Am Freitag, 20. April 2012, 12:17:51 schrieb Ming Lei:
> On Fri, Apr 20, 2012 at 3:57 PM, Oliver Neukum <[email protected]> wrote:
> >
> > You are racing with hid_irq_out(). It calls hid_submit_out()
> > under lock. So if hid_irq_out() is running between dropping
> > the lock and usb_unlink_urb() you may kill the newly submitted
> > urb, not the old urb that has timed out.
>
> Yes, it is the race I missed, :-(
>
> > You must make sure that between the times you check usbhid->last_out
> > and calling unlink hid_submit_out() cannot be called.
> > You can't just drop the lock (at least on SMP)
>
> Looks it is not easy to avoid the race if the lock is to be dropped.
>
> So how about not acquiring the lock during unlinking as below?
>
> diff --git a/drivers/hid/usbhid/hid-core.c b/drivers/hid/usbhid/hid-core.c
> index aa1c503..b437223 100644
> --- a/drivers/hid/usbhid/hid-core.c
> +++ b/drivers/hid/usbhid/hid-core.c
> @@ -429,7 +429,8 @@ static void hid_irq_out(struct urb *urb)
> urb->status);
> }
>
> - spin_lock_irqsave(&usbhid->lock, flags);
> + if (!test_cpu_bit(HID_PCPU_TIMEOUT, usbhid->cpuiofl))
> + spin_lock_irqsave(&usbhid->lock, flags);
>
> if (unplug)
> usbhid->outtail = usbhid->outhead;
> @@ -438,12 +439,14 @@ static void hid_irq_out(struct urb *urb)
>
> if (usbhid->outhead != usbhid->outtail && !hid_submit_out(hid)) {
> /* Successfully submitted next urb in queue */
> - spin_unlock_irqrestore(&usbhid->lock, flags);
> + if (!test_cpu_bit(HID_PCPU_TIMEOUT, usbhid->cpuiofl))
> + spin_unlock_irqrestore(&usbhid->lock, flags);
> return;
> }
No. This introduces a race where you can drop a lock you've not taken
and vice versa.
And why are you using per cpu structures here?
To be blunt, I'd prefer a guarantee that allows usb_unlink_urb() to be
called with spinlocks held.
Well, if we can't get a good usb_unlink_urb() then the lock must be dropped.
Your idea of setting a flag is good.
You'd do in hid_irq_out():
if (usbhid->outhead != usbhid->outtail && !usbhid->timeout_detected &&
!hid_submit_out(hid))
and you set usbhid->timeout_detected under lock. That way we can never
accidentally kill a new urb. This however means that then we need to kill the
urb and restart the queue preferably from a workqueue and this must be handled
in suspend/resume/close/pre-/postreset/disconnect
As I said, I'd very much appreciate sane semantics for usb_unlink_urb().
Regards
Oliver
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