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

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Message-Id: <20161114162417.GJ4127@linux.vnet.ibm.com>
Date:   Mon, 14 Nov 2016 08:24:17 -0800
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Cong Wang <xiyou.wangcong@...il.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 Sun, Nov 13, 2016 at 10:47:01PM -0800, Cong Wang wrote:
> On Fri, Nov 11, 2016 at 4:55 PM, Cong Wang <xiyou.wangcong@...il.com> wrote:
> > 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...
> 
> 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.

I must confess that I don't understand how it would help to use an
expedited grace period when some other process is holding RTNL.
In contrast, I do well understand how it helps when the current process
is holding RTNL.

So what am I missing here?

							Thanx, Paul