Re: [PATCH v2 5/5] vhost: add device started check in migration set log

[Dima Stepanov <dimastep@yandex-team.ru>]

On Tue, May 12, 2020 at 11:32:50AM +0800, Jason Wang wrote:
> 
> On 2020/5/11 下午5:25, Dima Stepanov wrote:
> >On Mon, May 11, 2020 at 11:15:53AM +0800, Jason Wang wrote:
> >>On 2020/4/30 下午9:36, Dima Stepanov wrote:
> >>>If vhost-user daemon is used as a backend for the vhost device, then we
> >>>should consider a possibility of disconnect at any moment. If such
> >>>disconnect happened in the vhost_migration_log() routine the vhost
> >>>device structure will be clean up.
> >>>At the start of the vhost_migration_log() function there is a check:
> >>>   if (!dev->started) {
> >>>       dev->log_enabled = enable;
> >>>       return 0;
> >>>   }
> >>>To be consistent with this check add the same check after calling the
> >>>vhost_dev_set_log() routine. This in general help not to break a
> >>>migration due the assert() message. But it looks like that this code
> >>>should be revised to handle these errors more carefully.
> >>>
> >>>In case of vhost-user device backend the fail paths should consider the
> >>>state of the device. In this case we should skip some function calls
> >>>during rollback on the error paths, so not to get the NULL dereference
> >>>errors.
> >>>
> >>>Signed-off-by: Dima Stepanov <dimastep@yandex-team.ru>
> >>>---
> >>>  hw/virtio/vhost.c | 39 +++++++++++++++++++++++++++++++++++----
> >>>  1 file changed, 35 insertions(+), 4 deletions(-)
> >>>
> >>>diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
> >>>index 3ee50c4..d5ab96d 100644
> >>>--- a/hw/virtio/vhost.c
> >>>+++ b/hw/virtio/vhost.c
> >>>@@ -787,6 +787,17 @@ static int vhost_dev_set_features(struct vhost_dev *dev,
> >>>  static int vhost_dev_set_log(struct vhost_dev *dev, bool enable_log)
> >>>  {
> >>>      int r, i, idx;
> >>>+
> >>>+    if (!dev->started) {
> >>>+        /*
> >>>+         * If vhost-user daemon is used as a backend for the
> >>>+         * device and the connection is broken, then the vhost_dev
> >>>+         * structure will be reset all its values to 0.
> >>>+         * Add additional check for the device state.
> >>>+         */
> >>>+        return -1;
> >>>+    }
> >>>+
> >>>      r = vhost_dev_set_features(dev, enable_log);
> >>>      if (r < 0) {
> >>>          goto err_features;
> >>>@@ -801,12 +812,19 @@ static int vhost_dev_set_log(struct vhost_dev *dev, bool enable_log)
> >>>      }
> >>>      return 0;
> >>>  err_vq:
> >>>-    for (; i >= 0; --i) {
> >>>+    /*
> >>>+     * Disconnect with the vhost-user daemon can lead to the
> >>>+     * vhost_dev_cleanup() call which will clean up vhost_dev
> >>>+     * structure.
> >>>+     */
> >>>+    for (; dev->started && (i >= 0); --i) {
> >>>          idx = dev->vhost_ops->vhost_get_vq_index(
> >>
> >>Why need the check of dev->started here, can started be modified outside
> >>mainloop? If yes, I don't get the check of !dev->started in the beginning of
> >>this function.
> >>
> >No dev->started can't change outside the mainloop. The main problem is
> >only for the vhost_user_blk daemon. Consider the case when we
> >successfully pass the dev->started check at the beginning of the
> >function, but after it we hit the disconnect on the next call on the
> >second or third iteration:
> >      r = vhost_virtqueue_set_addr(dev, dev->vqs + i, idx, enable_log);
> >The unix socket backend device will call the disconnect routine for this
> >device and reset the structure. So the structure will be reset (and
> >dev->started set to false) inside this set_addr() call.
> 
> 
> I still don't get here. I think the disconnect can not happen in the middle
> of vhost_dev_set_log() since both of them were running in mainloop. And even
> if it can, we probably need other synchronization mechanism other than
> simple check here.
Disconnect isn't happened in the separate thread it is happened in this
routine inside vhost_dev_set_log. When for instance vhost_user_write()
call failed:
  vhost_user_set_log_base()
    vhost_user_write()
      vhost_user_blk_disconnect()
        vhost_dev_cleanup()
          vhost_user_backend_cleanup()
So the point is that if we somehow got a disconnect with the
vhost-user-blk daemon before the vhost_user_write() call then it will
continue clean up by running vhost_user_blk_disconnect() function. I
wrote a more detailed backtrace stack in the separate thread, which is
pretty similar to what we have here:
  Re: [PATCH v2 4/5] vhost: check vring address before calling unmap
The places are different but the problem is pretty similar.

