谈谈守护进程与僵尸进程

前两天在网上看到许多帖子以及一些unix书籍，认为一次fork后产生守护进程足够了，各有道理吧，不过多了一次fork到底是出于什么目的呢？

进程也就是task，看看内核里维护进程的数据结构task_struct，这里有两个成员：

struct task_struct {
    volatile long state;
    int exit_state;
    ...
}

看看include/linux/sched.h里的value取值：

#define TASK_RUNNING        0
#define TASK_INTERRUPTIBLE  1
#define TASK_UNINTERRUPTIBLE    2
#define __TASK_STOPPED      4
#define __TASK_TRACED       8
/* in tsk->exit_state */
#define EXIT_ZOMBIE     16
#define EXIT_DEAD       32
/* in tsk->state again */
#define TASK_DEAD       64
#define TASK_WAKEKILL       128
#define TASK_WAKING     256
#define TASK_STATE_MAX      512

可以看到，进程状态里除了大家都理解的running/interuptible/uninterruptible/stop等状态外，还有一个ZOMBIE状态，这个状态是怎么回事呢？

这是因为linux里的进程都属于一颗树，树的根结点是linux系统初始化结束阶段时启动的init进程，这个进程的pid是1，所有的其他进程都是它的子孙。除了init，任何进程一定有他的父进程，而父进程会负责分配（fork）、回收（wait4）它申请的进程资源。这个树状关系也比较健壮，当某个进程还在运行时，它的父进程却退出了，这个进程却没有成为孤儿进程，因为linux有一个机制，init进程会接管它，成为它的父进程。这也是守护进程的由来了，因为守护进程的其中一个要求就是希望init成为守护进程的父进程。

如果某个进程自身终止了，在调用exit清理完相关的内容文件等资源后，它就会进入ZOMBIE状态，它的父进程会调用wait4来回收这个task_struct，但是，如果父进程一直没有调用wait4去释放子进程的task_struct，问题就来了，这个task_struct谁来回收呢？永远没有人，除非父进程终止后，被init进程接管这个ZOMBIE进程，然后调用wait4来回收进程描述符。如果父进程一直在运行着，这个ZOMBIE会永远的占用系统资源，用KILL发任何信号量也不能释放它。这是很可怕的，因为服务器上可能会出现无数ZOMBIE进程导致机器挂掉。

来看看内核代码吧。进程在退出时执行sys_exit（C程序里在main函数返回会执行到），而它会调用do_exit，do_exit首先清理进程使用的资源，然后调用exit_notify方法，将进程置为僵尸ZOMBIE状态，决定是否要以init进程做为当前进程的父进程，最后通知当前进程的父进程：

kernel/exit.c

static void exit_notify(struct task_struct *tsk)
{
    int state;
    struct task_struct *t;
    struct list_head ptrace_dead, *_p, *_n;

    if (signal_pending(tsk) && !tsk->signal->group_exit
        && !thread_group_empty(tsk)) {
        /*
         * This occurs when there was a race between our exit
         * syscall and a group signal choosing us as the one to
         * wake up.  It could be that we are the only thread
         * alerted to check for pending signals, but another thread
         * should be woken now to take the signal since we will not.
         * Now we'll wake all the threads in the group just to make
         * sure someone gets all the pending signals.
         */
        read_lock(&tasklist_lock);
        spin_lock_irq(&tsk->sighand->siglock);
        for (t = next_thread(tsk); t != tsk; t = next_thread(t))
            if (!signal_pending(t) && !(t->flags & PF_EXITING)) {
                recalc_sigpending_tsk(t);
                if (signal_pending(t))
                    signal_wake_up(t, 0);
            }
        spin_unlock_irq(&tsk->sighand->siglock);
        read_unlock(&tasklist_lock);
    }

    write_lock_irq(&tasklist_lock);

    /*
     * This does two things:
     *
     * A.  Make init inherit all the child processes
     * B.  Check to see if any process groups have become orphaned
     *  as a result of our exiting, and if they have any stopped
     *  jobs, send them a SIGHUP and then a SIGCONT.  (POSIX 3.2.2.2)
     */

