Re: [PATCH 06/10] virtio: console: fix race in port_fops_poll() and port unplug

[Amit Shah <amit.shah@redhat.com>]

On (Mon) 22 Jul 2013 [15:15:34], Rusty Russell wrote:
> Amit Shah <amit.shah@redhat.com> writes:
> > On (Fri) 19 Jul 2013 [18:17:32], Jason Wang wrote:
> >> On 07/19/2013 03:48 PM, Amit Shah wrote:
> >> > On (Fri) 19 Jul 2013 [15:03:50], Jason Wang wrote:
> >> >> On 07/19/2013 04:16 AM, Amit Shah wrote:
> >> >>> Between poll() being called and processed, the port can be unplugged.
> >> >>> Check if this happened, and bail out.
> >> >>>
> >> >>> Signed-off-by: Amit Shah <amit.shah@redhat.com>
> >> >>> ---
> >> >>>  drivers/char/virtio_console.c | 4 ++++
> >> >>>  1 file changed, 4 insertions(+)
> >> >>>
> >> >>> diff --git a/drivers/char/virtio_console.c b/drivers/char/virtio_console.c
> >> >>> index 7728af9..1d4b748 100644
> >> >>> --- a/drivers/char/virtio_console.c
> >> >>> +++ b/drivers/char/virtio_console.c
> >> >>> @@ -967,6 +967,10 @@ static unsigned int port_fops_poll(struct file *filp, poll_table *wait)
> >> >>>  	unsigned int ret;
> >> >>>  
> >> >>>  	port = filp->private_data;
> >> >>> +	if (!port->guest_connected) {
> >> >>> +		/* Port was unplugged before we could proceed */
> >> >>> +		return POLLHUP;
> >> >>> +	}
> >> >>>  	poll_wait(filp, &port->waitqueue, wait);
> >> >>>  
> >> >>>  	if (!port->guest_connected) {
> >> >> Looks still racy here. Unlike port_fops_read() which check
> >> >> will_read_block(). If unplug happens after the check but before the
> >> >> poll_wait(), caller will be blocked forever.
> >> > unplug_port() calls wake_up_interruptible on the waitqueue.
> >> 
> >> I mean the following cases:
> >
> > (formatting to fit properly:)
> >
> >> 
> >> CPU0:                                CPU1: unplug_port()
> >> 
> >> if (!port->guest_connected) {
> >>     return POLLHUP;
> >> }
> >>                                      wake_up_interruptiable()
> >> 
> >> poll_wait(filp, &port->waitqueue, wait);
> >
> > Agreed, this can happen.  I can't think of a way to resolve this.  One
> > way would be to remove the waitqueue (port->waitqueue = NULL in
> > unplug_port()), but I'm not sure of the effect on the other parts
> > yet.  I'll leave this one for later analysis.
> 
> No, you are confused by the name, I think,
> 
> poll_wait() doesn't actually wait.  It's more like a poll_enqueue().

Ah!  Thanks, yes.

I'll drop this patch.

		Amit