Amit Shah <[email protected]> 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 <[email protected]> >> >>> --- >> >>> 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 [email protected] https://lists.linuxfoundation.org/mailman/listinfo/virtualization
