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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: |
Wed, 28 Feb 2018 09:40:37 +0000 |
User-agent: |
Mutt/1.9.2 (2017-12-15) |
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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