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Date:   Tue, 3 Sep 2019 13:06:07 -0400 (EDT)
From:   Alan Stern <stern@rowland.harvard.edu>
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To:     David Howells <dhowells@redhat.com>
cc:     Guenter Roeck <linux@roeck-us.net>, <viro@zeniv.linux.org.uk>,
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Subject: Re: [PATCH 08/11] usb: Add USB subsystem notifications [ver #7]
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On Tue, 3 Sep 2019, David Howells wrote:

> Guenter Roeck <linux@roeck-us.net> wrote:
> 
> > > > This added call to usbdev_remove() results in a crash when running
> > > > the qemu "tosa" emulation. Removing the call fixes the problem.
> > > 
> > > Yeah - I'm going to drop the bus notification messages for now.
> > > 
> > It is not the bus notification itself causing problems. It is the
> > call to usbdev_remove().
> 
> Unfortunately, I don't know how to fix it and don't have much time to
> investigate it right now - and it's something that can be added back later.

The cause of your problem is quite simple:

 static int usbdev_notify(struct notifier_block *self,
 			       unsigned long action, void *dev)
 {
 	switch (action) {
 	case USB_DEVICE_ADD:
+		post_usb_device_notification(dev, NOTIFY_USB_DEVICE_ADD, 0);
 		break;
 	case USB_DEVICE_REMOVE:
+		post_usb_device_notification(dev, NOTIFY_USB_DEVICE_REMOVE, 0);
+		usbdev_remove(dev);
+		break;
+	case USB_BUS_ADD:
+		post_usb_bus_notification(dev, NOTIFY_USB_BUS_ADD, 0);
+		break;
+	case USB_BUS_REMOVE:
+		post_usb_bus_notification(dev, NOTIFY_USB_BUS_REMOVE, 0);
 		usbdev_remove(dev);
 		break;
 	}

The original code had usbdev_remove(dev) under the USB_DEVICE_REMOVE
case.  The patch mistakenly moves it, putting it under the
USB_BUS_REMOVE case.

If the usbdev_remove() call were left where it was originally, the 
problem would be solved.

Alan Stern