Re: [PATCH 3/4] vsock/virtio: fix flush of works during the .remove()

[Stefano Garzarella]

On Thu, Jun 13, 2019 at 04:57:15PM +0800, Jason Wang wrote:
> 
> On 2019/6/6 äå4:11, Stefano Garzarella wrote:
> > On Fri, May 31, 2019 at 05:56:39PM +0800, Jason Wang wrote:
> > > On 2019/5/31 äå4:18, Stefano Garzarella wrote:
> > > > On Thu, May 30, 2019 at 07:59:14PM +0800, Jason Wang wrote:
> > > > > On 2019/5/30 äå6:10, Stefano Garzarella wrote:
> > > > > > On Thu, May 30, 2019 at 05:46:18PM +0800, Jason Wang wrote:
> > > > > > > On 2019/5/29 äå6:58, Stefano Garzarella wrote:
> > > > > > > > On Wed, May 29, 2019 at 11:22:40AM +0800, Jason Wang wrote:
> > > > > > > > > On 2019/5/28 äå6:56, Stefano Garzarella wrote:
> > > > > > > > > > @@ -690,6 +693,9 @@ static void virtio_vsock_remove(struct virtio_device *vdev)
> > > > > > > > > >       	vsock->event_run = false;
> > > > > > > > > >       	mutex_unlock(&vsock->event_lock);
> > > > > > > > > > +	/* Flush all pending works */
> > > > > > > > > > +	virtio_vsock_flush_works(vsock);
> > > > > > > > > > +
> > > > > > > > > >       	/* Flush all device writes and interrupts, device will not use any
> > > > > > > > > >       	 * more buffers.
> > > > > > > > > >       	 */
> > > > > > > > > > @@ -726,6 +732,11 @@ static void virtio_vsock_remove(struct virtio_device *vdev)
> > > > > > > > > >       	/* Delete virtqueues and flush outstanding callbacks if any */
> > > > > > > > > >       	vdev->config->del_vqs(vdev);
> > > > > > > > > > +	/* Other works can be queued before 'config->del_vqs()', so we flush
> > > > > > > > > > +	 * all works before to free the vsock object to avoid use after free.
> > > > > > > > > > +	 */
> > > > > > > > > > +	virtio_vsock_flush_works(vsock);
> > > > > > > > > Some questions after a quick glance:
> > > > > > > > > 
> > > > > > > > > 1) It looks to me that the work could be queued from the path of
> > > > > > > > > vsock_transport_cancel_pkt() . Is that synchronized here?
> > > > > > > > > 
> > > > > > > > Both virtio_transport_send_pkt() and vsock_transport_cancel_pkt() can
> > > > > > > > queue work from the upper layer (socket).
> > > > > > > > 
> > > > > > > > Setting the_virtio_vsock to NULL, should synchronize, but after a careful look
> > > > > > > > a rare issue could happen:
> > > > > > > > we are setting the_virtio_vsock to NULL at the start of .remove() and we
> > > > > > > > are freeing the object pointed by it at the end of .remove(), so
> > > > > > > > virtio_transport_send_pkt() or vsock_transport_cancel_pkt() may still be
> > > > > > > > running, accessing the object that we are freed.
> > > > > > > Yes, that's my point.
> > > > > > > 
> > > > > > > 
> > > > > > > > Should I use something like RCU to prevent this issue?
> > > > > > > > 
> > > > > > > >         virtio_transport_send_pkt() and vsock_transport_cancel_pkt()
> > > > > > > >         {
> > > > > > > >             rcu_read_lock();
> > > > > > > >             vsock = rcu_dereference(the_virtio_vsock_mutex);
> > > > > > > RCU is probably a way to go. (Like what vhost_transport_send_pkt() did).
> > > > > > > 
> > > > > > Okay, I'm going this way.
> > > > > > 
> > > > > > > >             ...
> > > > > > > >             rcu_read_unlock();
> > > > > > > >         }
> > > > > > > > 
> > > > > > > >         virtio_vsock_remove()
> > > > > > > >         {
> > > > > > > >             rcu_assign_pointer(the_virtio_vsock_mutex, NULL);
> > > > > > > >             synchronize_rcu();
> > > > > > > > 
> > > > > > > >             ...
> > > > > > > > 
> > > > > > > >             free(vsock);
> > > > > > > >         }
> > > > > > > > 
> > > > > > > > Could there be a better approach?
> > > > > > > > 
> > > > > > > > 
> > > > > > > > > 2) If we decide to flush after dev_vqs(), is tx_run/rx_run/event_run still
> > > > > > > > > needed? It looks to me we've already done except that we need flush rx_work
> > > > > > > > > in the end since send_pkt_work can requeue rx_work.
> > > > > > > > The main reason of tx_run/rx_run/event_run is to prevent that a worker
> > > > > > > > function is running while we are calling config->reset().
> > > > > > > > 
> > > > > > > > E.g. if an interrupt comes between virtio_vsock_flush_works() and
> > > > > > > > config->reset(), it can queue new works that can access the device while
> > > > > > > > we are in config->reset().
> > > > > > > > 
> > > > > > > > IMHO they are still needed.
> > > > > > > > 
> > > > > > > > What do you think?
> > > > > > > I mean could we simply do flush after reset once and without tx_rx/rx_run
> > > > > > > tricks?
> > > > > > > 
> > > > > > > rest();
> > > > > > > 
> > > > > > > virtio_vsock_flush_work();
> > > > > > > 
> > > > > > > virtio_vsock_free_buf();
> > > > > > My only doubt is:
> > > > > > is it safe to call config->reset() while a worker function could access
> > > > > > the device?
> > > > > > 
> > > > > > I had this doubt reading the Michael's advice[1] and looking at
> > > > > > virtnet_remove() where there are these lines before the config->reset():
> > > > > > 
> > > > > > 	/* Make sure no work handler is accessing the device. */
> > > > > > 	flush_work(&vi->config_work);
> > > > > > 
> > > > > > Thanks,
> > > > > > Stefano
> > > > > > 
> > > > > > [1] https://lore.kernel.org/netdev/20190521055650-mutt-send-email-mst@xxxxxxxxxx
> > > > > Good point. Then I agree with you. But if we can use the RCU to detect the
> > > > > detach of device from socket for these, it would be even better.
> > > > > 
> > > > What about checking 'the_virtio_vsock' in the worker functions in a RCU
> > > > critical section?
> > > > In this way, I can remove the rx_run/tx_run/event_run.
> > > > 
> > > > Do you think it's cleaner?
> > > 
> > > Yes, I think so.
> > > 
> > Hi Jason,
> > while I was trying to use RCU also for workers, I discovered that it can
> > not be used if we can sleep. (Workers have mutex, memory allocation, etc.).
> > There is SRCU, but I think the rx_run/tx_run/event_run is cleaner.
> > 
> > So, if you agree I'd send a v2 using RCU only for the
> > virtio_transport_send_pkt() or vsock_transport_cancel_pkt(), and leave
> > this patch as is to be sure that no one is accessing the device while we
> > call config->reset().
> > 
> > Thanks,
> > Stefano
> 
> 
> If it work, I don't object to use that consider it was suggested by Michael.
> You can go this way and let's see.

Okay, I'll try if it works.

> 
> Personally I would like something more cleaner. E.g RCU + some kind of
> reference count (kref?).

I'll try to check if kref can help to have a cleaner solution in this
case.

Thanks for your comments,
Stefano