MIT6.828 HW8 uthreads

　　在这次作业中，我们将实现1个简单的用户级线程包。
　　
Switching threads
　　下载uthread.c和uthread_switch.S到xv6目录，在Makefile文件的_forktest规则之后添加如下规则：

_uthread: uthread.o uthread_switch.o    $(LD) $(LDFLAGS) -N -e main -Ttext 0 -o _uthread uthread.o uthread_switch.o $(ULIB)    $(OBJDUMP) -S _uthread > uthread.asm

　　在UPROGS目标下添加uthread依赖项。
　　
　　当你完成uthread_switch.S时，你将获得如下的输出：

~/classes/6828/xv6$ make CPUS=1 qemu-noxdd if=/dev/zero of=xv6.img count=1000010000+0 records in10000+0 records out5120000 bytes transferred in 0.037167 secs (137756344 bytes/sec)dd if=bootblock of=xv6.img conv=notrunc1+0 records in1+0 records out512 bytes transferred in 0.000026 secs (19701685 bytes/sec)dd if=kernel of=xv6.img seek=1 conv=notrunc307+1 records in307+1 records out157319 bytes transferred in 0.003590 secs (43820143 bytes/sec)qemu -nographic -hdb fs.img xv6.img -smp 1 -m 512 Could not open option rom 'sgabios.bin': No such file or directoryxv6...cpu0: startinginit: starting sh$ uthreadmy thread runningmy thread 0x2A30my thread runningmy thread 0x4A40my thread 0x2A30my thread 0x4A40my thread 0x2A30my thread 0x4A40....

　　uthread创建2个线程，然后互相交替执行。每个线程打印”my thread …”，然后退让给其他线程机会去执行。
　　首先必须熟悉一下uthread.c文件中的内容。uthread.c中有2个全局变量current_thread和next_thread，是指向thread结构体的指针。而thread结构体的定义如下：

struct thread {  int        sp;             /* curent stack pointer */  char stack[STACK_SIZE];    /* the thread's stack */  int        state;          /* running, runnable, waiting */};

　　每个thread都有1个stack和指向stack的指针sp，在内存中的布局如下：
　　
　　在main函数中，首先进行线程初始化，然后创建2个线程，并开始调度。其中main也是1个线程，但是只在第1次调度时，被涉及，后面调度时，由于一直是running状态，所以不会被调度。

int main(int argc, char *argv[]) {  thread_init();  thread_create(mythread);  thread_create(mythread);  thread_schedule();  return 0;}

　　thread_schedule函数实现从线程链表中寻找1个可运行的线程，然后进行切换运行。类似于进程调度，最后用thread_switch进行上下文保存和替换工作。

static void thread_schedule(void){  thread_p t;  /* Find another runnable thread. */  for (t = all_thread; t < all_thread + MAX_THREAD; t++) {    if (t->state == RUNNABLE && t != current_thread) {      next_thread = t;      break;    }  }  if (t >= all_thread + MAX_THREAD && current_thread->state == RUNNABLE) {    /* The current thread is the only runnable thread; run it. */    next_thread = current_thread;  }  if (next_thread == 0) {    printf(2, "thread_schedule: no runnable threads; deadlock\n");    exit();  }  if (current_thread != next_thread) {         /* switch threads?  */    next_thread->state = RUNNING;    thread_switch();  } else    next_thread = 0;}

　　thread_switch函数就是我们要实现的内容，它的任务是保存当前运行的线程状态到current_thread指针指向的thread结构体，然后从next_thread指针指向的结构体中恢复将要运行的线程状态，同时将current_thread指向next_thread指向的结构体，将next_thread的值清零。

thread_switch:        /* YOUR CODE HERE */        pushal        movl current_thread, %eax        movl %esp, (%eax)        movl next_thread, %ebx        movl %ebx, current_thread        movl (%ebx), %esp        popal        movl $0x0, next_thread        ret                /* pop return address from stack */

　　首先将通用寄存器压入堆栈，保存sp到current_thread指针指向额结构体，然后从next_thread指针指向的结构体中恢复堆栈指针和通用寄存器，改变2个指针的值。