[PATCH v2 1/2] virtio-balloon: tweak config_changed implementation

[Wei Wang]

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.

Reported-by: Christian Borntraeger <borntraeger@xxxxxxxxxx>
Signed-off-by: Wei Wang <wei.w.wang@xxxxxxxxx>
---
 drivers/virtio/virtio_balloon.c | 81 +++++++++++++++++++++++++++--------------
 1 file changed, 53 insertions(+), 28 deletions(-)

diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
index 728ecd1..35ee762 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;
@@ -77,6 +81,8 @@ 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;
@@ -390,37 +396,31 @@ 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)
@@ -609,6 +609,16 @@ static int get_free_page_and_send(struct virtio_balloon *vb)
 	return 0;
 }
 
+static void virtio_balloon_read_cmd_id_received(struct virtio_balloon *vb)
+{
+	if (!test_and_clear_bit(VIRTIO_BALLOON_CONFIG_READ_CMD_ID,
+				&vb->config_read_bitmap))
+		return;
+
+	virtio_cread(vb->vdev, struct virtio_balloon_config,
+		     free_page_report_cmd_id, &vb->cmd_id_received);
+}
+
 static int send_free_pages(struct virtio_balloon *vb)
 {
 	int err;
@@ -620,6 +630,7 @@ static int send_free_pages(struct virtio_balloon *vb)
 		 * stop the reporting.
 		 */
 		cmd_id_active = virtio32_to_cpu(vb->vdev, vb->cmd_id_active);
+		virtio_balloon_read_cmd_id_received(vb);
 		if (cmd_id_active != vb->cmd_id_received)
 			break;
 
@@ -637,11 +648,9 @@ 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 +668,22 @@ 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);
+
+	virtio_balloon_read_cmd_id_received(vb);
+	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(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
-- 
2.7.4