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

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Message-ID: <CAM_iQpUDnPfVayQv7Q+ZtC_2_ZbX9MRX=514gFpF3E6XY+GtSA@mail.gmail.com>
Date:   Fri, 11 Nov 2016 16:55:11 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     Rolf Neugebauer <rolf.neugebauer@...ker.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Justin Cormack <justin.cormack@...ker.com>,
        Ian Campbell <ian.campbell@...ker.com>
Subject: Re: Long delays creating a netns after deleting one (possibly RCU related)

On Fri, Nov 11, 2016 at 4:23 PM, Paul E. McKenney
<paulmck@...ux.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();
        else
                synchronize_rcu();