virtio_pmem_host_ack() reclaims virtqueue descriptors with
virtqueue_get_buf(). The -ENOSPC waiter wakeup is tied to completing the
returned token. If token completion is skipped for any reason, reclaimed
descriptors may not wake a waiter and the submitter may sleep forever
waiting for a free slot. Always wake one -ENOSPC waiter for each virtqueue
completion before touching the returned token.

Signed-off-by: Li Chen <me@linux.beauty>
---
v2->v3:
- Split out the waiter wakeup ordering change from READ_ONCE()/WRITE_ONCE()
  updates (now patch 2/5), per Pankaj's suggestion.

 drivers/nvdimm/nd_virtio.c | 25 ++++++++++++++++---------
 1 file changed, 16 insertions(+), 9 deletions(-)

diff --git a/drivers/nvdimm/nd_virtio.c b/drivers/nvdimm/nd_virtio.c
index af82385be7c6..a1ad8d67ad2d 100644
--- a/drivers/nvdimm/nd_virtio.c
+++ b/drivers/nvdimm/nd_virtio.c
@@ -9,26 +9,33 @@
 #include "virtio_pmem.h"
 #include "nd.h"
 
+static void virtio_pmem_wake_one_waiter(struct virtio_pmem *vpmem)
+{
+	struct virtio_pmem_request *req_buf;
+
+	if (list_empty(&vpmem->req_list))
+		return;
+
+	req_buf = list_first_entry(&vpmem->req_list,
+				   struct virtio_pmem_request, list);
+	req_buf->wq_buf_avail = true;
+	wake_up(&req_buf->wq_buf);
+	list_del(&req_buf->list);
+}
+
  /* The interrupt handler */
 void virtio_pmem_host_ack(struct virtqueue *vq)
 {
 	struct virtio_pmem *vpmem = vq->vdev->priv;
-	struct virtio_pmem_request *req_data, *req_buf;
+	struct virtio_pmem_request *req_data;
 	unsigned long flags;
 	unsigned int len;
 
 	spin_lock_irqsave(&vpmem->pmem_lock, flags);
 	while ((req_data = virtqueue_get_buf(vq, &len)) != NULL) {
+		virtio_pmem_wake_one_waiter(vpmem);
 		req_data->done = true;
 		wake_up(&req_data->host_acked);
-
-		if (!list_empty(&vpmem->req_list)) {
-			req_buf = list_first_entry(&vpmem->req_list,
-					struct virtio_pmem_request, list);
-			req_buf->wq_buf_avail = true;
-			wake_up(&req_buf->wq_buf);
-			list_del(&req_buf->list);
-		}
 	}
 	spin_unlock_irqrestore(&vpmem->pmem_lock, flags);
 }
-- 
2.52.0