进程管理（三）

（一）：进程创建

linux不同于其他操作系统，linux在进程的创建的时候，将进程的创建和执行程序分成了两个函数，fork()和exec()。进程在创建的过程中，首先通过fork()函数拷贝一份当前进程来创建一个子进程。子进程和父进程的区别仅仅在于PID,PPID(父进程的进程号，子进程将其设置为被拷贝进程的进程号)和某些资源以及统计量（被挂起的信号等）。exec()函数负责执行负责执行可执行文件并将其载入地址空间开始运行。

1：写时拷贝
在传统的fotk()函数中，直接将进程的所有的资源复制给新创建的进程，这样有一些不好的地方，首先，这样会使得进程创建缓慢，其次就是有很多没有必要继承的数据被无辜继承下来，后来还需要修改，这样就会造成效率低下。

现在有了写时拷贝（copy-on-write），这是一种推迟甚至免除拷贝数据的技术，在进程被创建的时候，内核并不是复制整个进程地址空间，而是让父进程和子进程共享同一个拷贝。只有在需要写入的时候，数据才会被复制。

2：fork()
Linux通过clone()系统调用实现fork()。这个调用通过一系列的参数标志来指明父，子进程需要共享的资源。其中fork(),vfork(),_clone()都根据各自需要的参数标志去调用clone(),然后clone()在去调用dk_fork()。最后do_fork()调用copy_process()让进程开始运行。下面我们来看一下该调用过程。其中基本上大部分定义在 kernel/fork.c中。

