Re: [PATCH v3 1/3] virtio-balloon: tweak config_changed implementation

[Michael S. Tsirkin]

On Mon, Jan 07, 2019 at 03:01:04PM +0800, Wei Wang wrote:
> virtio-ccw has deadlock issues with reading the config space inside the
> interrupt context, so we tweak the virtballoon_changed implementation
> by moving the config read operations into the related workqueue contexts.
> The config_read_bitmap is used as a flag to the workqueue callbacks
> about the related config fields that need to be read.
> 
> The cmd_id_received is also renamed to cmd_id_received_cache, and
> the value should be obtained via virtio_balloon_cmd_id_received.
> 
> Reported-by: Christian Borntraeger <borntraeger@xxxxxxxxxx>
> Signed-off-by: Wei Wang <wei.w.wang@xxxxxxxxx>
> Reviewed-by: Cornelia Huck <cohuck@xxxxxxxxxx>
> Reviewed-by: Halil Pasic <pasic@xxxxxxxxxxxxx>

Please include a Fixes tag to list the problematic commit.
See Documentation/process/submitting-patches.rst for the appropriate format.

> ---
>  drivers/virtio/virtio_balloon.c | 98 +++++++++++++++++++++++++++--------------
>  1 file changed, 65 insertions(+), 33 deletions(-)
> 
> diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
> index 728ecd1..fb12fe2 100644
> --- a/drivers/virtio/virtio_balloon.c
> +++ b/drivers/virtio/virtio_balloon.c
> @@ -61,6 +61,10 @@ enum virtio_balloon_vq {
>  	VIRTIO_BALLOON_VQ_MAX
>  };
>  
> +enum virtio_balloon_config_read {
> +	VIRTIO_BALLOON_CONFIG_READ_CMD_ID = 0,
> +};
> +
>  struct virtio_balloon {
>  	struct virtio_device *vdev;
>  	struct virtqueue *inflate_vq, *deflate_vq, *stats_vq, *free_page_vq;
> @@ -77,14 +81,20 @@ struct virtio_balloon {
>  	/* Prevent updating balloon when it is being canceled. */
>  	spinlock_t stop_update_lock;
>  	bool stop_update;
> +	/* Bitmap to indicate if reading the related config fields are needed */
> +	unsigned long config_read_bitmap;
>  
>  	/* The list of allocated free pages, waiting to be given back to mm */
>  	struct list_head free_page_list;
>  	spinlock_t free_page_list_lock;
>  	/* The number of free page blocks on the above list */
>  	unsigned long num_free_page_blocks;
> -	/* The cmd id received from host */
> -	u32 cmd_id_received;
> +	/*
> +	 * The cmd id received from host.
> +	 * Read it via virtio_balloon_cmd_id_received to get the latest value
> +	 * sent from host.
> +	 */
> +	u32 cmd_id_received_cache;
>  	/* The cmd id that is actively in use */
>  	__virtio32 cmd_id_active;
>  	/* Buffer to store the stop sign */
> @@ -390,37 +400,31 @@ static unsigned long return_free_pages_to_mm(struct virtio_balloon *vb,
>  	return num_returned;
>  }
>  
> +static void virtio_balloon_queue_free_page_work(struct virtio_balloon *vb)
> +{
> +	if (!virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_FREE_PAGE_HINT))
> +		return;
> +
> +	/* No need to queue the work if the bit was already set. */
> +	if (test_and_set_bit(VIRTIO_BALLOON_CONFIG_READ_CMD_ID,
> +			     &vb->config_read_bitmap))
> +		return;
> +
> +	queue_work(vb->balloon_wq, &vb->report_free_page_work);
> +}
> +
>  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);
> -		if (vb->cmd_id_received == VIRTIO_BALLOON_CMD_ID_DONE) {
> -			/* Pass ULONG_MAX to give back all the free pages */
> -			return_free_pages_to_mm(vb, ULONG_MAX);
> -		} else if (vb->cmd_id_received != VIRTIO_BALLOON_CMD_ID_STOP &&
> -			   vb->cmd_id_received !=
> -			   virtio32_to_cpu(vdev, vb->cmd_id_active)) {
> -			spin_lock_irqsave(&vb->stop_update_lock, flags);
> -			if (!vb->stop_update) {
> -				queue_work(vb->balloon_wq,
> -					   &vb->report_free_page_work);
