On Tue 16-09-14 11:10:31, Petr Mládek wrote:
> Anyway, the solution for the race between kick_hub_wq() and
> hub_event() might be to get the reference already in kick_hub_wq().
> I mean something like:
>
> static void kick_hub_wq(struct usb_hub *hub)
> {
> if (hub->disconnected || work_pending(&hub->events))
> return;
> /*
> * Suppress autosuspend until the event is proceed.
> *
> * Be careful and make sure that the symmetric operation is
> * always called. We are here only when there is no pending
> * work for this hub. Therefore put the interface either when
> * the new work is called or when it is canceled.
> */
> usb_autopm_get_interface_no_resume(to_usb_interface(hub->intfdev));
> kref_get(&hub->kref);
>
> if (queue_work(hub_wq, &hub->events))
> return;
>
> /* the work could not be scheduled twice */
> kref_put(&hub->kref, hub_release);
> usb_autopm_put_interface_no_suspend(intf);
> }
I have just realized that I was working in the Linus' three where the
commit c605f3cdff53a743f6d87 ("usb: hub: take hub->hdev reference when
processing from eventlist") was missing :-(
It means that we would need to call also usb_get_dev(hdev) in
kick_hub_wq() to make sure that everything is valid when the work
hub_event() is proceed.
Hmm, it stopped being nice. So, I was looking for another solution and
found an interesting behavior of cancel_work_sync() that is used in
hub_disconnect(). It either removes the work item from the queue or it
waits until the work is done. Anyway, no work is pending when the
function returns. It means that struct usb_hub never will be released
when hub_event() is in progress and we do not need to get the
references in advance at all.
I am really sorry for sending so many mails. I am still getting familiar with
kernel hacking. I am often surprised how complex it is. I do not want
to spam and lose your interest, definitely. So, I am going to calm
down and be more patient when doing the homework.
Best Regards,
Petr
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