我们首先看一下copy_process()函数：

/* * This creates a new process as a copy of the old one, * but does not actually start it yet. *  * 该方法会创建一个旧进程的拷贝，但是他并没有真实运行 * * It copies the registers, and all the appropriate * parts of the process environment (as per the clone * flags). The actual kick-off is left to the caller. * * 他会复制寄存器的内容，以及进程环境所有可能的部分。 * */static struct task_struct *copy_process(unsigned long clone_flags,                    unsigned long stack_start,                    struct pt_regs *regs,                    unsigned long stack_size,                    int __user *parent_tidptr,                    int __user *child_tidptr,                    int pid){    int retval;    struct task_struct *p = NULL;    if ((clone_flags & (CLONE_NEWNS|CLONE_FS)) == (CLONE_NEWNS|CLONE_FS))        return ERR_PTR(-EINVAL);    /*     * Thread groups must share signals as well, and detached threads     * can only be started up within the thread group.     *     * 线程组一定要共享信号，并且分离的线程也仅仅能在线程组中运行。     *     */    if ((clone_flags & CLONE_THREAD) && !(clone_flags & CLONE_SIGHAND))        return ERR_PTR(-EINVAL);    /*     * Shared signal handlers imply shared VM. By way of the above,     * thread groups also imply shared VM. Blocking this case allows     * for various simplifications in other code.     */    if ((clone_flags & CLONE_SIGHAND) && !(clone_flags & CLONE_VM))        return ERR_PTR(-EINVAL);    retval = security_task_create(clone_flags);    if (retval)        goto fork_out;    retval = -ENOMEM;    /*     * 使用dump_task_struct()为新进程创建一个内核栈，thread_info结构和task_struct，这些值     * 与当前进程相同，此时，子进程和父进程的描述符是完全一样的。     *     */    p = dup_task_struct(current);    /*     * 创建完成之后，检查新创建进程的正确性，以及当前拥有的进程数目没有     * 超过给他分配的资源的限制。     *     * 子进程开始着手将自己与父进程区别开来，进程描述符中的很多成员需要被清0     * 或被设置为初始值。     */    if (!p)        goto fork_out;#ifdef CONFIG_TRACE_IRQFLAGS    DEBUG_LOCKS_WARN_ON(!p->hardirqs_enabled);    DEBUG_LOCKS_WARN_ON(!p->softirqs_enabled);#endif    retval = -EAGAIN;    if (atomic_read(&p->user->processes) >=            p->signal->rlim[RLIMIT_NPROC].rlim_cur) {        if (!capable(CAP_SYS_ADMIN) && !capable(CAP_SYS_RESOURCE) &&                p->user != &root_user)            goto bad_fork_free;    }    atomic_inc(&p->user->__count);    atomic_inc(&p->user->processes);    get_group_info(p->group_info);    /*     * If multiple threads are within copy_process(), then this check     * triggers too late. This doesn't hurt, the check is only there     * to stop root fork bombs.     */    if (nr_threads >= max_threads)        goto bad_fork_cleanup_count;    if (!try_module_get(task_thread_info(p)->exec_domain->module))        goto bad_fork_cleanup_count;    if (p->binfmt && !try_module_get(p->binfmt->module))        goto bad_fork_cleanup_put_domain;    p->did_exec = 0;    delayacct_tsk_init(p);  /* Must remain after dup_task_struct() */    /*     * copy_flags()函数被调用，来更新task_struct的flags成员。表明进程     * 是否拥有超级权限的PF_SUPERPRIV标志被清0,表明进程还没有调用exec()     * 函数的PF_FORKNOEXEC标志被设置     */    copy_flags(clone_flags, p);    /*     * 由于之前函数刚开始的时候，调用函数alloc_pid()函数已经为该进程     * 分配了pid，在这里只需设置新创建进程的pid即可。     *     */    p->pid = pid;    retval = -EFAULT;    // 根据传递的参数标志，copy_process()拷贝或共享打开的文件，文件系统信息，    // 信号处理函数，进程地址空间和命名空间等。    if (clone_flags & CLONE_PARENT_SETTID)        if (put_user(p->pid, parent_tidptr))            goto bad_fork_cleanup_delays_binfmt;    INIT_LIST_HEAD(&p->children);    INIT_LIST_HEAD(&p->sibling);    p->vfork_done = NULL;    spin_lock_init(&p->alloc_lock);    clear_tsk_thread_flag(p, TIF_SIGPENDING);    init_sigpending(&p->pending);    p->utime = cputime_zero;    p->stime = cputime_zero;    p->sched_time = 0;    p->rchar = 0;       /* I/O counter: bytes read */    p->wchar = 0;       /* I/O counter: bytes written */    p->syscr = 0;       /* I/O counter: read syscalls */    p->syscw = 0;       /* I/O counter: write syscalls */    acct_clear_integrals(p);    p->it_virt_expires = cputime_zero;    p->it_prof_expires = cputime_zero;    p->it_sched_expires = 0;    INIT_LIST_HEAD(&p->cpu_timers[0]);    INIT_LIST_HEAD(&p->cpu_timers[1]);    INIT_LIST_HEAD(&p->cpu_timers[2]);    