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Subject: Re: [Xen-devel] [PATCH] xen-netfront: wait xenbus state change when
 load module manually
To:     Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Juergen Gross <jgross@suse.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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From:   Jiri Slaby <jslaby@suse.cz>
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Message-ID: <172007cc-6a82-99dd-2d1e-5103a6c1b9b0@suse.cz>
Date:   Fri, 7 Sep 2018 13:06:25 +0200
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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?

I could fix it, but untested changes are "a bit" worse than tested changes.

thanks,
-- 
js
suse labs