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

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Message-ID: <20100427162201.GA5826@linux.vnet.ibm.com>
Date:	Tue, 27 Apr 2010 09:22:01 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Miles Lane <miles.lane@...il.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 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
--
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