Re: [PATCH] virtio-rng: complete have_data completion in removing device

[Amos Kong <akong@redhat.com>]

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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