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From: Hannes Frederic Sowa <hannes@stressinduktion.org>
To: Cong Wang <xiyou.wangcong@gmail.com>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Rolf Neugebauer <rolf.neugebauer@docker.com>,
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Subject: Re: Long delays creating a netns after deleting one (possibly RCU
 related)
Date: Mon, 14 Nov 2016 18:29:37 +0100
In-Reply-To: <CAM_iQpUDnPfVayQv7Q+ZtC_2_ZbX9MRX=514gFpF3E6XY+GtSA@mail.gmail.com>
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Hi Cong,

On Sat, Nov 12, 2016, at 01:55, Cong Wang wrote:
> On Fri, Nov 11, 2016 at 4:23 PM, Paul E. McKenney
> <paulmck@linux.vnet.ibm.com> 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();

I don't think we should depend on lockdep for this check but rather use
mutex_is_locked here (I think it would fail to build like this without
CONFIG_LOCKDEP).

Bye,
Hannes