Subject: Re: [PATCH net-next v3 2/3] xdp: Always use a devmap for XDP_REDIRECT to a device

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Mon, 04 Mar 2019 23:28:14 +0100

Jakub Kicinski <[email-masked]> 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?

-Toke