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

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Wed, Feb 28, 2018 at 09:58:13AM +0800, Fam Zheng wrote:
> 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.

You are right.

This race condition also affects the existing 'stop' command, where
ioeventfd is disabled in the same way.  I'll send a v2 with a patch to
fix this.

Stefan

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