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Project 4: xv6 Threads

[TOC]

Overview

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 (in the user space), with a thread_create() and thread_join() call. Additionally we need to add the support for locks and conditional variables. Hence, we need lock_ini(), lock_acquire() and lock_release() for locks and cv_init(), cv_wait() and cv_signal() for conditional variables. And now, for some details.

Details

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.

The other 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 thread_create(void (*start_routine)(void*), void *arg) routine. This routine should call malloc() to create a new user stack, use clone() to create the child thread and get it running. A thread_join() call should also be used, which calls the underlying join() system call, frees the user stack, and then returns.

Your thread library should also have a simple spin lock. There should be a type lock_t that one uses to declare a lock, and two routines lock_acquire(lock_t *) and lock_release(lock_t *) , which acquire and release the lock. The spin lock should use x86 atomic exchange to built the spin lock (see the xv6 kernel for an example of something close to what you need to do). One last routine, lock_init(lock_t *) , is used to initialize the lock as need be.

Finally, you should have a simple condition variable and related routines: cond_t and cv_wait(cond_t *, lock_t *) and cv_signal(cond_t *). These routines should do what is expected: either put the caller to sleep (and release the lock) or wake a sleeping thread, respectively. You will have to add additional system calls to achieve this. Just left it to the end of the project.

To test your code, use the provided 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.

Reference

As usual, code and book.

You may also find this book useful: Programming from the Ground Up . Particular attention should be paid to the first few chapters, including the calling convention (i.e., what's on the stack when you make a function call, and how it all is manipulated). Since the book uses x86, it should be perfect to translate your MIPS knowledge to xv6 context. Although it sounds scary, the only place where we need to tinkering with this is when clone() has to start the fcn with the right args. We don need to write a single line in assembler, just to put the things in the stack in the proper order to guarantee that fcn works as desired!

Environment

As usual