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    SubjectRe: 2.6.34-rc1: rcu lockdep bug?
    From
    2010/3/15 Américo Wang <xiyou.wangcong@gmail.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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