Re: [Xen-devel] [PATCH] xen-netfront: wait xenbus state change when load module manually

On 08/24/2018, 04:26 PM, Boris Ostrovsky wrote:

On 08/24/2018 07:26 AM, Juergen Gross wrote:

quoted

On 24/08/18 13:12, Jiri Slaby wrote:

quoted

On 07/30/2018, 10:18 AM, Xiao Liang wrote:

quoted

On 07/29/2018 11:30 PM, David Miller wrote:

quoted

From: Xiao Liang <redacted>
Date: Fri, 27 Jul 2018 17:56:08 +0800

quoted

@@ -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.

quoted

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