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

[Paolo Bonzini <pbonzini@redhat.com>]

On 15/03/2016 14:18, Cornelia Huck wrote:
> On Tue, 15 Mar 2016 20:45:30 +0800
> Fam Zheng <famz@redhat.com> 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 <famz@redhat.com>
>> Date: Thu, 10 Mar 2016 10:26:36 +0800
>> Subject: [PATCH] virtio-blk dataplane start crash fix
>>
>> Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
>> Signed-off-by: Fam Zheng <famz@redhat.com>
>> ---
>>  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);

I'm not sure that this is necessary.  An empty section should be the
same as plain old bdrv_drain.

>>      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?

Yes.

>> +
>> +    /* 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?

No, but really we shouldn't have needed it before either. :)  So a task
for another day.

>> +        qemu_mutex_unlock(&s->start_lock);
>>          return;
>>      }
>>
>>  /* Context: QEMU global mutex held */
> 
> Do you also need to do something in _stop()?

_stop definitely needs to take the mutex too.