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Date: Fri, 23 Feb 2018 16:20:44 +0800
From: Fam Zheng <famz@redhat.com>
Message-ID: <20180223082044.GB31530@lemon.usersys.redhat.com>
References: <20180220131014.8998-1-stefanha@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH] vl: introduce vm_shutdown()
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To: Stefan Hajnoczi <stefanha@redhat.com>
Cc: qemu-devel@nongnu.org, qemu-block@nongnu.org, Kevin Wolf <kwolf@redhat.com>, pbonzini@redhat.com

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?

If so I think we additionally need to an "s->ioeventfd_stopped" flag set in
virtio_scsi_stop_ioeventfd() and check it in
virtio_scsi_data_plane_handle_cmd().

Fam