> -			}
> -			spin_unlock_irqrestore(&vb->stop_update_lock, flags);
> -		}
> +	spin_lock_irqsave(&vb->stop_update_lock, flags);
> +	if (!vb->stop_update) {
> +		queue_work(system_freezable_wq,
> +			   &vb->update_balloon_size_work);
> +		virtio_balloon_queue_free_page_work(vb);
>  	}
> +	spin_unlock_irqrestore(&vb->stop_update_lock, flags);
>  }
>  
>  static void update_balloon_size(struct virtio_balloon *vb)
> @@ -527,6 +531,17 @@ static int init_vqs(struct virtio_balloon *vb)
>  	return 0;
>  }
>  
> +static u32 virtio_balloon_cmd_id_received(struct virtio_balloon *vb)
> +{
> +	if (test_and_clear_bit(VIRTIO_BALLOON_CONFIG_READ_CMD_ID,
> +			       &vb->config_read_bitmap))
> +		virtio_cread(vb->vdev, struct virtio_balloon_config,
> +			     free_page_report_cmd_id,
> +			     &vb->cmd_id_received_cache);
> +
> +	return vb->cmd_id_received_cache;
> +}
> +
>  static int send_cmd_id_start(struct virtio_balloon *vb)
>  {
>  	struct scatterlist sg;
> @@ -537,7 +552,8 @@ static int send_cmd_id_start(struct virtio_balloon *vb)
>  	while (virtqueue_get_buf(vq, &unused))
>  		;
>  
> -	vb->cmd_id_active = cpu_to_virtio32(vb->vdev, vb->cmd_id_received);
> +	vb->cmd_id_active = virtio32_to_cpu(vb->vdev,
> +					virtio_balloon_cmd_id_received(vb));
>  	sg_init_one(&sg, &vb->cmd_id_active, sizeof(vb->cmd_id_active));
>  	err = virtqueue_add_outbuf(vq, &sg, 1, &vb->cmd_id_active, GFP_KERNEL);
>  	if (!err)
> @@ -620,7 +636,8 @@ static int send_free_pages(struct virtio_balloon *vb)
>  		 * stop the reporting.
>  		 */
>  		cmd_id_active = virtio32_to_cpu(vb->vdev, vb->cmd_id_active);
> -		if (cmd_id_active != vb->cmd_id_received)
> +		if (unlikely(cmd_id_active !=
> +			     virtio_balloon_cmd_id_received(vb)))
>  			break;
>  
>  		/*
> @@ -637,11 +654,9 @@ static int send_free_pages(struct virtio_balloon *vb)
>  	return 0;
>  }
>  
> -static void report_free_page_func(struct work_struct *work)
> +static void virtio_balloon_report_free_page(struct virtio_balloon *vb)
>  {
>  	int err;
> -	struct virtio_balloon *vb = container_of(work, struct virtio_balloon,
> -						 report_free_page_work);
>  	struct device *dev = &vb->vdev->dev;
>  
>  	/* Start by sending the received cmd id to host with an outbuf. */
> @@ -659,6 +674,23 @@ static void report_free_page_func(struct work_struct *work)
>  		dev_err(dev, "Failed to send a stop id, err = %d\n", err);
>  }
>  
> +static void report_free_page_func(struct work_struct *work)
> +{
> +	struct virtio_balloon *vb = container_of(work, struct virtio_balloon,
> +						 report_free_page_work);
> +	u32 cmd_id_received;
> +
> +	cmd_id_received = virtio_balloon_cmd_id_received(vb);
> +	if (cmd_id_received == VIRTIO_BALLOON_CMD_ID_DONE) {
> +		/* Pass ULONG_MAX to give back all the free pages */
> +		return_free_pages_to_mm(vb, ULONG_MAX);
> +	} else if (cmd_id_received != VIRTIO_BALLOON_CMD_ID_STOP &&
> +		   cmd_id_received !=
> +		   virtio32_to_cpu(vb->vdev, vb->cmd_id_active)) {
> +		virtio_balloon_report_free_page(vb);
> +	}
> +}
> +
>  #ifdef CONFIG_BALLOON_COMPACTION
>  /*
>   * virtballoon_migratepage - perform the balloon page migration on behalf of
> @@ -885,7 +917,7 @@ static int virtballoon_probe(struct virtio_device *vdev)
>  			goto out_del_vqs;
>  		}
>  		INIT_WORK(&vb->report_free_page_work, report_free_page_func);
> -		vb->cmd_id_received = VIRTIO_BALLOON_CMD_ID_STOP;
> +		vb->cmd_id_received_cache = VIRTIO_BALLOON_CMD_ID_STOP;
>  		vb->cmd_id_active = cpu_to_virtio32(vb->vdev,
>  						  VIRTIO_BALLOON_CMD_ID_STOP);
>  		vb->cmd_id_stop = cpu_to_virtio32(vb->vdev,
> -- 
> 2.7.4