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标题: 1.0内核--fork.c [打印本页]
作者: blackghost    时间: 2002-11-18 23:41     标题: 1.0内核--fork.c

/* * linux/kernel/fork.c * * Copyright (C) 1991, 1992 Linus Torvalds */ /* * 'fork.c' contains the help-routines for the 'fork' system call * (see also system_call.s). * Fork is rather simple, once you get the hang of it, but the memory * management can be a bitch. See 'mm/mm.c': 'copy_page_tables()' */ #include #include #include #include #include #include #include #include #include #include #include #include asmlinkage void ret_from_sys_call(void) __asm__("ret_from_sys_call"); /* These should maybe be in */ #define MAX_TASKS_PER_USER (NR_TASKS/2)//定义每个用户的最大可创建的进程数!!!!! #define MIN_TASKS_LEFT_FOR_ROOT 4//给root留得最少的进程数 extern int shm_fork(struct task_struct *, struct task_struct *);//共享内存复制 long last_pid=0; static int find_empty_process(void) { int free_task; int i, tasks_free; int this_user_tasks; repeat: if ((++last_pid) & 0xffff8000) last_pid=1; this_user_tasks = 0; tasks_free = 0; free_task = -EAGAIN; i = NR_TASKS; while (--i > 0) { if (!task) { free_task = i; tasks_free++; continue; } if (task->uid == current->uid) this_user_tasks++; if (task->pid == last_pid || task->pgrp == last_pid || task->session == last_pid) goto repeat; } if (tasks_free <= MIN_TASKS_LEFT_FOR_ROOT || this_user_tasks > MAX_TASKS_PER_USER) if (current->uid) return -EAGAIN; return free_task; }//找到空闲的task结构,其中要判断这个用户能创建的进程是不是超过了它能创建的进程总数 static struct file * copy_fd(struct file * old_file)//复制打开文件描述符,并且为新的进程分配相应的file结构 { struct file * new_file = get_empty_filp(); int error; if (new_file) { memcpy(new_file,old_file,sizeof(struct file)); new_file->f_count = 1; if (new_file->f_inode) new_file->f_inode->i_count++; if (new_file->f_op && new_file->f_op->open) { error = new_file->f_op->open(new_file->f_inode,new_file); if (error) { iput(new_file->f_inode); new_file->f_count = 0; new_file = NULL; } } } return new_file; } int dup_mmap(struct task_struct * tsk)//复制内存映射 { struct vm_area_struct * mpnt, **p, *tmp; tsk->mmap = NULL; tsk->stk_vma = NULL; p = &tsk->mmap; for (mpnt = current->mmap ; mpnt ; mpnt = mpnt->vm_next) { tmp = (struct vm_area_struct *) kmalloc(sizeof(struct vm_area_struct), GFP_KERNEL);//在内核空间分配空间 if (!tmp) return -ENOMEM; *tmp = *mpnt; tmp->vm_task = tsk; tmp->vm_next = NULL; if (tmp->vm_inode) tmp->vm_inode->i_count++;//将相应的i节点引用数加一 *p = tmp; p = &tmp->vm_next; if (current->stk_vma == mpnt) tsk->stk_vma = tmp; } return 0; } #define IS_CLONE (regs.orig_eax == __NR_clone) #define copy_vm(p) ((clone_flags & COPYVM)?copy_page_tables(p):clone_page_tables(p)) /* * Ok, this is the main fork-routine. It copies the system process * information (task[nr]) and sets up the necessary registers. It * also copies the data segment in its entirety. */ asmlinkage int sys_fork(struct pt_regs regs) { struct pt_regs * childregs; struct task_struct *p; int i,nr; struct file *f; unsigned long clone_flags = COPYVM | SIGCHLD; if(!