cleanup in workq and dst_destroy

[Denis V. Lunev]

Daniel Lezcano wrote:
> 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 ?
> 

we are doing this staff in the special thread. There are a lot of
difficult things to perform like synchronize_net & netdev_run_todo inside

Regards,
	Den