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SubjectRe: [Xen-devel] [PATCHv2] xen-netfront: remove warning when unloading module
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Hi, Eduardo!

I am working on a frontend driver (PV DRM) and also seeing some strange

things on driver unloading:

xt# rmmod -f drm_xen_front.ko
[ 3236.462497] [drm] Unregistering XEN PV vdispl
[ 3236.485745] [drm:xen_drv_remove [drm_xen_front]] *ERROR* Backend
state is InitWait while removing driver
[ 3236.486950] vdispl vdispl-0: 22 freeing event channel 11
[ 3236.496123] vdispl vdispl-0: failed to write error node for
device/vdispl/0 (22 freeing event channel 11)
[ 3236.496271] vdispl vdispl-0: 22 freeing event channel 12
[ 3236.501633] vdispl vdispl-0: failed to write error node for
device/vdispl/0 (22 freeing event channel 12)

These are somewhat different from your use-case with grant references,
but I have a question:

do you really see that XenbusStateClosed and XenbusStateClosing are

called? In my driver I can't see those and once I tried to dig deeper
into the problem

I saw that on driver removal it is disconnected from XenBus, so no
backend state

change events come in via .otherend_changed callback.

The only difference I see here is that the backend is a user-space
application

Thank you,
Oleksandr

On 11/23/2017 04:18 PM, Eduardo Otubo wrote:
> v2:
> * Replace busy wait with wait_event()/wake_up_all()
> * Cannot garantee that at the time xennet_remove is called, the
> xen_netback state will not be XenbusStateClosed, so added a
> condition for that
> * There's a small chance for the xen_netback state is
> XenbusStateUnknown by the time the xen_netfront switches to Closed,
> so added a condition for that.
>
> When unloading module xen_netfront from guest, dmesg would output
> warning messages like below:
>
> [ 105.236836] xen:grant_table: WARNING: g.e. 0x903 still in use!
> [ 105.236839] deferring g.e. 0x903 (pfn 0x35805)
>
> This problem relies on netfront and netback being out of sync. By the time
> netfront revokes the g.e.'s netback didn't have enough time to free all of
> them, hence displaying the warnings on dmesg.
>
> The trick here is to make netfront to wait until netback frees all the g.e.'s
> and only then continue to cleanup for the module removal, and this is done by
> manipulating both device states.
>
> Signed-off-by: Eduardo Otubo <otubo@redhat.com>
> ---
> drivers/net/xen-netfront.c | 18 ++++++++++++++++++
> 1 file changed, 18 insertions(+)
>
> diff --git a/drivers/net/xen-netfront.c b/drivers/net/xen-netfront.c
> index 8b8689c6d887..391432e2725d 100644
> --- a/drivers/net/xen-netfront.c
> +++ b/drivers/net/xen-netfront.c
> @@ -87,6 +87,8 @@ struct netfront_cb {
> /* IRQ name is queue name with "-tx" or "-rx" appended */
> #define IRQ_NAME_SIZE (QUEUE_NAME_SIZE + 3)
>
> +static DECLARE_WAIT_QUEUE_HEAD(module_unload_q);
> +
> struct netfront_stats {
> u64 packets;
> u64 bytes;
> @@ -2021,10 +2023,12 @@ static void netback_changed(struct xenbus_device *dev,
> break;
>
> case XenbusStateClosed:
> + wake_up_all(&module_unload_q);
> if (dev->state == XenbusStateClosed)
> break;
> /* Missed the backend's CLOSING state -- fallthrough */
> case XenbusStateClosing:
> + wake_up_all(&module_unload_q);
> xenbus_frontend_closed(dev);
> break;
> }
> @@ -2130,6 +2134,20 @@ static int xennet_remove(struct xenbus_device *dev)
>
> dev_dbg(&dev->dev, "%s\n", dev->nodename);
>
> + if (xenbus_read_driver_state(dev->otherend) != XenbusStateClosed) {
> + xenbus_switch_state(dev, XenbusStateClosing);
> + wait_event(module_unload_q,
> + xenbus_read_driver_state(dev->otherend) ==
> + XenbusStateClosing);
> +
> + xenbus_switch_state(dev, XenbusStateClosed);
> + wait_event(module_unload_q,
> + xenbus_read_driver_state(dev->otherend) ==
> + XenbusStateClosed ||
> + xenbus_read_driver_state(dev->otherend) ==
> + XenbusStateUnknown);
> + }
> +
> xennet_disconnect_backend(info);
>
> unregister_netdev(info->netdev);

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