(p = (struct task_struct*)__get_free_page(GFP_KERNEL))) goto bad_fork; nr = find_empty_process(); if (nr < 0) goto bad_fork_free; task[nr] = p;//分配进程结构 *p = *current;// p->did_exec = 0;//不可执行 p->kernel_stack_page = 0; p->state = TASK_UNINTERRUPTIBLE; p->flags &= ~(PF_PTRACED|PF_TRACESYS); p->pid = last_pid;//给进程分配的id p->swappable = 1;//可以交换到swap分区 p->p_pptr = p->p_opptr = current;//将当前进程结构指针付给子进程的父进程指针 p->p_cptr = NULL;//没有子进程 SET_LINKS(p); p->signal = 0; p->it_real_value = p->it_virt_value = p->it_prof_value = 0; p->it_real_incr = p->it_virt_incr = p->it_prof_incr = 0; p->leader = 0;/* process leadership doesn't inherit */ p->utime = p->stime = 0; p->cutime = p->cstime = 0; p->min_flt = p->maj_flt = 0; p->cmin_flt = p->cmaj_flt = 0; p->start_time = jiffies;//开始时间,是从进程创建的时候开始的!!!!!!!!!!!!!!!!!! /* * set up new TSS and kernel stack */ if (!(p->kernel_stack_page = __get_free_page(GFP_KERNEL)))//分配新的堆栈,当然不能与父进程共用了 goto bad_fork_cleanup; p->tss.es = KERNEL_DS; p->tss.cs = KERNEL_CS; p->tss.ss = KERNEL_DS; p->tss.ds = KERNEL_DS; p->tss.fs = USER_DS; p->tss.gs = KERNEL_DS; p->tss.ss0 = KERNEL_DS; p->tss.esp0 = p->kernel_stack_page + PAGE_SIZE;//因为堆栈是从高地址开始的 p->tss.tr = _TSS(nr); childregs = ((struct pt_regs *) (p->kernel_stack_page + PAGE_SIZE)) - 1; p->tss.esp = (unsigned long) childregs; p->tss.eip = (unsigned long) ret_from_sys_call;//我个人认为这里是fork()得最经典的地方,可谓设计的十分巧妙,当年 *childregs = regs; //阅读unix的源码的的时候就是这样的感觉!!!!!!!!1 childregs->eax = 0; p->tss.back_link = 0; p->tss.eflags = regs.eflags & 0xffffcfff;/* iopl is always 0 for a new process */ if (IS_CLONE) { if (regs.ebx) childregs->esp = regs.ebx; clone_flags = regs.ecx; if (childregs->esp == regs.esp) clone_flags |= COPYVM; } p->exit_signal = clone_flags & CSIGNAL; p->tss.ldt = _LDT(nr); if (p->ldt) { p->ldt = (struct desc_struct*) vmalloc(LDT_ENTRIES*LDT_ENTRY_SIZE); if (p->ldt != NULL) memcpy(p->ldt, current->ldt, LDT_ENTRIES*LDT_ENTRY_SIZE); } p->tss.bitmap = offsetof(struct tss_struct,io_bitmap); for (i = 0; i < IO_BITMAP_SIZE+1 ; i++) /* IO bitmap is actually SIZE+1 */ p->tss.io_bitmap = ~0; if (last_task_used_math == current) __asm__("clts ; fnsave %0 ; frstor %0":"=m" (p->tss.i387)); p->semun = NULL; p->shm = NULL; if (copy_vm(p) || shm_fork(current, p)) goto bad_fork_cleanup; if (clone_flags & COPYFD) { for (i=0; ifilp) != NULL) p->filp = copy_fd(f); } else { for (i=0; ifilp) != NULL) f->f_count++; } if (current->pwd) current->pwd->i_count++; if (current->root) current->root->i_count++; if (current->executable) current->executable->i_count++; dup_mmap(p); set_tss_desc(gdt+(nr<<1)+FIRST_TSS_ENTRY,&(p->tss)); if (p->ldt) set_ldt_desc(gdt+(nr<<1)+FIRST_LDT_ENTRY,p->ldt, 512); else set_ldt_desc(gdt+(nr<<1)+FIRST_LDT_ENTRY,&default_ldt, 1); p->counter = current->counter >> 1; p->state = TASK_RUNNING;/* do this last, just in case */ return p->pid; bad_fork_cleanup: task[nr] = NULL; REMOVE_LINKS(p); free_page(p->kernel_stack_page); bad_fork_free: free_page((long) p); bad_fork: return -EAGAIN; }



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