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

[Jason Wang <jasowang@redhat.com>]

On 07/22/2013 01:45 PM, 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().

Yes, but the caller will wait then and since the wakeup was called
before adding into waitqueue. It may block forever?

>
> Cheers,
> Rusty.