On Wed, 7 May 2014, Dan Williams wrote:
> In general we do not want khubd to act on port status changes that are
> the result of in progress resets or USB runtime PM operations.
> Specifically port power control testing has been able to trigger an
> unintended disconnect in hub_port_connect_change(), paraphrasing:
>
> if ((portstatus & USB_PORT_STAT_CONNECTION) && udev &&
> udev->state != USB_STATE_NOTATTACHED) {
> if (portstatus & USB_PORT_STAT_ENABLE) {
> /* Nothing to do */
> } else if (udev->state == USB_STATE_SUSPENDED &&
> udev->persist_enabled) {
> ...
> } else {
> /* Don't resuscitate */;
> }
> }
>
> ...by falling to the "Don't resuscitate" path or missing
> USB_PORT_STAT_CONNECTION because usb_port_resume() was in the middle of
> modifying the port status.
>
> So, we want a new lock to hold off khubd for a given port while the
> child device is being suspended, resumed, or reset. The lock ordering
> rules are now usb_lock_device() => usb_lock_port(). This is mandated by
> the device core which may hold the device_lock on the usb_device before
> invoking usb_port_{suspend|resume} which in turn take the status_lock on
> the usb_port. We attempt to hold the status_lock for the duration of a
> port_event() run, and drop/re-acquire it when needing to take the
> device_lock. The lock is also dropped/re-acquired during
> hub_port_reconnect().
>
> This patch also deletes hub->busy_bits as all use cases are now covered
> by port PM runtime synchronization or the port->status_lock and it
> pushes down usb_device_lock() into usb_remote_wakeup().
>
> Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
Acked-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
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