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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>, Xiao Liang <xiliang@redhat.com>,
        David Miller <davem@davemloft.net>
Cc:     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>
 <83ad1e3c-5e3b-8b24-430b-13e12b59ae8b@suse.cz>
From:   Juergen Gross <jgross@suse.com>
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Message-ID: <830d73b5-8bbb-4d9a-a004-7aeea0b35067@suse.com>
Date:   Fri, 24 Aug 2018 13:26:38 +0200
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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.


Juergen