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

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Date:   Mon, 14 Nov 2016 09:44:35 -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 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. :-/