On Tue, 27 Jun 2006, Oliver Neukum wrote:
> Am Dienstag, 27. Juni 2006 16:09 schrieb Franck Bui-Huu:
> > Oliver Neukum wrote:
> > > Am Dienstag, 27. Juni 2006 09:52 schrieb Franck Bui-Huu:
> > >> + expire = timeout ? msecs_to_jiffies(timeout) :
> > >> MAX_SCHEDULE_TIMEOUT;
> > >> + if (!wait_for_completion_timeout(&done, expire)) {
> > >> + usb_unlink_urb(urb);
> > >> }
> > >> -
> > >> + status = urb->status;
> > >
> > > Hm, is this racy?
> > > What happens if usb_unlink_urb() and the completion handler race?
> > > If usb_unlink_urb() fails to unlink the URB because it is too late,
> > > is it guaranteed that urb->status is valid?
> > > In the old version this could not happen because the completion was
> > > always done.
> >
> > well, I'm not sure to understand. Do you mean "what happen if the timeout
> > has
> > expired, but just before calling usb_unlink_urb() the urb is completed by
> > the
> > hcd" ?
> >
> > If so, I think that usb_unlink_urb() handles this case by checking (with
> > lock
> > held) that the urb is still queued.
>
> Precisely. If the URB is not queued it'll notice. But you read urb->status
> even in that case.
The race between usb_unlink_urb and the completion routine doesn't matter.
The core guarantees that urb->status is set correctly no matter what.
However, it's important that the URB be completely finished before the
routine returns. Remember that the completion handler _will_ be called
eventually, no matter what. And urb->status isn't valid until the URB has
completed.
So if the initial wait_for_completion_timeout() times out, you need to do
a second wait_for_completion().
Alan Stern
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