cleanup in workq and dst_destroy

[Daniel Lezcano <dlezcano-NmTC/0ZBporQT0dZR+AlfA@public.gmane.org>]

Hi all,

while doing ipv6 namespace, we were faced to a problem with the loopback 
and the dst_destroy function.

When the network namespace exits, the cleanup function is called by 
schedule_work and this function will browse the net ops list to call the 
different exit methods for the registered subsystems.

The different subsystems will shutdown their resources and in particular 
addrconf subsystem will ifdown the loopback. This function will call
rt6_ifdown
  -> fib6_clean_all
   -> fib6_clean_node
    -> fib6_clean_tree
     -> fib6_clean_node
      -> fib6_del
       -> fib6_del_route
        -> rt6_release
         ->dst_free
          -> __dst_free

The __dst_free function will schedule_delayed_work the dst_gc_work function.

The dst_gc_work will call dst_destroy and finally this one will call 
dst->ops->destroy ops function which is ip6_dst_destroy.

The problem here is we have the workq blocked because we are running 
inside the netns cleanup function. So the delayed work will not run 
until we exits the cleanup function. But the loopback is still 
referenced by the ip6 routes, the netdev_unregister will loop 
indefinitly => dead lock.

By the way, this bug appears with ipv6 but it is perhaps pending with ipv4.

Benjamin as proposed to create a separate workq for the network 
namespace, so in the worst case we have the unregister looping until the 
ip6 route are shut downed. Is it an acceptable solution ?