linux-kernel - Re: [BUG] 2.6.38-rc2: Circular Locking Dependency

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Message-Id: <20110208.000721.193716330.davem@davemloft.net>
Date:	Tue, 08 Feb 2011 00:07:21 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	Knut_Petersen@...nline.de
Cc:	paulus@...ba.org, linux-kernel@...r.kernel.org,
	mostrows@...thlink.net, linux-ppp@...r.kernel.org,
	xiaosuo@...il.com, eric.dumazet@...il.com
Subject: Re: [BUG] 2.6.38-rc2: Circular Locking Dependency

From: Knut Petersen <Knut_Petersen@...nline.de>
Date: Tue, 08 Feb 2011 08:51:22 +0100

> I bisected the problem with the following result:
> 
> aa9421041128abb4d269ee1dc502ff65fb3b7d69 is the first bad commit
> commit aa9421041128abb4d269ee1dc502ff65fb3b7d69
> Author: Changli Gao <xiaosuo@...il.com>
> Date:   Sat Dec 4 02:31:41 2010 +0000
> 
>     net: init ingress queue
> 
>     The dev field of ingress queue is forgot to initialized, then NULL
>     pointer dereference happens in qdisc_alloc().
> 
>     Move inits of tx queues to netif_alloc_netdev_queues().
> 
>     Signed-off-by: Changli Gao <xiaosuo@...il.com>
>     Acked-by: Eric Dumazet <eric.dumazet@...il.com>
>     Signed-off-by: David S. Miller <davem@...emloft.net>
> 
> :040000 040000 dcbb6ab41c4308cba1bc6823d200dcf92aa402d8
> b5e190ec681d26ffe62d1d0214c4ef77b8034189 M      net

Indeed, this initialization is now too early for the sake
of getting the lockdep bits right.  The problem is that at
the point in which we call netif_alloc_netdev_queue() we
haven't initialized dev->type yet, it is therefore always
zero when we setup the lockdep class for ->_xmit_lock.
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