On Fri, 12 Sep 2014, Petr Mladek wrote:
> There is no need to have separate kthread for handling USB hub events.
> It is more elegant to use the workqueue framework.
>
> The workqueue is allocated as unbound, cpu intensive, and freezable.
> There does not seem to be any big advantage to run it on the same CPU.
> The handler is taking a lock and thus could block for a longer time.
> And finally, the original thread was freezable as well.
>
> struct usb_hub is passed via the work item. Therefore we do not need
> hub_event_list.
>
> hub_events() is modified to process the given work item. It is renamed to
> hub_event(). The while cycle will be removed in a followup patch. It helps
> to see the real change here.
>
> One nice thing is that we do not need hub_event_lock any longer. It was needed
> when doing operations with hub_event_list and for balancing the calls
> usb_autopm_get_interface_no_resume() and usb_autopm_put_interface_no_suspend().
> It still works because the workqueue operations have their own locking.
> Also cancel_work_sync() tells us whether any work item was canceled.
> It means that we could put the interface either in hub_event() handler or when
> the work item was successfully canceled.
I don't think you can eliminate the lock quite so easily. This patch
introduces some nasty races.
> @@ -577,18 +571,20 @@ static int hub_port_status(struct usb_hub *hub, int port1,
>
> static void kick_khubd(struct usb_hub *hub)
> {
> - unsigned long flags;
> -
> - spin_lock_irqsave(&hub_event_lock, flags);
> - if (!hub->disconnected && list_empty(&hub->event_list)) {
> - list_add_tail(&hub->event_list, &hub_event_list);
> -
> - /* Suppress autosuspend until khubd runs */
> + if (!hub->disconnected && !work_pending(&hub->events)) {
Here you test hub->disconnected, with no lock for protection.
(Also, note that work_pending is not synchronized with anything. What
happens if two threads call this routine at the same time?)
> @@ -1647,13 +1643,9 @@ static void hub_disconnect(struct usb_interface *intf)
> int port1;
>
> /* Take the hub off the event list and don't let it be added again */
> - spin_lock_irq(&hub_event_lock);
> - if (!list_empty(&hub->event_list)) {
> - list_del_init(&hub->event_list);
> + if (cancel_work_sync(&hub->events))
> usb_autopm_put_interface_no_suspend(intf);
> - }
> hub->disconnected = 1;
And here you set hub->disconnected with no lock for protection. So
what happens if one thread calls kick_khubd at the same time as another
thread calls hub_disconnect?
Alan Stern
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