Re: [PATCH 4/4] usb: libusual: locking cleanup

Andrew Morton
2007-12-22T06:33:55Z

On Fri, 21 Dec 2007 22:24:28 -0800 Pete Zaitcev <zaitcev@redhat.com> wrote:

> On Fri, 21 Dec 2007 00:00:04 -0800, Daniel Walker <dwalker@mvista.com> wrote:
>
> > I converted the usu_init_notify semaphore to normal mutex usage, and it
> > should still prevent the request_module before the init routine is
> > complete. Before it acted more like a complete, now the mutex protects
> > two distinct section from running at the same time..
>
> Let's see.
>
> > @@ -178,10 +179,7 @@ static int usu_probe_thread(void *arg)
> > int rc;
> > unsigned long flags;
> >
> > - /* A completion does not work here because it's counted. */
> > - down(&usu_init_notify);
> > - up(&usu_init_notify);
> > -
> > + mutex_lock(&usu_probe_mutex);
> > rc = request_module(bias_names[type]);
>
> When I tried it, usb-storage would not load with unresolved symbols.
> It happens if child (usu_probe_thread) runs ahead of its parent
> (usb_usual_init -> usb_register -> usu_probe). It's entirely possible,
> depending on your scheduler.

afaict Daniel's change will fix that?

He releases usu_probe_mutex once the usb_register() has completed and this
then allows the usb_probe_thread() to start working.

I'm still wondering if that theory has been tested though.

> I hate this down-up trick too

I'd be worried if you were proud of it ;)