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    Subjectrace in usbnet.c in full RT

    seems there is a race in drivers/net/usbnet.c in full RT mode. To be
    honest I haven't hardly checked this on the latest kernel and latest RT
    patch but just took a look at usbnet.c and latest RT patch and haven't
    observed any related changes.

    The usbnet_start_xmit() contains the following code:

    spin_lock_irqsave (&dev->txq.lock, flags);

    #ifdef CONFIG_USB_NET1080
    if (info->flags & FLAG_FRAMING_NC) {
    header->packet_id = cpu_to_le16 ((u16)dev->dev_packet_id++);
    put_unaligned (header->packet_id, &trailer->packet_id);
    #if 0
    devdbg (dev, "frame >tx h %d p %d id %d",
    header->hdr_len, header->packet_len,
    #endif /* CONFIG_USB_NET1080 */

    switch ((retval = usb_submit_urb (urb, GFP_ATOMIC))) {
    case -EPIPE:
    netif_stop_queue (net);
    defer_kevent (dev, EVENT_TX_HALT);
    devdbg (dev, "tx: submit urb err %d", retval);
    case 0:
    net->trans_start = jiffies;
    __skb_queue_tail (&dev->txq, skb);
    if (dev->txq.qlen >= TX_QLEN (dev))
    netif_stop_queue (net);
    spin_unlock_irqrestore (&dev->txq.lock, flags);

    THe race in full RT is between tx_complete() routine and
    __skb_queue_tail (&dev->txq, skb): skb->list gets initialized in
    __skb_queue_tail() but may be dereferenced before initialization at
    defer_bh() called from tx_complete() (since tx_complete() and
    usbnet_start_xmit() are completely asynchronous routines).

    in non-RT case spin_lock_irqsave (&dev->txq.lock, flags) disables
    interrupts and thus code from usb_submit_urb() call upto
    __skb_queue_tail (&dev->txq, skb) executes atomically. But in RT case
    interrupts are not disabled and usb_submit_urb() triggers an interrupt
    which may cause tx_complete() execution before __skb_queue_tail () call.
    And since skb->list gets initialized just at __skb_queue_tail(), call to
    tx_complete() (via defer_bh() which thus executes before
    __skb_queue_tail) dereferences NULL (skb->list) pointer.

    Thus looks tx_complete() and usbnet_start_xmit() require a
    serialization. Please find proposed fix attached though not sure the
    patch will apply cleanly to the latest kernel.

    --- a/drivers/usb/net/usbnet.c 2004-12-25 00:34:58.000000000 +0300
    +++ b/drivers/usb/net/usbnet.c 2005-05-26 21:02:58.000000000 +0400
    @@ -2820,6 +2820,8 @@

    urb->dev = NULL;
    entry->state = tx_done;
    + spin_lock_rt (&dev->txq.lock);
    + spin_unlock_rt(&dev->txq.lock);
    defer_bh (dev, skb);
     \ /
      Last update: 2005-06-08 12:16    [W:0.021 / U:7.324 seconds]
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