[PATCH v2] virtio-rng: fix stuck of hot-unplugging busy device

When we try to hot-remove a busy virtio-rng device from QEMU monitor,
the device can't be hot-removed. Because virtio-rng driver hangs at
wait_for_completion_killable().

This patch exits the waiting by completing have_data completion before
unregistering, resets data_avail to avoid the hwrng core use wrong
buffer bytes. Before real unregistering we should return -ENODEV for
reading.

When we hot-unplug the device, dd process in guest will exit.
  $ dd if=/dev/hwrng of=/dev/null
  dd: error reading ‘/dev/hwrng’: No such device

Signed-off-by: Amos Kong <redacted>
Cc: stable@vger.kernel.org
---
V2: reset data_avail (Amit)
    adjust unregister order
---
 drivers/char/hw_random/virtio-rng.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/drivers/char/hw_random/virtio-rng.c b/drivers/char/hw_random/virtio-rng.c
index 2e3139e..e76433b 100644
--- a/drivers/char/hw_random/virtio-rng.c
+++ b/drivers/char/hw_random/virtio-rng.c

@@ -68,6 +68,10 @@ static int virtio_read(struct hwrng *rng, void *buf, size_t size, bool wait)
 	int ret;
 	struct virtrng_info *vi = (struct virtrng_info *)rng->priv;
 
+	if (!vi->hwrng_register_done) {
+		return -ENODEV;
+	}
+
 	if (!vi->busy) {
 		vi->busy = true;
 		init_completion(&vi->have_data);

@@ -137,10 +141,14 @@ static void remove_common(struct virtio_device *vdev)
 {
 	struct virtrng_info *vi = vdev->priv;
 
+	vi->data_avail = 0;
+	complete(&vi->have_data);
 	vdev->config->reset(vdev);
 	vi->busy = false;
-	if (vi->hwrng_register_done)
+	if (vi->hwrng_register_done) {
+		vi->hwrng_register_done = false;
 		hwrng_unregister(&vi->hwrng);
+	}
 	vdev->config->del_vqs(vdev);
 	ida_simple_remove(&rng_index_ida, vi->index);
 	kfree(vi);

-- 
1.9.3

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