On 23.08.2016 18:10, Alan Stern wrote:
On Tue, 23 Aug 2016, Mathias Nyman wrote:
The Dell XPS 9550 has an additional xhci controller for handling the type-C port.
This controller is hotplug removed from the PCI bus when the last USB type-c
device is disconnected.
xhci driver, and usb core it seems is not really designed with this in mind.
xhci driver will suddenly start reading ffffffff from PCI.
I've been looking at issues related to this. Currently there is at least one similar case with mass storage
where we see the device release function being called for the mass storage interface device _after_ we
freed all memory related to both xhci hcd's. bug for that is here:
https://bugzilla.kernel.org/show_bug.cgi?id=120241
usb devices with their children should be synchronously removed before hcd's are freed, but seems
that is not the case, at least not for the device release function for the interface device.
Be careful what words you use. USB devices and their children are
indeed synchronously _removed_ before hcd's are freed. However, they
may not be _released_ until later.
What you encountered in that bug report was probably usb_release_dev()
calling bus_to_hcd() and usb_put_hcd(). This action is part of a
release, and it's allowed to happen long after everything has been
removed (i.e., unregistered).
Yes, it's the release function which is called later,
I did mix up the concepts of remove and release.
Reference counting is supposed to keep everything important from being
deallocated until all the releases are finished. In particular, if the
hcd structure was already deallocated when usb_put_hcd() was called
then there is a refcounting bug somewhere.
That seems like a possible cause, I'll look into the refcouting.
This helps a lot. I was getting stuck with that bug.
Thanks
-Mathias
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