linux-kernel - Re: [PATCH] RCU: don't turn off lockdep when find suspicious rcu_dereference

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Message-ID: <g2ya44ae5cd1004271058p9f73fba1v3f611693e81869c3@mail.gmail.com>
Date:	Tue, 27 Apr 2010 13:58:30 -0400
From:	Miles Lane <miles.lane@...il.com>
To:	paulmck@...ux.vnet.ibm.com
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Vivek Goyal <vgoyal@...hat.com>,
	Eric Paris <eparis@...hat.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>, nauman@...gle.com,
	eric.dumazet@...il.com, netdev@...r.kernel.org,
	Jens Axboe <jens.axboe@...cle.com>,
	Gui Jianfeng <guijianfeng@...fujitsu.com>,
	Li Zefan <lizf@...fujitsu.com>,
	Johannes Berg <johannes@...solutions.net>
Subject: Re: [PATCH] RCU: don't turn off lockdep when find suspicious 
	rcu_dereference_check() usage

On Tue, Apr 27, 2010 at 12:22 PM, Paul E. McKenney
<paulmck@...ux.vnet.ibm.com> wrote:
> On Mon, Apr 26, 2010 at 09:27:44PM -0700, Paul E. McKenney wrote:
>> On Mon, Apr 26, 2010 at 11:35:10AM -0700, Eric W. Biederman wrote:
>> > "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> writes:
>> >
>> > > Eric Dumazet traced these down to a commit from Eric Biederman.
>> > >
>> > > If I don't hear from Eric Biederman in a few days, I will attempt a
>> > > patch, but it would be more likely to be correct coming from someone
>> > > with a better understanding of the code.  ;-)
>> >
>> > I already replied.
>> >
>> > http://lkml.org/lkml/2010/4/21/420
>>
>> You did indeed!!!  This experience is giving me an even better appreciation
>> of the maintainers' ability to keep all their patches straight!
>>
>> I will put together something based on your suggestion.
>
> How about the following?
>
>                                                        Thanx, Paul
>
> ------------------------------------------------------------------------
>
> commit 85fa42bd568ab99c375f018761ae6345249942cd
> Author: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> Date:   Mon Apr 26 21:40:05 2010 -0700
>
>    net: suppress RCU lockdep false positive in twsk_net()
>
>    Calls to twsk_net() are in some cases protected by reference counting
>    as an alternative to RCU protection.  Cases covered by reference counts
>    include __inet_twsk_kill(), inet_twsk_free(), inet_twdr_do_twkill_work(),
>    inet_twdr_twcal_tick(), and tcp_timewait_state_process().  RCU is used
>    by inet_twsk_purge().  Locking is used by established_get_first()
>    and established_get_next().  Finally, __inet_twsk_hashdance() is an
>    initialization case.
>
>    It appears to be non-trivial to locate the appropriate locks and
>    reference counts from within twsk_net(), so used rcu_dereference_raw().
>
>    Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
>
> diff --git a/include/net/inet_timewait_sock.h b/include/net/inet_timewait_sock.h
> index 79f67ea..a066fdd 100644
> --- a/include/net/inet_timewait_sock.h
> +++ b/include/net/inet_timewait_sock.h
> @@ -224,7 +224,9 @@ static inline
>  struct net *twsk_net(const struct inet_timewait_sock *twsk)
>  {
>  #ifdef CONFIG_NET_NS
> -       return rcu_dereference(twsk->tw_net);
> +       return rcu_dereference_raw(twsk->tw_net); /* protected by locking, */
> +                                                 /* reference counting, */
> +                                                 /* initialization, or RCU. */
>  #else
>        return &init_net;
>  #endif
>

Worked for me.  Thanks!

           Miles
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