linux-kernel - Re: debugging oops after disconnecting Nexio USB touchscreen

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Date:	Wed, 2 Dec 2009 10:58:28 -0500 (EST)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Ondrej Zary <linux@...nbow-software.org>
cc:	linux-usb@...r.kernel.org, <linux-kernel@...r.kernel.org>
Subject: Re: debugging oops after disconnecting Nexio USB touchscreen

On Wed, 2 Dec 2009, Ondrej Zary wrote:

> It shows "prev is NULL, qh=f6581080, ehci->async=f6581000".
> 
> The problem is that activating usbmon causes the problem to disappear. No errors
> in maybe 15 attempts. It appeared on 2nd attempt after unloading usbmon.

Nevertheless, the usbmon output might be useful.  Try going through a 
couple of cycles with usbmon installed, then unload usbmon and create a 
crash.

Also, try adding some more debugging output (and let's hope it doesn't
also make the problem disappear).  In start_unlink_async(), just before
your "after:" label, add

	ehci_info(ehci, "unlink qh %p %p\n", qh, qh->qh_next);

In qh_link_async(), just after the wmb(), add

	ehci_info(ehci, "link qh %p %p\n", qh, qh->qh_next);

In end_unlink_async(), just after the iaa_watchdog_done(ehci), add

	ehci_info(ehci, "end unlink qh %p %p\n", qh, qh->next);

And in qh_make(), just before the end, add

	ehci_info(ehci, "create qh %p, dev %s, ep %x\n",
		qh, urb->dev->devpath, urb->ep->desc.bEndpointAddress);

Alan Stern

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