Re: [Qemu-devel] [PATCH 1/1] virtio-blk: fix race on guest notifiers

[Halil Pasic <pasic@linux.vnet.ibm.com>]

On 03/02/2017 04:32 PM, Paolo Bonzini wrote:
> 
> 
> On 02/03/2017 15:49, Cornelia Huck wrote:
>> On Thu,  2 Mar 2017 14:04:22 +0100
>> Halil Pasic <pasic@linux.vnet.ibm.com> wrote:
>>
>>> diff --git a/hw/block/dataplane/virtio-blk.c b/hw/block/dataplane/virtio-blk.c
>>> index 5556f0e..13dd14d 100644
>>> --- a/hw/block/dataplane/virtio-blk.c
>>> +++ b/hw/block/dataplane/virtio-blk.c
>>> @@ -258,9 +258,16 @@ void virtio_blk_data_plane_stop(VirtIODevice *vdev)
>>>          virtio_queue_aio_set_host_notifier_handler(vq, s->ctx, NULL);
>>>      }
>>>
>>> -    /* Drain and switch bs back to the QEMU main loop */
>>> +    /* Drain and switch bs back to the QEMU main loop. After drain, the
>>> +     * device will not submit (nor comple) any requests until dataplane
>>
>> s/comple/complete/
>>

Will fix.

>>> +     * starts again.
>>> +     */
>>>      blk_set_aio_context(s->conf->conf.blk, qemu_get_aio_context());

I'm wondering if synchronization is needed for batch_notify_vqs. I think
the set_bit can be from the iothread, but the notify_guest_bh below is main
event loop. Is it OK like this (could we miss bits set in other thread)?
Does blk_set_aio_context include a barrier?

>>>
>>> +    /* Notify guest before the guest notifiers get cleaned up */
>>> +    qemu_bh_cancel(s->bh);
>>> +    notify_guest_bh(s);
>>> +
>>
>> Hm... does virtio-scsi dataplane need a similar treatment? Or am I
>> missing something?
> 
> No, the BH optimization is specific to virtio-blk.  Thanks for the patch
> Halil!
> 

You are welcome ;)!

Regards,
Halil