Re: Long delays creating a netns after deleting one (possibly RCU related)

[Cong Wang]

On Fri, Nov 11, 2016 at 4:55 PM, Cong Wang <xiyou.wangcong@xxxxxxxxx> wrote:
> On Fri, Nov 11, 2016 at 4:23 PM, Paul E. McKenney
> <paulmck@xxxxxxxxxxxxxxxxxx> wrote:
>>
>> Ah!  This net_mutex is different than RTNL.  Should synchronize_net() be
>> modified to check for net_mutex being held in addition to the current
>> checks for RTNL being held?
>>
>
> Good point!
>
> Like commit be3fc413da9eb17cce0991f214ab0, checking
> for net_mutex for this case seems to be an optimization, I assume
> synchronize_rcu_expedited() and synchronize_rcu() have the same
> behavior...

Thinking a bit more, I think commit be3fc413da9eb17cce0991f
gets wrong on rtnl_is_locked(), the lock could be locked by other
process not by the current one, therefore it should be
lockdep_rtnl_is_held() which, however, is defined only when LOCKDEP
is enabled... Sigh.

I don't see any better way than letting callers decide if they want the
expedited version or not, but this requires changes of all callers of
synchronize_net(). Hm.