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

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Message-Id: <20161114181425.GN4127@linux.vnet.ibm.com>
Date:   Mon, 14 Nov 2016 10:14:25 -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 Mon, Nov 14, 2016 at 09:44:35AM -0800, Cong Wang wrote:
> On Mon, Nov 14, 2016 at 8:24 AM, Paul E. McKenney
> <paulmck@...ux.vnet.ibm.com> wrote:
> > 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.
> 
> Yeah, this is exactly my point. And same for ASSERT_RTNL() which checks
> rtnl_is_locked(), clearly we need to assert "it is held by the current process"
> rather than "it is locked by whatever process".
> 
> But given *_is_held() is always defined by LOCKDEP, so we probably need
> mutex to provide such a helper directly, mutex->owner is not always defined
> either. :-/

There is always the option of making acquisition and release set a per-task
variable that can be tested.  (Where did I put that asbestos suit, anyway?)

							Thanx, Paul