Re: [Qemu-devel] [Qemu-block] [PATCH 4/4] virtio-blk: Clean up start/stop with mutex and BH

[Christian Borntraeger <borntraeger@de.ibm.com>]

On 03/23/2016 10:08 AM, Paolo Bonzini wrote:
> 
> 
> On 23/03/2016 09:10, Cornelia Huck wrote:
>>> -    /* Kick right away to begin processing requests already in vring */
>>> -    event_notifier_set(virtio_queue_get_host_notifier(s->vq));
>>> +    vblk->dataplane_started = true;
>>>
>>> -    /* Get this show started by hooking up our callbacks */
>>> +    /* 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);
>>> +    atomic_dec(&s->starting);
>>> +
>>> +    /* Kick right away to begin processing requests already in vring */
>>> +    event_notifier_set(virtio_queue_get_host_notifier(s->vq));
>>
>> I'm wondering whether moving this event_notifier_set() masks something?
>> IOW, may we run into trouble if the event notifier is set from some
>> other path before the callbacks are set up properly?
> 
> The reentrancy check should catch that...  But:
> 
> 1) the patch really makes no difference, your fix is enough for me


Tu Bo, can you test with master + Cornelias 6 refactoring patches and nothing on top?


 
> 2) vblk->dataplane_started becomes true before the callbacks are set;
> that should be enough.
> 
> 3) this matches what I tested, but it would of course be better if the
> assertions on s->starting suffice
> 
>>> -    if (!vblk->dataplane_started || s->stopping) {
>>> +    if (!vblk->dataplane_started) {
>>
>> No fear of reentrancy here?
> 
> No, because this is only invoked from reset, hence only from the CPU
> thread and only under the BQL.
> 
> On start, reentrancy happens between iothread (running outside BQL) and
> a CPU thread (running within BQL).
> 
> Paolo
> 
>>>          return;
>>>      }
>>>
>>> @@ -255,7 +251,7 @@ void virtio_blk_data_plane_stop(VirtIOBlockDataPlane *s)
>>>          vblk->dataplane_started = false;
>>>          return;
>>>      }
>>> -    s->stopping = true;
>>> +
>>>      trace_virtio_blk_data_plane_stop(s);
>>>
>>>      aio_context_acquire(s->ctx);
>>> @@ -274,5 +270,4 @@ void virtio_blk_data_plane_stop(VirtIOBlockDataPlane *s)
>>>      k->set_guest_notifiers(qbus->parent, 1, false);
>>>
>>>      vblk->dataplane_started = false;
>>> -    s->stopping = false;
>>>  }
>>>
>>
>