Project 4b: xv6 Threads


Some test cases can be found here


In this project, you'll be adding real kernel threads to xv6. Sound like fun? Well, it should. Because you are on your way to becoming a real kernel hacker. And what could be more fun than that?

Specifically, you'll do three things. First, you'll define a new system call to create a kernel thread, called clone(), as well as one to wait for a thread called join(). Then, you'll use clone() to build a little thread library, with a thread_create() and a thread_join(). Finally, you'll show these things work by using the TA's tests. That's it! And now, for some details.

Note: Start with a clean kernel; no need for your new fancy address space with the stack at the bottom, for example.


Your new clone system call should look like this:

int clone(void(*fcn)(void*), void *arg, void *stack) .

This call creates a new kernel thread which shares the calling process's address space. File descriptors are copied as in fork. The new process uses stack as its user stack, which is passed the given argument arg and uses a fake return PC (0xffffffff). The stack should be one page in size and page-aligned. The new thread starts executing at the address specified by fcn. As with fork(), the PID of the new thread is returned to the parent.

Another new system call is

int join(void **stack) 

This call waits for a child thread that shares the address space with the calling process. It returns the PID of waited-for child or -1 if none. The location of the child's user stack is copied into the argument stack.

You also need to think about the semantics of a couple of existing system calls. For example, int wait() should wait for a child process that does not share the address space with this process. It should also free the address space if this is last reference to it. Finally, exit() should work as before but for both processes and threads; little change is required here.

Your thread library will be built on top of this, and just have a simple routine

thread_create(void (*start_routine)(void*), void *arg) 

This routine should call malloc() to create a new user stack, use clone() to create the child thread and get it running. A paired routine

int thread_join() 

should also be used, which calls the underlying join() system call, frees the user stack, and then returns.

To test your code, use the TAs tests, as usual! But of course you should write your own little code snippets to test pieces as you go.

One thing you need to be careful with is when an address space is grown by a thread in a multi-threaded process. Trace this code path carefully and see where a new lock is needed and what else needs to be updated to grow an address space in a multi-threaded process correctly.

Have fun!


It might be good to read the xv6 book a bit.

For preparing stacks for threads (what's on the stack when you make a function call, and how it all is manipulated), you need to know a bit about calling convensions of x86. The following two links 1, 2 and the first few chapters of this book (Programming from the Ground Up) might be userful.