Re: [Qemu-devel] [PATCH 5/8] virtio-blk: fix "disabled data plane" mode

[Cornelia Huck]

On Tue, 15 Mar 2016 20:45:30 +0800
Fam Zheng <address@hidden> wrote:

> On Fri, 03/11 11:28, Paolo Bonzini wrote:

> > But secondarily, I'm thinking of making the logic simpler to understand 
> > in two ways:
> > 
> > 1) adding a mutex around virtio_blk_data_plane_start/stop.
> > 
> > 2) moving
> > 
> >     event_notifier_set(virtio_queue_get_host_notifier(s->vq));
> >     virtio_queue_aio_set_host_notifier_handler(s->vq, s->ctx, true, true);
> > 
> > to a bottom half (created with aio_bh_new in s->ctx).  The bottom half
> > takes the mutex, checks again "if (vblk->dataplane_started)" and if it's
> > true starts the processing.
> 
> Like this? If it captures your idea, could Bo or Christian help test?
> 
> ---
> 
> From b5b8886693828d498ee184fc7d4e13d8c06cdf39 Mon Sep 17 00:00:00 2001
> From: Fam Zheng <address@hidden>
> Date: Thu, 10 Mar 2016 10:26:36 +0800
> Subject: [PATCH] virtio-blk dataplane start crash fix
> 
> Suggested-by: Paolo Bonzini <address@hidden>
> Signed-off-by: Fam Zheng <address@hidden>
> ---
>  block.c                         |  4 +++-
>  hw/block/dataplane/virtio-blk.c | 39 ++++++++++++++++++++++++++++++++-------
>  2 files changed, 35 insertions(+), 8 deletions(-)
> 
> diff --git a/block.c b/block.c
> index ba24b8e..e37e8f7 100644
> --- a/block.c
> +++ b/block.c
> @@ -4093,7 +4093,9 @@ void bdrv_attach_aio_context(BlockDriverState *bs,
> 
>  void bdrv_set_aio_context(BlockDriverState *bs, AioContext *new_context)
>  {
> -    bdrv_drain(bs); /* ensure there are no in-flight requests */
> +    /* ensure there are no in-flight requests */
> +    bdrv_drained_begin(bs);
> +    bdrv_drained_end(bs);
> 
>      bdrv_detach_aio_context(bs);
> 
> diff --git a/hw/block/dataplane/virtio-blk.c b/hw/block/dataplane/virtio-blk.c
> index 36f3d2b..6db5c22 100644
> --- a/hw/block/dataplane/virtio-blk.c
> +++ b/hw/block/dataplane/virtio-blk.c
> @@ -49,6 +49,8 @@ struct VirtIOBlockDataPlane {
> 
>      /* Operation blocker on BDS */
>      Error *blocker;
> +
> +    QemuMutex start_lock;
>  };
> 
>  /* Raise an interrupt to signal guest, if necessary */
> @@ -150,6 +152,7 @@ void virtio_blk_data_plane_create(VirtIODevice *vdev, 
> VirtIOBlkConf *conf,
>      s = g_new0(VirtIOBlockDataPlane, 1);
>      s->vdev = vdev;
>      s->conf = conf;
> +    qemu_mutex_init(&s->start_lock);
> 
>      if (conf->iothread) {
>          s->iothread = conf->iothread;
> @@ -184,15 +187,38 @@ void virtio_blk_data_plane_destroy(VirtIOBlockDataPlane 
> *s)
>      g_free(s);
>  }
> 
> +typedef struct {
> +    VirtIOBlockDataPlane *s;
> +    QEMUBH *bh;
> +} VirtIOBlockStartData;
> +
> +static void virtio_blk_data_plane_start_bh_cb(void *opaque)
> +{
> +    VirtIOBlockStartData *data = opaque;
> +    VirtIOBlockDataPlane *s = data->s;

Won't you need to check here whether ->started is still set?

> +
> +    /* Kick right away to begin processing requests already in vring */
> +    event_notifier_set(virtio_queue_get_host_notifier(s->vq));
> +
> +    /* Get this show started by hooking up our callbacks */
> +    virtio_queue_aio_set_host_notifier_handler(s->vq, s->ctx, true, true);
> +
> +    qemu_bh_delete(data->bh);
> +    g_free(data);
> +}
> +
>  /* Context: QEMU global mutex held */
>  void virtio_blk_data_plane_start(VirtIOBlockDataPlane *s)
>  {
>      BusState *qbus = BUS(qdev_get_parent_bus(DEVICE(s->vdev)));
>      VirtioBusClass *k = VIRTIO_BUS_GET_CLASS(qbus);
>      VirtIOBlock *vblk = VIRTIO_BLK(s->vdev);
> +    VirtIOBlockStartData *data;
>      int r;
> 
> +    qemu_mutex_lock(&s->start_lock);
>      if (vblk->dataplane_started || s->starting) {

Do we still need ->starting with the new mutex?

> +        qemu_mutex_unlock(&s->start_lock);
>          return;
>      }
> 
> @@ -221,13 +247,11 @@ void virtio_blk_data_plane_start(VirtIOBlockDataPlane 
> *s)
> 
>      blk_set_aio_context(s->conf->conf.blk, s->ctx);
> 
> -    /* Kick right away to begin processing requests already in vring */
> -    event_notifier_set(virtio_queue_get_host_notifier(s->vq));
> -
> -    /* Get this show started by hooking up our callbacks */
> -    aio_context_acquire(s->ctx);
> -    virtio_queue_aio_set_host_notifier_handler(s->vq, s->ctx, true, true);
> -    aio_context_release(s->ctx);
> +    data = g_new(VirtIOBlockStartData, 1);
> +    data->s = s;
> +    data->bh = aio_bh_new(s->ctx, virtio_blk_data_plane_start_bh_cb, data);
> +    qemu_bh_schedule(data->bh);
> +    qemu_mutex_unlock(&s->start_lock);
>      return;
> 
>    fail_host_notifier:
> @@ -236,6 +260,7 @@ void virtio_blk_data_plane_start(VirtIOBlockDataPlane *s)
>      s->disabled = true;
>      s->starting = false;
>      vblk->dataplane_started = true;
> +    qemu_mutex_unlock(&s->start_lock);
>  }
> 
>  /* Context: QEMU global mutex held */

Do you also need to do something in _stop()?