Re: Re: [PATCH v1 1/2] virtio-balloon: tweak config_changed implementation

Wei Wang
2019-01-04T05:14:27Z

On 01/04/2019 12:42 AM, Michael S. Tsirkin wrote:
> On Thu, Jan 03, 2019 at 01:31:01PM +0800, Wei Wang wrote:
>> virtio-ccw has deadlock issues with reading config registers inside the
>> interrupt context, so we tweak the virtballoon_changed implementation
>> by moving the config read operations into the related workqueue contexts.
>>
>> Signed-off-by: Wei Wang <wei.w.wang@intel.com>
>> ---
>> drivers/virtio/virtio_balloon.c | 54 ++++++++++++++++++++---------------------
>> 1 file changed, 26 insertions(+), 28 deletions(-)
>>
>> diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
>> index 728ecd1..9a82a11 100644
>> --- a/drivers/virtio/virtio_balloon.c
>> +++ b/drivers/virtio/virtio_balloon.c
>> @@ -394,33 +394,15 @@ static void virtballoon_changed(struct virtio_device *vdev)
>> {
>> struct virtio_balloon *vb = vdev->priv;
>> unsigned long flags;
>> - s64 diff = towards_target(vb);
>> -
>> - if (diff) {
>> - spin_lock_irqsave(&vb->stop_update_lock, flags);
>> - if (!vb->stop_update)
>> - queue_work(system_freezable_wq,
>> - &vb->update_balloon_size_work);
>> - spin_unlock_irqrestore(&vb->stop_update_lock, flags);
>> - }
>>
>> - if (virtio_has_feature(vdev, VIRTIO_BALLOON_F_FREE_PAGE_HINT)) {
>> - virtio_cread(vdev, struct virtio_balloon_config,
>> - free_page_report_cmd_id, &vb->cmd_id_received);
> There's one problem with this approach:
>
> previously updating the cmd_id_received here would immediately
> stop the report in send_free_pages.
>
> With this approach we are waiting for the wq to schedule,
> which might be blocked waiting for report to complete.
> So host can no longer quickly stop the report in progress.
>
> A simple work-around would be to set some kind of flag whenever there
> is a change interrupt, then have send_free_pages test it
> and re-read cmd_id_received.
>
> Needs to be an atomic I guess ...
>

Yes, sounds better..will update the patch.

Best,
Wei