Amit Shah <amit.shah@xxxxxxxxxx> 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@xxxxxxxxxx>
>> >>> ---
>> >>> 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().
Cheers,
Rusty.
_______________________________________________
Virtualization mailing list
Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx
https://lists.linuxfoundation.org/mailman/listinfo/virtualization
