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Re: [Qemu-block] [PATCH] vl: introduce vm_shutdown()
From: |
Stefan Hajnoczi |
Subject: |
Re: [Qemu-block] [PATCH] vl: introduce vm_shutdown() |
Date: |
Tue, 27 Feb 2018 15:30:25 +0000 |
User-agent: |
Mutt/1.9.2 (2017-12-15) |
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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