote:
> From: Ido Schimmel <idosch@...lanox.com>
> 
> Listeners of the FIB notification chain are expected to be able to take
> and release a reference on notified IPv6 routes. This is needed in the
> case of drivers capable of offloading these routes to a capable device.
> 
> Since notifications are sent in an atomic context, these drivers need to
> take a reference on the route, prepare a work item to offload the route
> and release the reference at the end of the work.
> 
> Currently, rt6i_ref is used to indicate in how many FIB nodes a route
> appears. Different code paths rely on rt6i_ref being 0 to indicate the
> route is no longer used by the FIB.
> 
> For example, whenever a route is deleted or replaced, fib6_purge_rt() is
> run to make sure the route is no longer present in intermediate nodes. A
> BUG_ON() at the end of the function is executed in case the reference
> count isn't 1, as it's only supposed to appear in the non-intermediate
> node from which it's going to be deleted.
> 
> Instead of changing the semantics of rt6i_ref, a new reference count is
> added, so that external users could also take a reference on routes
> without modifying rt6i_ref.
> 
> To make sure external users don't release routes used by the FIB, the
> reference count is set to 1 upon creation of a route and decremented by
> the FIB upon rt6_release().
> 
> The reference count is atomic, as it's not protected by any locks and
> placed in the 40 bytes hole after the existing rt6i_ref.

I'd rather not add another reference counter. Debugging reference leaks
is a huge PITA now; adding another counter just makes it worse.

Why can't the BUG_ON in fib6_purge_rt be removed since there are other
reference holders now?