On Mon, Sep 08, 2014 at 11:29:51PM +0800, Amos Kong wrote:
> 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?
> >
Hi Amit
> > 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().
I finally understand content, we need to set vi->data_avail to 0
before returning virtio_read(), it might enter wait_event_completion_killable()
when we try to remove the device.
We call complete() in remove_common(), then wait_event_completion_killable()
will exit, but virtio_read() will be re-entered if the device still
isn't unregistered, then re-stuck inside wait_event_completion_killable().
I tested some complex condition (both quick/slow backend), I found
some problem in below two patches.
I will post another fix later. The test result is expected.
1. Hotplug remove virtio-rng0, dd process will exit with an error:
"dd: error reading ‘/dev/hwrng’: Operation not permitted"
virtio-rng0 disappear from 'info pci'
2. Re-read by dd, hotplug virtio-rng1, dd process exit with same
error, virtio-rng1 disappear
Thanks, Amos
> >
> > 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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