Re: 2.6.31-rc5 regression: Oops when USB Serial disconnected while in use

Bruno Prémont
2009-08-20T19:36:16Z

On Wed, 19 August 2009 Alan Stern <stern@rowland.harvard.edu> wrote:
> On Wed, 19 Aug 2009, Alan Cox wrote:
>
> > > So Alan, what's the explanation? An unsuccessful open method call
> > > should not lead to a close method call.
> >
> > For tty it always has since back in 0.9x, and most drivers depend
> > upon this behaviour.
>
> Curiouser and curiouser...
>
> Okay Bruno, try out this patch and let's see if it helps. It should
> fix the problem with the pl2303. I'm not so sure about the FTDI
> device because that driver maintains its own reference count for its
> private data.
>
> Alan Stern

No, that one does not really help (well it avoids dereferencing a bad
pointer (NULL for pl2302 or invalid for ftdi) with serial_do_free() after
destroy_serial() but is does underflow module refcount (refcount ends
up at 2^32-1) and lets the kernel stall/panic a short time later without
any useful information at that moment (only very first line of trace
ever appears) [ftdi for the panic/freeze].

So I would say at best it's just a step towards the solution...
there are still unbalanced operations left (module refcount being the
visible one, then probably the already mentioned private book-keeping
of ftdi)!

Bruno

> Index: usb-2.6/drivers/usb/serial/usb-serial.c
> ===================================================================
> --- usb-2.6.orig/drivers/usb/serial/usb-serial.c
> +++ usb-2.6/drivers/usb/serial/usb-serial.c
> @@ -209,8 +209,13 @@ static int serial_open (struct tty_struc
> * to be acquired while serial->disc_mutex is held.
> */
> mutex_unlock(&serial->disc_mutex);
> +
> + /* For reasons that have been lost in antiquity, the TTY
> layer calls
> + * our close method even if the open call fails. Hence we
> must not
> + * drop the reference obtained by usb_serial_get_by_index().
> + */
> if (retval)
> - goto bailout_serial_put;
> + return retval;
>
> if (mutex_lock_interruptible(&port->mutex)) {
> retval = -ERESTARTSYS;
> @@ -276,8 +281,6 @@ bailout_mutex_unlock:
> mutex_unlock(&port->mutex);
> bailout_port_put:
> put_device(&port->dev);
> -bailout_serial_put:
> - usb_serial_put(serial);
> return retval;
> }
>
>