On Fri, Dec 04, 2009 at 10:58:48PM +0100, Oliver Neukum wrote:
> Am Freitag, 4. Dezember 2009 22:36:22 schrieb Alan Stern:
> > > > Typically the driver would take a reference during open() and drop it
> > > > during close().
> > >
> > >
> > > You can do that but then you must not do IO prior to open() or after
> > > close(). That is you must actually wait for IO to finish in close() and
> > > cannot prefill your buffers before open().
> >
> > If open() or close() is called before disconnect() then you don't have
> > to worry.
> >
> > If close() is called after disconnect() there's nothing to wait for,
> > because disconnect() should call usb_kill_urb() on all outstanding
> > transfers (actually usbcore will do that for you). Likewise with
> > open().
> >
> > The problem in this example stems from the fact that you are using
> > instance->dev at a time when you don't know that it is valid -- in
> > fact, you have good reason to believe it _isn't_ valid because
> > instance->disconnected is set.
>
> OK, yes. It's a bad example. However this is tricky.
>
> This is a bug then:
>
> mutex_lock(...);
>
> if (instance->error) {
> rv = instance->error;
> instance->error = 0;
> dev_dbg(instance->dev,...);
> goto err_out;
> }
>
> rv = -ENODEV;
> if (instance->disconnected)
> goto err_out;
>
> > One approach is to set instance->dev to NULL in disconnect(). That
> > wouldn't do much good for your dev_dbg(), though. A better solution is
> > to refcount the instance->dev pointer: Take a reference to the device
> > when setting instance->dev and drop it when clearing instance->dev (or
> > when instance is freed).
>
> That would mean that I am forced to adopt refcounting just to print
> something. This seems very inelegant.
Don't print anything if you are disconnecting :)
thanks,
greg k-h
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