netdev - Re: 2.6.34-rc1: rcu lockdep bug?

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Date:	Mon, 15 Mar 2010 11:10:06 +0800
From:	Américo Wang <xiyou.wangcong@...il.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Américo Wang <xiyou.wangcong@...il.com>,
	Eric Dumazet <eric.dumazet@...il.com>,
	David Miller <davem@...emloft.net>, peterz@...radead.org,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: 2.6.34-rc1: rcu lockdep bug?

2010/3/15 Américo Wang <xiyou.wangcong@...il.com>:
> On Sat, Mar 13, 2010 at 01:58:38PM -0800, Paul E. McKenney wrote:
>>On Sat, Mar 13, 2010 at 01:33:56PM +0800, Américo Wang wrote:
>>> On Fri, Mar 12, 2010 at 02:37:38PM +0100, Eric Dumazet wrote:
>>> >Le vendredi 12 mars 2010 à 21:11 +0800, Américo Wang a écrit :
>>> >
>>> >> Oh, but lockdep complains about rcu_read_lock(), it said
>>> >> rcu_read_lock() can't be used in softirq context.
>>> >>
>>> >> Am I missing something?
>>> >
>>> >Well, lockdep might be dumb, I dont know...
>>> >
>>> >I suggest you read rcu_read_lock_bh kernel doc :
>>> >
>>> >/**
>>> > * rcu_read_lock_bh - mark the beginning of a softirq-only RCU critical
>>> >section
>>> > *
>>> > * This is equivalent of rcu_read_lock(), but to be used when updates
>>> > * are being done using call_rcu_bh(). Since call_rcu_bh() callbacks
>>> > * consider completion of a softirq handler to be a quiescent state,
>>> > * a process in RCU read-side critical section must be protected by
>>> > * disabling softirqs. Read-side critical sections in interrupt context
>>> > * can use just rcu_read_lock().
>>> > *
>>> > */
>>> >
>>> >
>>> >Last sentence being perfect :
>>> >
>>> >Read-side critical sections in interrupt context
>>> >can use just rcu_read_lock().
>>> >
>>>
>>> Yeah, right, then it is more likely to be a bug of rcu lockdep.
>>> Paul is looking at it.
>>
>>Except that it seems to be working correctly for me...
>>
>
> Hmm, then I am confused. The only possibility here is that this is
> a lockdep bug...
>

I believe so...

Peter, this looks odd:

 kernel:  (usbfs_mutex){+.?...}, at: [<ffffffff8146419f>]
netif_receive_skb+0xe7/0x819

netif_receive_skb() never has a chance to take usbfs_mutex. How can this
comes out?
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