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Re: [Qemu-devel] [Qemu-block] [PATCH] vl: introduce vm_shutdown()
From: |
Fam Zheng |
Subject: |
Re: [Qemu-devel] [Qemu-block] [PATCH] vl: introduce vm_shutdown() |
Date: |
Wed, 28 Feb 2018 09:58:13 +0800 |
User-agent: |
Mutt/1.9.2 (2017-12-15) |
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