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

From: Amos Kong <hidden>
Date: 2014-09-09 10:47:42
Also in: kvm, virtualization

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:

quoted

On (Wed) 06 Aug 2014 [16:05:41], Amos Kong wrote:

quoted

On Wed, Aug 06, 2014 at 01:35:15AM +0800, Amos Kong wrote:

quoted

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.

quoted

Is it expected result?

Hi Amit

quoted

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

quoted hunk ↗ jump to hunk

quoted

		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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