When using lprof (svn at date of bug submission) with a GretagMacbeth Eye1 2 to set the display black level, the UI hangs when trying to measure the black region. The hang continues until it is broken by dramatically increasing the brightness of the region under the colour meter (eg by moving a bright window over it).
ps shows that lprof is in a system call (wchan=usb_start_wait_urb). I obtained a kernel backtrace using alt+sqsrq+t and extracted the thread information for lprof. It's attached as lprof_task_trace.txt .
I can't / don't know how to get a user-space backtrace on the process while it's in a system call. However, lprof_gdb_bt.txt shows what the stack looks like if I hit CTL-C in gdb while it's waiting in the kernel, then move a bright window over the colour meter.
I'm using libusb 0.1.12-5 from Debian Etch on a 2.6.18 kernel (full uname 2.6.18-4-686 #1 SMP Wed May 9 23:03:12 UTC 2007 i686 GNU/Linux). I'll be updating to 2.6.21 shortly to see if that affects the problem.