Re: [PATCH v3 4/5] vhost-user-scsi: support reconnect to backend

[Markus Armbruster]

Li Feng <fengli@smartx.com> writes:

> If the backend crashes and restarts, the device is broken.
> This patch adds reconnect for vhost-user-scsi.
>
> Tested with spdk backend.
>
> Signed-off-by: Li Feng <fengli@smartx.com>
> ---
>  hw/scsi/vhost-user-scsi.c           | 199 +++++++++++++++++++++++++---
>  include/hw/virtio/vhost-user-scsi.h |   4 +
>  2 files changed, 184 insertions(+), 19 deletions(-)
>
> diff --git a/hw/scsi/vhost-user-scsi.c b/hw/scsi/vhost-user-scsi.c
> index ee99b19e7a..5bf012461b 100644
> --- a/hw/scsi/vhost-user-scsi.c
> +++ b/hw/scsi/vhost-user-scsi.c
> @@ -43,26 +43,54 @@ enum VhostUserProtocolFeature {
>      VHOST_USER_PROTOCOL_F_RESET_DEVICE = 13,
>  };
>  
> +static int vhost_user_scsi_start(VHostUserSCSI *s)
> +{
> +    VHostSCSICommon *vsc = VHOST_SCSI_COMMON(s);
> +    int ret;
> +
> +    ret = vhost_scsi_common_start(vsc);
> +    s->started_vu = (ret < 0 ? false : true);
> +
> +    return ret;
> +}
> +
> +static void vhost_user_scsi_stop(VHostUserSCSI *s)
> +{
> +    VHostSCSICommon *vsc = VHOST_SCSI_COMMON(s);
> +
> +    if (!s->started_vu) {
> +        return;
> +    }
> +    s->started_vu = false;
> +
> +    vhost_scsi_common_stop(vsc);
> +}
> +
>  static void vhost_user_scsi_set_status(VirtIODevice *vdev, uint8_t status)
>  {
>      VHostUserSCSI *s = (VHostUserSCSI *)vdev;
> +    DeviceState *dev = &s->parent_obj.parent_obj.parent_obj.parent_obj;
>      VHostSCSICommon *vsc = VHOST_SCSI_COMMON(s);
> -    bool start = (status & VIRTIO_CONFIG_S_DRIVER_OK) && vdev->vm_running;
> +    VirtIOSCSICommon *vs = VIRTIO_SCSI_COMMON(dev);
> +    bool should_start = virtio_device_should_start(vdev, status);
> +    int ret;
>  
> -    if (vhost_dev_is_started(&vsc->dev) == start) {
> +    if (!s->connected) {
>          return;
>      }
>  
> -    if (start) {
> -        int ret;
> +    if (vhost_dev_is_started(&vsc->dev) == should_start) {
> +        return;
> +    }
>  
> -        ret = vhost_scsi_common_start(vsc);
> +    if (should_start) {
> +        ret = vhost_user_scsi_start(s);
>          if (ret < 0) {
>              error_report("unable to start vhost-user-scsi: %s", 
> strerror(-ret));
> -            exit(1);
> +            qemu_chr_fe_disconnect(&vs->conf.chardev);
>          }
>      } else {
> -        vhost_scsi_common_stop(vsc);
> +        vhost_user_scsi_stop(s);
>      }
>  }
>  
> @@ -89,14 +117,126 @@ static void vhost_dummy_handle_output(VirtIODevice 
> *vdev, VirtQueue *vq)
>  {
>  }
>  
> +static int vhost_user_scsi_connect(DeviceState *dev, Error **errp)
> +{
> +    VirtIODevice *vdev = VIRTIO_DEVICE(dev);
> +    VHostUserSCSI *s = VHOST_USER_SCSI(vdev);
> +    VHostSCSICommon *vsc = VHOST_SCSI_COMMON(s);
> +    VirtIOSCSICommon *vs = VIRTIO_SCSI_COMMON(dev);
> +    int ret = 0;
> +
> +    if (s->connected) {
> +        return 0;
> +    }
> +    s->connected = true;
> +
> +    vsc->dev.num_queues = vs->conf.num_queues;
> +    vsc->dev.nvqs = VIRTIO_SCSI_VQ_NUM_FIXED + vs->conf.num_queues;
> +    vsc->dev.vqs = s->vhost_vqs;
> +    vsc->dev.vq_index = 0;
> +    vsc->dev.backend_features = 0;
> +
> +    ret = vhost_dev_init(&vsc->dev, &s->vhost_user, VHOST_BACKEND_TYPE_USER, 
> 0,
> +                         errp);
> +    if (ret < 0) {
> +        return ret;
> +    }
> +
> +    /* restore vhost state */
> +    if (virtio_device_started(vdev, vdev->status)) {
> +        ret = vhost_user_scsi_start(s);
> +        if (ret < 0) {
> +            return ret;
> +        }

Fails without setting an error, violating the function's (tacit)
postcondition.  Callers:

* vhost_user_scsi_event()

  Dereferences the null error and crashes.

