Am Montag, 17. Mai 2004 18:56 schrieb Greg KH:
> On Mon, May 17, 2004 at 12:41:42PM -0400, Alan Stern wrote:
> > +void usb_kill_urb(struct urb *urb)
> > +{
> > + if (!(urb->dev && urb->dev->bus && urb->dev->bus->op))
> > + return;
> > + urb->reject = 1;
> > + urb->dev->bus->op->unlink_urb(urb, -ENOENT);
> > + wait_event(usb_kill_urb_queue, atomic_read(&urb->kref.refcount) <= 1);
> > + urb->reject = 0;
> > }
>
> Heh, no, you can't just sit and spin on the refcount, because that will
> not work if I do:
> usb_free_urb(urb);
> usb_kill_urb(urb);
Arguably you must not kill what you've freed, as you have no reference.
> And that last urb->reject = 0 will oops if I do that :)
> (and yes, some drivers do that already today with a call to
> usb_submit_urb() followed by a call to usb_free_urb().)
>
> Or what if I do:
> usb_get_urb(urb);
> usb_get_urb(urb);
> usb_get_urb(urb);
> usb_kill_urb(urb);
> which will cause this function to deadlock.
>
> You need to grab a reference to the urb at the beginning of the call,
> and decrement it at the end of it. But in the middle there, you can't
> count on a paticular refcount value, so you will have to trigger off of
> something else.
Right. However, you need a _submission_ counter only if giveback_urb()
can be reentered. If we are ready to ban resubmission to another device
in completion handlers, a single bit will do.
Alan wrote:
>You are right. There's no choice but to add another field for the Reject
>flag.
But there is. Use urb->lock to protect transfer_flags.
Regards
Oliver
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