On Wed, Aug 06, 2014 at 01:55:29PM +0530, Amit Shah wrote:
> On (Wed) 06 Aug 2014 [16:05:41], Amos Kong wrote:
> > On Wed, Aug 06, 2014 at 01:35:15AM +0800, Amos Kong wrote:
> > > 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 fixed the hang by completing have_data completion before
> > > unregistering a virtio-rng device.
> >
> > Hi Amit,
> >
> > Before applying this patch, it's blocking insider
> > wait_for_completion_killable()
> > Applied this patch, wait_for_completion_killable() returns 0,
> > and vi->data_avail becomes 0, then rng_get_date() will return 0.
>
> Thanks for checking this.
>
> > Is it expected result?
>
> I think what will happen is vi->data_avail will be set to whatever it
> was set last. In case of a previous successful read request, the
> data_avail will be set to whatever number of bytes the host gave. On
> doing a hot-unplug on the succeeding wait, the value in data_avail
> will be re-used, and the hwrng core will wrongly take some bytes in
> the buffer as input from the host.
>
> So, I think we need to set vi->data_avail = 0; before calling
> wait_event_completion_killable().
>
> Amit
In my latest debugging, I found the hang is caused by unexpected reading
when we started to remove the device.
I have two draft fix, 1) is skip unexpected reading by checking a
remove flag. 2) is unregistering device at the beginning of
remove_common(). I think second patch is better if it won't cause
new problem.
The original patch (complete in remove_common()) is still necessary.
Test results: hotplug issue disappeared (dd process will quit).
diff --git a/drivers/char/hw_random/virtio-rng.c
b/drivers/char/hw_random/virtio-rng.c
index 2e3139e..028797c 100644
--- a/drivers/char/hw_random/virtio-rng.c
+++ b/drivers/char/hw_random/virtio-rng.c
@@ -35,6 +35,7 @@ struct virtrng_info {
unsigned int data_avail;
int index;
bool busy;
+ bool remove;
bool hwrng_register_done;
};
@@ -68,6 +69,9 @@ 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->remove)
+ return 0;
+
if (!vi->busy) {
vi->busy = true;
init_completion(&vi->have_data);
@@ -137,6 +141,8 @@ static void remove_common(struct virtio_device
*vdev)
{
struct virtrng_info *vi = vdev->priv;
+ vi->remove = true;
+ complete(&vi->have_data);
vdev->config->reset(vdev);
vi->busy = false;
if (vi->hwrng_register_done)
diff --git a/drivers/char/hw_random/virtio-rng.c
b/drivers/char/hw_random/virtio-rng.c
index 2e3139e..9b8c2ce 100644
--- a/drivers/char/hw_random/virtio-rng.c
+++ b/drivers/char/hw_random/virtio-rng.c
@@ -137,10 +137,11 @@ static void remove_common(struct virtio_device
*vdev)
{
struct virtrng_info *vi = vdev->priv;
- vdev->config->reset(vdev);
- vi->busy = false;
if (vi->hwrng_register_done)
hwrng_unregister(&vi->hwrng);
+ complete(&vi->have_data);
+ vdev->config->reset(vdev);
+ vi->busy = false;
vdev->config->del_vqs(vdev);
ida_simple_remove(&rng_index_ida, vi->index);
kfree(vi);
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