* vhost_user_scsi_realize_connect()

  Also fails without setting an error.  Caller:

  - vhost_user_scsi_realize()

    1. Doesn't emit the "Reconnecting after error: " message.  See
       also below.

    2. Succeeds instead of failing!

> +    }
> +
> +    return 0;
> +}
> +
> +static void vhost_user_scsi_event(void *opaque, QEMUChrEvent event);
> +
> +static void vhost_user_scsi_disconnect(DeviceState *dev)
> +{
> +    VirtIODevice *vdev = VIRTIO_DEVICE(dev);
> +    VHostUserSCSI *s = VHOST_USER_SCSI(vdev);
> +    VHostSCSICommon *vsc = VHOST_SCSI_COMMON(s);
> +    VirtIOSCSICommon *vs = VIRTIO_SCSI_COMMON(dev);
> +
> +    if (!s->connected) {
> +        return;
> +    }
> +    s->connected = false;
> +
> +    vhost_user_scsi_stop(s);
> +
> +    vhost_dev_cleanup(&vsc->dev);
> +
> +    /* Re-instate the event handler for new connections */
> +    qemu_chr_fe_set_handlers(&vs->conf.chardev, NULL, NULL,
> +                             vhost_user_scsi_event, NULL, dev, NULL, true);
> +}
> +
> +static void vhost_user_scsi_event(void *opaque, QEMUChrEvent event)
> +{
> +    DeviceState *dev = opaque;
> +    VirtIODevice *vdev = VIRTIO_DEVICE(dev);
> +    VHostUserSCSI *s = VHOST_USER_SCSI(vdev);
> +    VHostSCSICommon *vsc = VHOST_SCSI_COMMON(s);
> +    VirtIOSCSICommon *vs = VIRTIO_SCSI_COMMON(dev);
> +    Error *local_err = NULL;
> +
> +    switch (event) {
> +    case CHR_EVENT_OPENED:
> +        if (vhost_user_scsi_connect(dev, &local_err) < 0) {
> +            error_report_err(local_err);
> +            qemu_chr_fe_disconnect(&vs->conf.chardev);
> +            return;
> +        }
> +        break;
> +    case CHR_EVENT_CLOSED:
> +        /* defer close until later to avoid circular close */
> +        vhost_user_async_close(dev, &vs->conf.chardev, &vsc->dev,
> +                               vhost_user_scsi_disconnect);
> +        break;
> +    case CHR_EVENT_BREAK:
> +    case CHR_EVENT_MUX_IN:
> +    case CHR_EVENT_MUX_OUT:
> +        /* Ignore */
> +        break;
> +    }
> +}
> +
> +static int vhost_user_scsi_realize_connect(VHostUserSCSI *s, Error **errp)
> +{
> +    DeviceState *dev = &s->parent_obj.parent_obj.parent_obj.parent_obj;
> +    VirtIOSCSICommon *vs = VIRTIO_SCSI_COMMON(dev);
> +    int ret;
> +
> +    s->connected = false;
> +
> +    ret = qemu_chr_fe_wait_connected(&vs->conf.chardev, errp);
> +    if (ret < 0) {
> +        return ret;
> +    }
> +
> +    ret = vhost_user_scsi_connect(dev, errp);
> +    if (ret < 0) {
> +        qemu_chr_fe_disconnect(&vs->conf.chardev);
> +        return ret;
> +    }
> +    assert(s->connected);
> +
> +    return 0;
> +}
> +
>  static void vhost_user_scsi_realize(DeviceState *dev, Error **errp)
>  {
>      VirtIOSCSICommon *vs = VIRTIO_SCSI_COMMON(dev);
>      VHostUserSCSI *s = VHOST_USER_SCSI(dev);
>      VHostSCSICommon *vsc = VHOST_SCSI_COMMON(s);
> -    struct vhost_virtqueue *vqs = NULL;
>      Error *err = NULL;
>      int ret;
> +    int retries = VU_REALIZE_CONN_RETRIES;
>  
>      if (!vs->conf.chardev.chr) {
>          error_setg(errp, "vhost-user-scsi: missing chardev");
> @@ -115,18 +255,28 @@ static void vhost_user_scsi_realize(DeviceState *dev, 
> Error **errp)
>          goto free_virtio;
>      }
>  
> -    vsc->dev.nvqs = VIRTIO_SCSI_VQ_NUM_FIXED + vs->conf.num_queues;
> -    vsc->dev.vqs = g_new0(struct vhost_virtqueue, vsc->dev.nvqs);
> -    vsc->dev.vq_index = 0;
> -    vsc->dev.backend_features = 0;
> -    vqs = vsc->dev.vqs;
> +    vsc->inflight = g_new0(struct vhost_inflight, 1);
> +    s->vhost_vqs = g_new0(struct vhost_virtqueue,
> +                          VIRTIO_SCSI_VQ_NUM_FIXED + vs->conf.num_queues);
> +
> +    assert(!*errp);

