hvc_console: Don't access hvc_task if not initialised

[Amit Shah]

On (Mon) 28 Mar 2011 [11:52:05], Milton Miller wrote:
> On Fri, 25 Mar 2011 about 14:17:14 +0530, Amit Shah wrote:
> > On (Thu) 24 Mar 2011 [08:58:04], Milton Miller wrote:
> > > On Thu, 24 Mar 2011 07:29:58 -0000, Amit Shah wrote:
> > > > hvc_open() can be called without having any backing device.  This
> > > > results in a call to hvc_kick() which calls wake_up_process on a NULL
> > > > pointer.  
> > > 
> > > How is hvc_open called without a hvc_driver registered to the tty layer?
> > 
> > This gets reproduced in a couple of scenarios, I'm trying to get more
> > information.

OK - I finally could reproduce myself, albiet it's a panic in
hvc_open, not the one mentioned earlier.

hvc_console is built into the kernel and virtio_console is a module.
This sequence triggers a panic:

- modprobe virtio_console
- agetty /dev/hvc0 9600 vt100
- rmmod virtio_console
- modprobe virtio_console
- agetty /dev/hvc0 9600 vt100

A patch that I had sent previously, to hvc_remove() a port when the
associated virtio_console port gets unplugged, fixes this panic.

Stricter checking in hvc_open(), as you mentioned, will solve the
other one as well.

Thanks!

		Amit