Re: cleanup in workq and dst_destroy

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

Denis V. Lunev wrote:
> 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

The special thread ? do you mean keventd_wq ?