[9fans] usb panic

["Michael Teichgräber" <mt@wmipf.in-berlin.de>]

Hi,

accidently I started usb/usbmouse a second time, when one copy was
already running and handling an (the only) USB mouse. As a result I
got the following panic message:

    panic: kernel fault: no user process pc=0xf010051a addr=0x0000003c
    dumpstack disabled
    cpu0: exiting

I can reproduce it easily on two different machines with the following
steps, even without starting usbmouse a second time, just writing to
the `ctl' file:

    % usb/usbd
    % usb/usbmouse

The `usbmouse' process is now reading from e.g. /dev/usb1/1/ep1data

    % cd /dev/usb1/1
    % echo 'ep 1 bulk r 3 32' > ctl    # from /sys/src/cmd/usb/misc/usbmouse.c

The `echo' simulates the accidently restarted usb/usbmouse program, it
tries to reconfigure endpoint 1, which is already used by a running program.
An error message: "write error: permission denied" occurs immediately,
and the ep1data file vanished.

If the mouse has not been moved so far, the system is still
running. If I now move the mouse a bit, i.e. generate some traffic on
the endpoint, the panic message occures and the system is halted.

What does this message mean? Is there a way to debug this? (I have not
much experience with debugging on plan 9 so far.)

Michael