netdev - Re: [PATCH net-next v3 2/3] xdp: Always use a devmap for XDP

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Date:   Mon, 4 Mar 2019 14:49:34 -0800
From:   Jakub Kicinski <jakub.kicinski@...ronome.com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>
Cc:     David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <ast@...nel.org>
Subject: Re: [PATCH net-next v3 2/3] xdp: Always use a devmap for
 XDP_REDIRECT to a device

On Mon, 04 Mar 2019 23:28:14 +0100, Toke Høiland-Jørgensen wrote:
> Jakub Kicinski <jakub.kicinski@...ronome.com> writes:
> > On Mon, 04 Mar 2019 20:05:30 +0100, Toke Høiland-Jørgensen wrote:  
> >> > Hm.  I think you'll still need a lock (mutex?) on the alloc path, but
> >> > the free path should be fine as long as you load the map pointer before
> >> > looking at the refcnt (atomic op ensuring the barrier there).    
> >> 
> >> Yeah, for the per-namespace refcnt it's pretty straight forward, the
> >> trouble is the global count that needs to iterate over all namespaces;
> >> probably need to put that all behind a (non-spin)lock, right?  
> >
> > Because net iteration is under RCU? You can switch to taking net_rwsem
> > for that one, no? I'm probably confused again ;)  
> 
> Because there's a single refcount that needs to trigger
> creation/deletion of *all* the default maps. I.e.
> 
> if (atomic_dec_return(&global_refcnt))
>   for_each_namespace(net)
>     destroy_default_map(net);
> 
> which needs to not step on the toes of a subsequent
> 
> if (atomic_inc_return(&global_refcnt) == 1)
>   for_each_namespace(net)
>     create_default_map(net);
> 
> (or vice versa, of course).
> 
> Not sure there's a way to do that without wrapping both of those
> constructs (including the refcnt inc/dec) in a mutex?

Seems like it :(