On Mon, May 17, 2004 at 07:15:24PM +0200, Oliver Neukum wrote:
> 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.
Ah, yeah, you are right. Ok, nevermind that point. But my other one
remains :)
> > 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.
Agreed.
thanks,
greg k-h
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