在上次的文章中詳細的介紹了幾個系統調用,它們最終都是調用了do_fork來實現進程的創建。do_fork主要完成了進程描述符的創建和pid的創建,以及進程描述符的拷貝。
本系列文章所用源碼均來自2.6.38.
源碼分析如下:
/* * Ok, this is the main fork-routine. * * It copies the process, and if successful kick-starts * it and waits for it to finish using the VM if required. */ /*這部分代碼是在2.6.38中實現的*/ /*參數clone_flags由兩部分組成,最低的一個字節為信號掩碼,用於指定子進程退出時 *子進程向父進程發出的信號,通過sys_fork和sys_vfork知道它們的信號就是SIGCHLD,而 *clone由用戶自己決定。對於第二部分表示資源和特性標志位,fork為0,vfork為CLONE_VFORK和CLONE_VM *而clone由用戶自己定義。 */ long do_fork(unsigned long clone_flags, unsigned long stack_start, struct pt_regs *regs, unsigned long stack_size, int __user *parent_tidptr, int __user *child_tidptr) { /*定義一個進程描述符*/ struct task_struct *p; int trace = 0; long nr;//子進程號 /* * Do some preliminary argument and permissions checking before we * actually start allocating stuff */ /* *一些必要的檢查工作,我們會發現在sys_fork,sys_vfork,kernel_thread中都沒有傳遞CLONE_NEWUSER可見 *以下這些代碼沒有執行,這個檢查主要是為sys_clone使用的。 */ if (clone_flags & CLONE_NEWUSER) { if (clone_flags & CLONE_THREAD)//跟蹤標志被設置,出錯。 return -EINVAL; /* hopefully this check will go away when userns support is * complete */ if (!capable(CAP_SYS_ADMIN) || !capable(CAP_SETUID) || !capable(CAP_SETGID)) return -EPERM; } /* * We hope to recycle these flags after 2.6.26 */ /*這些代碼也是就一些檢查工作*/ if (unlikely(clone_flags & CLONE_STOPPED)) { static int __read_mostly count = 100; if (count > 0 && printk_ratelimit()) { char comm[TASK_COMM_LEN]; count--; printk(KERN_INFO "fork(): process `%s' used deprecated " "clone flags 0x%lx\n", get_task_comm(comm, current), clone_flags & CLONE_STOPPED); } } /* 查看本欄目更多精彩內容:http://www.bianceng.cn/OS/unix/ * When called from kernel_thread, don't do user tracing stuff. */ if (likely(user_mode(regs))) trace = tracehook_prepare_clone(clone_flags); /*copy_process來完成具體進程的創建,在系統資源豐富的條件下,來完成進程描述符的拷貝,當然進程號不同*/ p = copy_process(clone_flags, stack_start, regs, stack_size, child_tidptr, NULL, trace); /* *調用完copy_process後如果沒有指定CLONE_STOPPED就會調用下面的wake_up_new_task把新建的進程放到 *運行隊列中。如果父子進程在同一個cpu中運行,且在沒有設置CLONE_VM標志,則會采用寫實復制技術,把子進程放到 *父進程的前面,如果子進程調用了exec就會避免一系列不必要的復制操作。 */ /* * Do this prior waking up the new thread - the thread pointer * might get invalid after that point, if the thread exits quickly. */ /*IS_ERR()判斷p是否正確*/ if (!IS_ERR(p)) { /*進程描述符創建成功後,根據clone_flags來設置進程狀態*/ struct completion vfork; trace_sched_process_fork(current, p); nr = task_pid_vnr(p); /* *在sys_fork,sys_vfork,kernel_thread中沒有CLONE_PARENT_SETTID且parent_tidptr=NULL *為sys_clone使用 */ if (clone_flags & CLONE_PARENT_SETTID) put_user(nr, parent_tidptr); /* *sys_fork 或 sys_clone檢查的,如果設置了就把父進程放進等待隊列中 */ if (clone_flags & CLONE_VFORK) { p->vfork_done = &vfork; init_completion(&vfork); } audit_finish_fork(p); tracehook_report_clone(regs, clone_flags, nr, p); /* * We set PF_STARTING at creation in case tracing wants to * use this to distinguish a fully live task from one that * hasn't gotten to tracehook_report_clone() yet. Now we * clear it and set the child going. */ p->flags &= ~PF_STARTING; if (unlikely(clone_flags & CLONE_STOPPED)) { /* * We'll start up with an immediate SIGSTOP. */ sigaddset(&p->pending.signal, SIGSTOP); set_tsk_thread_flag(p, TIF_SIGPENDING); __set_task_state(p, TASK_STOPPED); } else { wake_up_new_task(p, clone_flags); } tracehook_report_clone_complete(trace, regs, clone_flags, nr, p); if (clone_flags & CLONE_VFORK) { freezer_do_not_count(); wait_for_completion(&vfork);//等待隊列 freezer_count(); tracehook_report_vfork_done(p, nr); } } else { nr = PTR_ERR(p); } return nr; }
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Author:csdn博客 muge0913