On Fri, 22 Feb 2013, Johan Hovold wrote: > We keep the usb-serial-port structure around until the last tty > reference is dropped, but the port private data is freed as part of > unregistering from the bus in disconnect. [ The subdrivers aren't > supposed to access the ports after port_remove is called as part of > unregistration. ] > > This wasn't always the case, but unregistering in serial_cleanup (when > the last tty reference is dropped) had other issues. In particular, we > would end up unregistering the parent device before its child (the > interface and usb device are unregistered when disconnect returns) > resulting in broken uevent devpaths: > > KERNEL[794.849051] remove /devices/pci0000:00/0000:00:1a.7/usb1/1-4/1-4.1/1-4.1:1.0 (usb) > KERNEL[794.851649] remove /devices/pci0000:00/0000:00:1a.7/usb1/1-4/1-4.1 (usb) > KERNEL[795.115623] remove /1-4.1:1.0/ttyUSB0/tty/ttyUSB0 (tty) > KERNEL[795.117429] remove /1-4.1:1.0/ttyUSB0 (usb-serial) > > This was reported here > > https://bugs.freedesktop.org/show_bug.cgi?id=20703 > > and fixed by Alan Stern in 2d93148ab6 (USB: serial: fix lifetime and > locking problems) by moving unregistration to disconnect. > > So we end up with an unregistered device still possibly referenced by > tty instead, and I suspect we can't do much else than deal with any > post-disconnect callbacks. [ These should be few, especially with my > latest TTY-patches applied. ] > > Alan, do you see any way around this? It's not possible (or desirable) > to pin the parent device (interface) until the last reference is > dropped, is it? On the contrary, it is customary to pin data structures until the last reference to them is gone. That's what krefs are for. On the other hand, the port private data was owned entirely by the serial sub-driver. Neither the serial core nor the tty layer is able to use it meaningfully -- they don't even know what type of structure it is. Therefore freeing the structure when the port is removed should be harmless -- unless the subdriver is called after the structure is deallocated. This means there still is one bug remaining. In usb_serial_device_remove(), the call to tty_unregister_device() should occur _before_ the call to driver->port_remove(), not _after_. Do you think changing the order cause any new problems? Alan Stern -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
