If you have the process that is currently blocked (note that "zombie" doesn't apply here -- zombies are processes that are dead but are waiting for someone to c

If you have the process that is currently blocked (note that "zombie" doesn't apply here -- zombies are processes that are dead but are waiting for someone to collect their exit code) you can look in /proc/$pid/stack to see what the current kernel stack is (the bottom-most entries tell you what syscall the process started by doing). This is the easiest way to start debugging a kernel problem (what is the function it's blocked in -- then go read that function and see whether the the pathology is obvious).