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Subject: Re: [Xen-devel] [PATCHv2] xen-netfront: remove warning when unloading
 module
To:     Eduardo Otubo <otubo@redhat.com>
Cc:     xen-devel@lists.xenproject.org, jgross@suse.com,
        wei.liu2@citrix.com, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org, paul.durrant@citrix.com,
        cheshi@redhat.com, vkuznets@redhat.com, mgamal@redhat.com,
        cavery@redhat.com, boris.ostrovsky@oracle.com
References: <20171123141835.5820-1-otubo@redhat.com>
 <fcbb9d8a-85b4-5543-6ebc-cd34a66729ff@gmail.com>
 <20180202085428.GA26899@vader>
From:   Oleksandr Andrushchenko <andr2000@gmail.com>
Message-ID: <d6f97273-a8e9-f56e-f6b7-30c15f4b1f3f@gmail.com>
Date:   Fri, 2 Feb 2018 11:45:21 +0200
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On 02/02/2018 10:54 AM, Eduardo Otubo wrote:
> On Wed, Jan 31, 2018 at 05:00:23PM +0200, Oleksandr Andrushchenko wrote:
>> 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
> To be honest, most of the things I assumed were true, according to some talks on
> IRC with maintainers. Since I assumed it was true I started to write code based
> on that and all the behaviors that followed were correct according to my
> assumptions (and discussions).
>
> But if you find something else weird, please let me know and we can fix it.
There is nothing wrong with the patch. One thing that
I cannot get in my driver is that .otherend_changed callback
is not called on .remove. Please see [1]
>> 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);
[1] https://patchwork.kernel.org/patch/10195163/