From mboxrd@z Thu Jan 1 00:00:00 1970 From: Rusty Russell Subject: Re: [PATCH 06/10] virtio: console: fix race in port_fops_poll() and port unplug Date: Mon, 22 Jul 2013 15:15:34 +0930 Message-ID: <87fvv7b11d.fsf@rustcorp.com.au> References: <51E8E4D6.7030807@redhat.com> <20130719072113.GL3087@amit-x200.redhat.com> <51E9123C.5040708@redhat.com> <20130719102941.GN3087@amit-x200.redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20130719102941.GN3087@amit-x200.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: Amit Shah , Jason Wang Cc: Virtualization List List-Id: virtualization@lists.linuxfoundation.org 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(). Cheers, Rusty.