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Subject: Re: [Xen-devel] [PATCH] xen-netfront: wait xenbus state change when
 load module manually
To:     Xiao Liang <xiliang@redhat.com>, David Miller <davem@davemloft.net>
Cc:     jgross@suse.com, netdev@vger.kernel.org,
        boris.ostrovsky@oracle.com, linux-kernel@vger.kernel.org,
        xen-devel@lists.xenproject.org
References: <20180727095608.25210-1-xiliang@redhat.com>
 <20180729.083040.53901922165497594.davem@davemloft.net>
 <60e93e09-7dc2-d2f6-379d-c1d086aa1f49@redhat.com>
From:   Jiri Slaby <jslaby@suse.cz>
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Message-ID: <83ad1e3c-5e3b-8b24-430b-13e12b59ae8b@suse.cz>
Date:   Fri, 24 Aug 2018 13:12:52 +0200
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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?

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)?

thanks,
-- 
js
suse labs