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Subject: Re: [Xen-devel] [PATCH] xen-netfront: wait xenbus state change when
 load module manually
To:     Jiri Slaby <jslaby@suse.cz>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Xiao Liang <xiliang@redhat.com>,
        David Miller <davem@davemloft.net>
Cc:     netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
        xen-devel@lists.xenproject.org
References: <20180727095608.25210-1-xiliang@redhat.com>
 <20180729.083040.53901922165497594.davem@davemloft.net>
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 <83ad1e3c-5e3b-8b24-430b-13e12b59ae8b@suse.cz>
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From:   Juergen Gross <jgross@suse.com>
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Date:   Fri, 7 Sep 2018 13:30:40 +0200
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On 07/09/18 13:06, Jiri Slaby wrote:
> On 08/24/2018, 04:26 PM, Boris Ostrovsky wrote:
>> On 08/24/2018 07:26 AM, Juergen Gross wrote:
>>> On 24/08/18 13:12, Jiri Slaby wrote:
>>>> On 07/30/2018, 10:18 AM, Xiao Liang wrote:
>>>>> On 07/29/2018 11:30 PM, David Miller wrote:
>>>>>> From: Xiao Liang <xiliang@redhat.com>
>>>>>> Date: Fri, 27 Jul 2018 17:56:08 +0800
>>>>>>
>>>>>>> @@ -1330,6 +1331,11 @@ static struct net_device
>>>>>>> *xennet_create_dev(struct xenbus_device *dev)
>>>>>>>       netif_carrier_off(netdev);
>>>>>>>         xenbus_switch_state(dev, XenbusStateInitialising);
>>>>>>> +    wait_event(module_load_q,
>>>>>>> +               xenbus_read_driver_state(dev->otherend) !=
>>>>>>> +               XenbusStateClosed &&
>>>>>>> +               xenbus_read_driver_state(dev->otherend) !=
>>>>>>> +               XenbusStateUnknown);
>>>>>>>       return netdev;
>>>>>>>      exit:
>>>>>> What performs the wakeups that will trigger for this sleep site?
>>>>> In my understanding, backend leaving closed/unknow state can trigger the
>>>>> wakeups. I mean to make sure both sides are ready for creating connection.
>>>> While backporting this to 4.12, I was surprised by the commit the same
>>>> as Boris and David.
>>>>
>>>> So I assume the explanation is that wake_up_all of module_unload_q in
>>>> netback_changed wakes also all the processes waiting on module_load_q?
>>>> If so, what makes sure that module_unload_q is queued and the process is
>>>> the same as for module_load_q?
>>> How could it? Either the thread is waiting on module_unload_q _or_ on
>>> module_load_q. It can't wait on two queues at the same time.
>>>
>>>> To me, it looks rather error-prone. Unless it is erroneous now, at least
>>>> for future changes. Wouldn't it make sense to wake up module_load_q
>>>> along with module_unload_q in netback_changed? Or drop module_load_q
>>>> completely and use only module_unload_q (i.e. in xennet_create_dev too)?
>>> To me this looks just wrong. A thread waiting on module_load_q won't be
>>> woken up again.
>>>
>>> I'd drop module_load_q in favor of module_unload_q.
>>
>>
>> Yes, use single queue, but rename it to something more neutral. module_wq?
> 
> Can somebody who is actually using the module fix this, please?

Already at it.


Juergen