From mboxrd@z Thu Jan 1 00:00:00 1970 From: Amit Shah Subject: Re: [PATCH 06/10] virtio: console: fix race in port_fops_poll() and port unplug Date: Fri, 19 Jul 2013 15:59:42 +0530 Message-ID: <20130719102941.GN3087@amit-x200.redhat.com> References: <51E8E4D6.7030807@redhat.com> <20130719072113.GL3087@amit-x200.redhat.com> <51E9123C.5040708@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Content-Disposition: inline In-Reply-To: <51E9123C.5040708@redhat.com> 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: Jason Wang Cc: Virtualization List List-Id: virtualization@lists.linuxfoundation.org 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. > But since it was existed even w/o this series. I agree to keep it as is > and fix on top. Yes, I think so too. > Other looks good. Thanks for your review! Amit