Re: [Qemu-devel] [Qemu-block] [PATCH] vl: introduce vm_shutdown()

[Fam Zheng <famz@redhat.com>]

On Tue, 02/27 15:30, Stefan Hajnoczi wrote:
> On Fri, Feb 23, 2018 at 04:20:44PM +0800, Fam Zheng wrote:
> > On Tue, 02/20 13:10, Stefan Hajnoczi wrote:
> > > 1. virtio_scsi_handle_cmd_vq() racing with iothread_stop_all() hits the
> > >    virtio_scsi_ctx_check() assertion failure because the BDS AioContext
> > >    has been modified by iothread_stop_all().
> > 
> > Does this patch fix the issue completely? IIUC virtio_scsi_handle_cmd can
> > already be entered at the time of main thread calling virtio_scsi_clear_aio(),
> > so this race condition still exists:
> > 
> >   main thread                           iothread
> > -----------------------------------------------------------------------------
> >   vm_shutdown
> >     ...
> >       virtio_bus_stop_ioeventfd
> >         virtio_scsi_dataplane_stop
> >                                         aio_poll()
> >                                           ...
> >                                             virtio_scsi_data_plane_handle_cmd()
> >           aio_context_acquire(s->ctx)
> >                                               virtio_scsi_acquire(s).enter
> >           virtio_scsi_clear_aio()
> >           aio_context_release(s->ctx)
> >                                               virtio_scsi_acquire(s).return
> >                                               virtio_scsi_handle_cmd_vq()
> >                                                 ...
> >                                                   virtqueue_pop()
> > 
> > Is it possible that the above virtqueue_pop() still returns one element that was
> > queued before vm_shutdown() was called?
> 
> No, it can't because virtio_scsi_clear_aio() invokes
> virtio_queue_host_notifier_aio_read(&vq->host_notifier) to process the
> virtqueue.  By the time we get back to iothread's
> virtio_scsi_data_plane_handle_cmd() the virtqueue is already empty.
> 
> Vcpus have been paused so no additional elements can slip into the
> virtqueue.

So there is:

static void virtio_queue_host_notifier_aio_read(EventNotifier *n)
{
    VirtQueue *vq = container_of(n, VirtQueue, host_notifier);
    if (event_notifier_test_and_clear(n)) {
        virtio_queue_notify_aio_vq(vq);
    }
}

Guest kicks after adding an element to VQ, but we check ioeventfd before trying
virtqueue_pop(). Is that a problem? If VCPUs are paused after enqueuing but
before kicking VQ, the ioeventfd is not set, the virtqueue is not processed
here.

Fam