Dereferencing *errp is wrong.  Quoting qapi/error.h's big comment:

 * = Why, when and how to use ERRP_GUARD() =
 *
 * Without ERRP_GUARD(), use of the @errp parameter is restricted:
 * - It must not be dereferenced, because it may be null.
 * - It should not be passed to error_prepend() or
 *   error_append_hint(), because that doesn't work with &error_fatal.
 * ERRP_GUARD() lifts these restrictions.

See there for how to use ERRP_GUARD().

> +    do {
> +        if (*errp) {
> +            error_prepend(errp, "Reconnecting after error: ");
> +            error_report_err(*errp);
> +            *errp = NULL;
> +        }
> +        ret = vhost_user_scsi_realize_connect(s, errp);
> +    } while (ret < 0 && retries--);

Reports "Reconnecting ..." to the HMP monitor when in HMP context, else
to stderr.  I.e. reports to stderr when we're in QMP context.  Is this
really what we want?

>  
> -    ret = vhost_dev_init(&vsc->dev, &s->vhost_user,
> -                         VHOST_BACKEND_TYPE_USER, 0, errp);
>      if (ret < 0) {
>          goto free_vhost;
>      }
>  
> +    /* we're fully initialized, now we can operate, so add the handler */
> +    qemu_chr_fe_set_handlers(&vs->conf.chardev,  NULL, NULL,
> +                             vhost_user_scsi_event, NULL, (void *)dev,
> +                             NULL, true);
>      /* Channel and lun both are 0 for bootable vhost-user-scsi disk */
>      vsc->channel = 0;
>      vsc->lun = 0;
> @@ -135,8 +285,12 @@ static void vhost_user_scsi_realize(DeviceState *dev, 
> Error **errp)
>      return;
>  
>  free_vhost:
> +    g_free(s->vhost_vqs);
> +    s->vhost_vqs = NULL;
> +    g_free(vsc->inflight);
> +    vsc->inflight = NULL;
>      vhost_user_cleanup(&s->vhost_user);
> -    g_free(vqs);
> +
>  free_virtio:
>      virtio_scsi_common_unrealize(dev);
>  }
> @@ -146,16 +300,23 @@ static void vhost_user_scsi_unrealize(DeviceState *dev)
>      VirtIODevice *vdev = VIRTIO_DEVICE(dev);
>      VHostUserSCSI *s = VHOST_USER_SCSI(dev);
>      VHostSCSICommon *vsc = VHOST_SCSI_COMMON(s);
> -    struct vhost_virtqueue *vqs = vsc->dev.vqs;
> +    VirtIOSCSICommon *vs = VIRTIO_SCSI_COMMON(dev);
>  
>      /* This will stop the vhost backend. */
>      vhost_user_scsi_set_status(vdev, 0);
> +    qemu_chr_fe_set_handlers(&vs->conf.chardev, NULL, NULL, NULL, NULL, NULL,
> +                             NULL, false);
>  
>      vhost_dev_cleanup(&vsc->dev);
> -    g_free(vqs);
> +    g_free(s->vhost_vqs);
> +    s->vhost_vqs = NULL;
> +
> +    vhost_dev_free_inflight(vsc->inflight);
> +    g_free(vsc->inflight);
> +    vsc->inflight = NULL;
>  
> -    virtio_scsi_common_unrealize(dev);
>      vhost_user_cleanup(&s->vhost_user);
> +    virtio_scsi_common_unrealize(dev);
>  }
>  
>  static Property vhost_user_scsi_properties[] = {
> diff --git a/include/hw/virtio/vhost-user-scsi.h 
> b/include/hw/virtio/vhost-user-scsi.h
> index 521b08e559..b405ec952a 100644
> --- a/include/hw/virtio/vhost-user-scsi.h
> +++ b/include/hw/virtio/vhost-user-scsi.h
> @@ -29,6 +29,10 @@ OBJECT_DECLARE_SIMPLE_TYPE(VHostUserSCSI, VHOST_USER_SCSI)
>  struct VHostUserSCSI {
>      VHostSCSICommon parent_obj;
>      VhostUserState vhost_user;
> +    bool connected;
> +    bool started_vu;
> +
> +    struct vhost_virtqueue *vhost_vqs;
>  };
>  
>  #endif /* VHOST_USER_SCSI_H */