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

[Cong Wang]

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...

diff --git a/net/core/dev.c b/net/core/dev.c
index eaad4c2..3415b6b 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -7762,7 +7762,7 @@ EXPORT_SYMBOL(free_netdev);
 void synchronize_net(void)
 {
        might_sleep();
-       if (rtnl_is_locked())
+       if (rtnl_is_locked() || lockdep_is_held(&net_mutex))
                synchronize_rcu_expedited();
        else
                synchronize_rcu();