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

[Stefan Hajnoczi <stefanha@gmail.com>]

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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.

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