So if vhost-user commands handshake then everything is fine and
reconnect will work as expected. The only problem is how to handle
reconnect properly between vhost-user command send/receive.

As i wrote we have a test:
  - run src VM with vhost-usr-blk daemon used
  - run fio inside it
  - perform reconnect every X seconds (just kill and restart daemon),
    X is random
  - run dst VM
  - perform migration
  - fio should complete in dst VM
And we cycle this test like forever.
So it fails once per ~25 iteration. By adding some delays inside qemu we
were able to make the race window larger.

> 
> 
> >  So
> >we shouldn't call the clean up calls because this virtqueues were clean
> >up in the disconnect call. But we should protect these calls somehow, so
> >it will not hit SIGSEGV and we will be able to pass migration.
> >
> >Just to summarize it:
> >For the vhost-user-blk devices we ca hit clean up calls twice in case of
> >vhost disconnect:
> >1. The first time during the disconnect process. The clean up is called
> >inside it.
> >2. The second time during roll back clean up.
> >So if it is the case we should skip p2.
> >
> >>>dev, dev->vq_index + i);
> >>>          vhost_virtqueue_set_addr(dev, dev->vqs + i, idx,
> >>>                                   dev->log_enabled);
> >>>      }
> >>>-    vhost_dev_set_features(dev, dev->log_enabled);
> >>>+    if (dev->started) {
> >>>+        vhost_dev_set_features(dev, dev->log_enabled);
> >>>+    }
> >>>  err_features:
> >>>      return r;
> >>>  }
> >>>@@ -832,7 +850,15 @@ static int vhost_migration_log(MemoryListener *listener, int enable)
> >>>      } else {
> >>>          vhost_dev_log_resize(dev, vhost_get_log_size(dev));
> >>>          r = vhost_dev_set_log(dev, true);
> >>>-        if (r < 0) {
> >>>+        /*
> >>>+         * The dev log resize can fail, because of disconnect
> >>>+         * with the vhost-user-blk daemon. Check the device
> >>>+         * state before calling the vhost_dev_set_log()
> >>>+         * function.
> >>>+         * Don't return error if device isn't started to be
> >>>+         * consistent with the check above.
> >>>+         */
> >>>+        if (dev->started && r < 0) {
> >>>              return r;
> >>>          }
> >>>      }
> >>>@@ -1739,7 +1765,12 @@ int vhost_dev_start(struct vhost_dev *hdev, VirtIODevice *vdev)
> >>>  fail_log:
> >>>      vhost_log_put(hdev, false);
> >>>  fail_vq:
> >>>-    while (--i >= 0) {
> >>>+    /*
> >>>+     * Disconnect with the vhost-user daemon can lead to the
> >>>+     * vhost_dev_cleanup() call which will clean up vhost_dev
> >>>+     * structure.
> >>>+     */
> >>>+    while ((--i >= 0) && (hdev->started)) {
> >>>          vhost_virtqueue_stop(hdev,
> >>>                               vdev,
> >>>                               hdev->vqs + i,
> >>
> >>This should be a separate patch.
> >Do you mean i should split this patch to two patches?
> 
> 
> Yes.
> 
> Thanks

Got it. Will do it in v3.

No other comments mixed in below.

Thanks.

> 
> 
> >
> >Thanks.
> >
> >>Thanks
> >>
>