p->lock_depth = -1;     /* -1 = no lock */    do_posix_clock_monotonic_gettime(&p->start_time);    p->security = NULL;    p->io_context = NULL;    p->io_wait = NULL;    p->audit_context = NULL;    cpuset_fork(p);#ifdef CONFIG_NUMA    p->mempolicy = mpol_copy(p->mempolicy);    if (IS_ERR(p->mempolicy)) {        retval = PTR_ERR(p->mempolicy);        p->mempolicy = NULL;        goto bad_fork_cleanup_cpuset;    }    mpol_fix_fork_child_flag(p);#endif#ifdef CONFIG_TRACE_IRQFLAGS    p->irq_events = 0;    p->hardirqs_enabled = 0;    p->hardirq_enable_ip = 0;    p->hardirq_enable_event = 0;    p->hardirq_disable_ip = _THIS_IP_;    p->hardirq_disable_event = 0;    p->softirqs_enabled = 1;    p->softirq_enable_ip = _THIS_IP_;    p->softirq_enable_event = 0;    p->softirq_disable_ip = 0;    p->softirq_disable_event = 0;    p->hardirq_context = 0;    p->softirq_context = 0;#endif#ifdef CONFIG_LOCKDEP    p->lockdep_depth = 0; /* no locks held yet */    p->curr_chain_key = 0;    p->lockdep_recursion = 0;#endif    rt_mutex_init_task(p);#ifdef CONFIG_DEBUG_MUTEXES    p->blocked_on = NULL; /* not blocked yet */#endif    p->tgid = p->pid;    if (clone_flags & CLONE_THREAD)        p->tgid = current->tgid;    if ((retval = security_task_alloc(p)))        goto bad_fork_cleanup_policy;    if ((retval = audit_alloc(p)))        goto bad_fork_cleanup_security;    /* copy all the process information */    if ((retval = copy_semundo(clone_flags, p)))        goto bad_fork_cleanup_audit;    if ((retval = copy_files(clone_flags, p)))        goto bad_fork_cleanup_semundo;    if ((retval = copy_fs(clone_flags, p)))        goto bad_fork_cleanup_files;    if ((retval = copy_sighand(clone_flags, p)))        goto bad_fork_cleanup_fs;    if ((retval = copy_signal(clone_flags, p)))        goto bad_fork_cleanup_sighand;    if ((retval = copy_mm(clone_flags, p)))        goto bad_fork_cleanup_signal;    if ((retval = copy_keys(clone_flags, p)))        goto bad_fork_cleanup_mm;    if ((retval = copy_namespace(clone_flags, p)))        goto bad_fork_cleanup_keys;    retval = copy_thread(0, clone_flags, stack_start, stack_size, p, regs);    if (retval)        goto bad_fork_cleanup_namespace;    p->set_child_tid = (clone_flags & CLONE_CHILD_SETTID) ? child_tidptr : NULL;    /*     * Clear TID on mm_release()?     */    p->clear_child_tid = (clone_flags & CLONE_CHILD_CLEARTID) ? child_tidptr: NULL;    p->robust_list = NULL;#ifdef CONFIG_COMPAT    p->compat_robust_list = NULL;#endif    INIT_LIST_HEAD(&p->pi_state_list);    p->pi_state_cache = NULL;    /*     * sigaltstack should be cleared when sharing the same VM     */    if ((clone_flags & (CLONE_VM|CLONE_VFORK)) == CLONE_VM)        p->sas_ss_sp = p->sas_ss_size = 0;    /*     * Syscall tracing should be turned off in the child regardless     * of CLONE_PTRACE.     */    clear_tsk_thread_flag(p, TIF_SYSCALL_TRACE);#ifdef TIF_SYSCALL_EMU    clear_tsk_thread_flag(p, TIF_SYSCALL_EMU);#endif    /* Our parent execution domain becomes current domain       These must match for thread signalling to apply */    p->parent_exec_id = p->self_exec_id;    /* ok, now we should be set up.. */    p->exit_signal = (clone_flags & CLONE_THREAD) ? -1 : (clone_flags & CSIGNAL);    p->pdeath_signal = 0;    p->exit_state = 0;    /*     * Ok, make it visible to the rest of the system.     * We dont wake it up yet.     */    p->group_leader = p;    INIT_LIST_HEAD(&p->thread_group);    INIT_LIST_HEAD(&p->ptrace_children);    INIT_LIST_HEAD(&p->ptrace_list);    /* Perform scheduler related setup. Assign this task to a CPU. */    sched_fork(p, clone_flags);    /* Need tasklist lock for parent etc handling! */    write_lock_irq(&tasklist_lock);    /*     * The task hasn't been attached yet, so its cpus_allowed mask will     * not be changed, nor will its assigned CPU.     *     * The cpus_allowed mask of the parent may have changed after it was     * copied first time - so re-copy it here, then check the child's CPU     * to ensure it is on a valid CPU (and if not, just force it back to     * parent's CPU). This avoids alot of nasty races.     */    p->cpus_allowed = current->cpus_allowed;    if (unlikely(!cpu_isset(task_cpu(p), p->cpus_allowed) ||            !cpu_online(task_cpu(p))))        set_task_cpu(p, smp_processor_id());    /* CLONE_PARENT re-uses the old parent */    if (clone_flags & (CLONE_PARENT|CLONE_THREAD))        p->real_parent = current->real_parent;    else        p->real_parent = current;    p->parent = p->real_parent;    spin_lock(¤t->sighand->siglock);    /*     * Process group and session signals need to be delivered to just the     * parent before the fork or both the parent and the child after the     * fork. Restart if a signal comes in before we add the new process to     * it's process group.     * A fatal signal pending means that current will exit, so the new     * thread can't slip out of an OOM kill (or normal SIGKILL).     */    recalc_sigpending();    if (signal_pending(current)) {        spin_unlock(¤t->sighand->siglock);        write_unlock_irq(&tasklist_lock);        retval = -ERESTARTNOINTR;        goto bad_fork_cleanup_namespace;    }    if (clone_flags & CLONE_THREAD) {        p->group_leader = current->group_leader;        list_add_tail_rcu(&p->thread_group, &p->group_leader->thread_group);        if (!cputime_eq(current->signal->it_virt_expires,                cputime_zero) ||            !cputime_eq(current->signal->it_prof_expires,                cputime_zero) ||            current->signal->rlim[RLIMIT_CPU].rlim_cur != RLIM_INFINITY ||            !list_empty(¤t->signal->cpu_timers[0]) ||            !list_empty(¤t->signal->cpu_timers[1]) ||            !list_empty(¤t->signal->cpu_timers[2])) {            /*             * Have child wake up on its first tick to check             * for process CPU timers.             */            p->it_prof_expires = jiffies_to_cputime(1);        }    }    /*     * inherit ioprioretval     */    p->ioprio = current->ioprio;    if (likely(p->pid)) {        add_parent(p);        if (unlikely(p->ptrace & PT_PTRACED))            __ptrace_link(p, current->parent);        if (thread_group_leader(p)) {            p->signal->tty = current->signal->tty;            p->signal->pgrp = process_group(current);            p->signal->session = current->signal->session;            attach_pid(p, PIDTYPE_PGID, process_group(p));            attach_pid(p, PIDTYPE_SID, p->signal->session);            list_add_tail_rcu(&p->tasks, &init_task.tasks);            __get_cpu_var(process_counts)++;        }        attach_pid(p, PIDTYPE_PID, p->pid);        nr_threads++;    }    total_forks++;    spin_unlock(¤t->sighand->siglock);    write_unlock_irq(&tasklist_lock);    proc_fork_connector(p);    //最后返回新创建进程描述符的指针    return p;bad_fork_cleanup_namespace:    exit_namespace(p);bad_fork_cleanup_keys:    exit_keys(p);bad_fork_cleanup_mm:    if (p->mm)        mmput(p->mm);bad_fork_cleanup_signal:    cleanup_signal(p);bad_fork_cleanup_sighand:    __cleanup_sighand(p->sighand);bad_fork_cleanup_fs:    exit_fs(p); /* blocking */bad_fork_cleanup_files:    exit_files(p); /* blocking */bad_fork_cleanup_semundo:    exit_sem(p);bad_fork_cleanup_audit:    audit_free(p);bad_fork_cleanup_security:    security_task_free(p);bad_fork_cleanup_policy:#ifdef CONFIG_NUMA    mpol_free(p->mempolicy);bad_fork_cleanup_cpuset:#endif    cpuset_exit(p);bad_fork_cleanup_delays_binfmt:    delayacct_tsk_free(p);    if (p->binfmt)        module_put(p->binfmt->module);bad_fork_cleanup_put_domain:    module_put(task_thread_info(p)->exec_domain->module);bad_fork_cleanup_count:    put_group_info(p->group_info);    atomic_dec(&p->user->processes);    free_uid(p->user);bad_fork_free:    free_task(p);fork_out:    return ERR_PTR(retval);}

这样，我们来梳理一下copy_process()的工作流程。

根据代码，当该函数被调用的时候，首先会调用alloc_pid()函数分配一个新的pid。接着进程下面的操作：

1：调用dump_task_struct()函数为为新进程创建一个内核栈，thread_info结构和task_struct，这些值和当前进程的值相同。此时，子进程和父进程的进程描述符是完全一样的。2：接着，程序会检查新创建的子进程的正确性，并且检查当前用户所拥有的进程数目没有超过给他分配的资源限制。3：子进程着手将自己与父进程去被开来。进程描述符内的很多成员都要被清0或者是设置为初始值。    4：子进程的状态被设置为TASK_UNINTERRUPTIBLE，来保证不会投入运行5：调用copy_flags()来更新task_struct的flags成员。表示用户是否拥有超级权限的标志PF_SUPERPRIV被清0.6：设置新进程的pid7：根据clone()传递的参数标志，copy_process()拷贝或共享打开的文件，文件系统信息，信号处理函数，进程地址空间和命名空间等。8：最后，返回一个指向子进程的指针。