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    Subjectauro deadlock (was Re: e100 lockdep irq lock inversion.)
    On Fri, 2006-07-07 at 13:19 -0400, Dave Jones wrote:
    > Another one triggered by a Fedora-development user..
    > e100: eth1: e100_watchdog: link up, 100Mbps, half-duplex
    > =========================================================
    > [ INFO: possible irq lock inversion dependency detected ]
    > ---------------------------------------------------------
    > ipcalc/1671 just changed the state of lock:
    > (&skb_queue_lock_key){-+..}, at: [<c05ebe2f>] udp_ioctl+0x3b/0x6e
    > but this lock was taken by another, hard-irq-safe lock in the past:
    > (&ai->aux_lock){+...}
    > and interrupts could create inverse lock ordering between them.

    ok the situation is this:

    the airo driver has a per card aux_lock.

    This lock is used in the hardirq handler, and thus all uses need to be
    irqsave (and they are)

    Act 1

    Enter the mpi_start_xmit() function, which is airo's xmit function.
    This function takes the aux_lock first, with irq's off, then calls
    skb_queue_tail(). skb_queue_tail takes the sk_receive_queue.lock (with
    irqsave as well).

    Act 2

    Enter the ipcalc program. This program calls an ioctl, which ends up
    calling udp_ioctl. udp_ioctl does

    This is a deadlock

    For brevity, lock A is the aux_lock, lock B is the sk_receive_queue.lock

    CPU 0 CPU 1
    user context user context
    udp_ioctl takes lock B with _bh
    (leaving irqs enabled)

    mpi_start_function takes
    lock A with _irqsave
    mpi_start_function takes
    lock B with _irqsave and spins

    interrupt happens
    the hard irq handler takes lock A
    .... and spins for cpu 0

    eg a classic AB-BA deadlock.

    Now a question for netdev: what is the interrupt-or-softirq rules for
    the sk_receive_queue.lock?

    Anyway, the patch below fixes this deadlock; it may or may not be the
    correct solution depending on the netdev answer, but the deadlock is
    gone ;)

    Signed-off-by: Arjan van de Ven

    Index: linux-2.6.18-rc1/net/ipv4/udp.c
    --- linux-2.6.18-rc1.orig/net/ipv4/udp.c
    +++ linux-2.6.18-rc1/net/ipv4/udp.c
    @@ -725,6 +725,7 @@ out:

    int udp_ioctl(struct sock *sk, int cmd, unsigned long arg)
    + unsigned long flags;
    case SIOCOUTQ:
    @@ -739,7 +740,7 @@ int udp_ioctl(struct sock *sk, int cmd,
    unsigned long amount;

    amount = 0;
    - spin_lock_bh(&sk->sk_receive_queue.lock);
    + spin_lock_irqsave(&sk->sk_receive_queue.lock, flags);
    skb = skb_peek(&sk->sk_receive_queue);
    if (skb != NULL) {
    @@ -749,7 +750,7 @@ int udp_ioctl(struct sock *sk, int cmd,
    amount = skb->len - sizeof(struct udphdr);
    - spin_unlock_bh(&sk->sk_receive_queue.lock);
    + spin_unlock_irqrestore(&sk->sk_receive_queue.lock, flags);
    return put_user(amount, (int __user *)arg);

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      Last update: 2006-07-07 20:15    [W:0.023 / U:31.932 seconds]
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