From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jason Wang Subject: Re: [PATCH 06/10] virtio: console: fix race in port_fops_poll() and port unplug Date: Tue, 23 Jul 2013 11:01:02 +0800 Message-ID: <51EDF1EE.4010801@redhat.com> References: <51E8E4D6.7030807@redhat.com> <20130719072113.GL3087@amit-x200.redhat.com> <51E9123C.5040708@redhat.com> <20130719102941.GN3087@amit-x200.redhat.com> <87fvv7b11d.fsf@rustcorp.com.au> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <87fvv7b11d.fsf@rustcorp.com.au> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: virtualization-bounces@lists.linux-foundation.org Errors-To: virtualization-bounces@lists.linux-foundation.org To: Rusty Russell Cc: Amit Shah , Virtualization List List-Id: virtualization@lists.linuxfoundation.org On 07/22/2013 01:45 PM, Rusty Russell wrote: > Amit Shah 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 >>>>>> --- >>>>>> 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.