    INIT_LIST_HEAD(&ptrace_dead);
    <strong><span style="color:#ff0000;">forget_original_parent(tsk, &ptrace_dead);</span></strong>
    BUG_ON(!list_empty(&tsk->children));
    BUG_ON(!list_empty(&tsk->ptrace_children));

    /*
     * Check to see if any process groups have become orphaned
     * as a result of our exiting, and if they have any stopped
     * jobs, send them a SIGHUP and then a SIGCONT.  (POSIX 3.2.2.2)
     *
     * Case i: Our father is in a different pgrp than we are
     * and we were the only connection outside, so our pgrp
     * is about to become orphaned.
     */

    t = tsk->real_parent;

    if ((process_group(t) != process_group(tsk)) &&
        (t->signal->session == tsk->signal->session) &&
        will_become_orphaned_pgrp(process_group(tsk), tsk) &&
        has_stopped_jobs(process_group(tsk))) {
        __kill_pg_info(SIGHUP, (void *)1, process_group(tsk));
        __kill_pg_info(SIGCONT, (void *)1, process_group(tsk));
    }

    /* Let father know we died
     *
     * Thread signals are configurable, but you aren't going to use
     * that to send signals to arbitary processes.
     * That stops right now.
     *
     * If the parent exec id doesn't match the exec id we saved
     * when we started then we know the parent has changed security
     * domain.
     *
     * If our self_exec id doesn't match our parent_exec_id then
     * we have changed execution domain as these two values started
     * the same after a fork.
     *
     */

    if (tsk->exit_signal != SIGCHLD && tsk->exit_signal != -1 &&
        ( tsk->parent_exec_id != t->self_exec_id  ||
          tsk->self_exec_id != tsk->parent_exec_id)
        && !capable(CAP_KILL))
        tsk->exit_signal = SIGCHLD;


    /* If something other than our normal parent is ptracing us, then
     * send it a SIGCHLD instead of honoring exit_signal.  exit_signal
     * only has special meaning to our real parent.
     */
    if (tsk->exit_signal != -1 && thread_group_empty(tsk)) {
        int signal = tsk->parent == tsk->real_parent ? tsk->exit_signal : SIGCHLD;
        <span style="color:#ff0000;">do_notify_parent(tsk, signal);</span>
    } else if (tsk->ptrace) {
        do_notify_parent(tsk, SIGCHLD);
    }

    <span style="color:#ff0000;"><strong>state = EXIT_ZOMBIE;</strong></span>
    if (tsk->exit_signal == -1 && tsk->ptrace == 0)
        state = EXIT_DEAD;
    tsk->exit_state = state;

    /*
     * Clear these here so that update_process_times() won't try to deliver
     * itimer, profile or rlimit signals to this task while it is in late exit.
     */
    tsk->it_virt_value = 0;
    tsk->it_prof_value = 0;

    write_unlock_irq(&tasklist_lock);

    list_for_each_safe(_p, _n, &ptrace_dead) {
        list_del_init(_p);
        t = list_entry(_p,struct task_struct,ptrace_list);
        release_task(t);
    }

    /* If the process is dead, release it - nobody will wait for it */
    if (state == EXIT_DEAD)
        release_task(tsk);

    /* PF_DEAD causes final put_task_struct after we schedule. */
    preempt_disable();
    tsk->flags |= PF_DEAD;
}

大家可以看到这段内核代码的注释非常全。forget_original_parent这个函数还会把该进程的所有子孙进程重设父进程，交给init进程接管。

回过头来，看看为什么守护进程要fork两次。这里有一个假定，父进程生成守护进程后，还有自己的事要做，它的人生意义并不只是为了生成守护进程。这样，如果父进程fork一次创建了一个守护进程，然后继续做其它事时阻塞了，这时守护进程一直在运行，父进程却没有正常退出。如果守护进程因为正常或非正常原因退出了，就会变成ZOMBIE进程。

如果fork两次呢？父进程先fork出一个儿子进程，儿子进程再fork出孙子进程做为守护进程，然后儿子进程立刻退出，守护进程被init进程接管，这样无论父进程做什么事，无论怎么被阻塞，都与守护进程无关了。所以，fork两次的守护进程很安全，避免了僵尸进程出现的可能性。