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linux進程管理之wait系統調用

六: wait4 ()系統調用

在父進程中,用wait4()可以獲得子進程的退出狀態,並且防止在父進程退出前,子進程退出造成僵死 狀態。這是我們這節分析的最後一個小節了。

關於wait4()在用戶空間的調用方式可以自行參考相關資料,在這裡只是討論內核對這個系統調用的實 現過程。

Wait4()的系統調用入口為sys_wait4().代碼如下所示:

asmlinkage long sys_wait4(pid_t pid, int __user *stat_addr,
        int options, struct rusage __user *ru)
{
   long ret;
   //options的標志為須為WNOHANG…__WALL的組合,否則會出錯
   //相關標志的作用在do_wait()中再進行分析
   if (options & ~(WNOHANG|WUNTRACED|WCONTINUED|
       __WNOTHREAD|__WCLONE|__WALL))
     return -EINVAL;
   ret = do_wait(pid, options | WEXITED, NULL, stat_addr, ru);
   /* avoid REGPARM breakage on x86: */
   prevent_tail_call(ret);
   return ret;
}

do_wait()是其中的核心處理函數。代碼如下:

static long do_wait(pid_t pid, int options, struct siginfo __user *infop,
       int __user *stat_addr, struct rusage __user *ru)
{
   //初始化一個等待隊列
   DECLARE_WAITQUEUE(wait, current);
   struct task_struct *tsk;
   int flag, retval;
   int allowed, denied;
   //將當前進程加入等待隊列,子進程退出給父進程發送信號會wake up些等待隊列
   add_wait_queue(&current->signal->wait_chldexit,&wait);
repeat:
   flag = 0;
   allowed = denied = 0;
   //設置進程狀態為TASK_INTERRUPTIBLE.下次調度必須要等到子進程喚醒才可以了
   current->state = TASK_INTERRUPTIBLE;
   read_lock(&tasklist_lock);
   tsk = current;
   do {
     struct task_struct *p;
     struct list_head *_p;
     int ret;
     //遍歷進程下的子進程
     list_for_each(_p,&tsk->children) {
       p = list_entry(_p, struct task_struct, sibling);
       //判斷是否是我們要wait 的子進程
       ret = eligible_child(pid, options, p);
       if (!ret)
          continue;
       if (unlikely(ret < 0)) {
          denied = ret;
          continue;
       }
       allowed = 1;
       switch (p->state) {
       //子進程為TASK_TRACED.即處於跟蹤狀態。則取子進程的相關信息
       case TASK_TRACED:
          flag = 1;
          //判斷是否是被父進程跟蹤的子進程
          //如果是則返回1..不是返回0
          if (!my_ptrace_child(p))
            continue;
          /*FALLTHROUGH*/
       case TASK_STOPPED:
          flag = 1;
          //WUNTRACED:子進程是停止的,也馬上返回
          //沒有定義WUNTRACED 參數.繼續遍歷子進程
          /*從此看出.生父進程是不會處理STOP狀態的子進程的.只有
            發起跟蹤的進程才會
           */
           
          if (!(options & WUNTRACED) &&
            !my_ptrace_child(p))
            continue;
          //WNOWAIT:不會將zombie子進程的退出狀態撤銷
          //下次調用wait系列函數的時候還可以繼續獲得這個退出狀態
          retval = wait_task_stopped(p, ret == 2,
                   (options & WNOWAIT),
                   infop,
                   stat_addr, ru);
          if (retval == -EAGAIN)
            goto repeat;
          if (retval != 0) /* He released the lock. */
            goto end;
          break;
       default:
       // case EXIT_DEAD:
          //不需要處理DEAD狀態
          if (p->exit_state == EXIT_DEAD)
            continue;
       // case EXIT_ZOMBIE:
          //子進程為僵屍狀態
          if (p->exit_state == EXIT_ZOMBIE) {
            if (ret == 2)
              goto check_continued;
            if (!likely(options & WEXITED))
continue;
            retval = wait_task_zombie(
              p, (options & WNOWAIT),
              infop, stat_addr, ru);
            /* He released the lock. */
            if (retval != 0)
              goto end;
            break;
          }
check_continued:
          /*
          * It's running now, so it might later
          * exit, stop, or stop and then continue.
          */
          flag = 1;
       //WCONTINUED:報告任何繼續運行的指定進程號的子進程的狀態
          if (!unlikely(options & WCONTINUED))
            continue;
        //取進程的相關狀態
        retval = wait_task_continued(
            p, (options & WNOWAIT),
            infop, stat_addr, ru);
          if (retval != 0) /* He released the lock. */
            goto end;
          break;
       }
     }
     //遍歷被跟蹤出去的子進程
     //從這裡可以看出.如果一個子進程被跟蹤出去了.那麼子進程的退出
     //操作並不是由生父進程進行了
     if (!flag) {
       list_for_each(_p, &tsk->ptrace_children) {
          p = list_entry(_p, struct task_struct,
              ptrace_list);
          if (!eligible_child(pid, options, p))
            continue;
          flag = 1;
          break;
       }
     }
     if (options & __WNOTHREAD)
       break;
     //也有可能是進程中的線程在wait其fork出來的子進程
     tsk = next_thread(tsk);
     BUG_ON(tsk->signal != current->signal);
   } while (tsk != current);
   //
   read_unlock(&tasklist_lock);
   if (flag) {
     retval = 0;
     //如果定義了WHNOHANG:馬上退出
     if (options & WNOHANG)
       goto end;
     retval = -ERESTARTSYS;
     if (signal_pending(current))
       goto end;
     schedule();
     goto repeat;
   }
   retval = -ECHILD;
   if (unlikely(denied) && !allowed)
     retval = denied;
end:
   //將進程設為運行狀態,從等待隊列中移除
   current->state = TASK_RUNNING;
   remove_wait_queue(&current->signal->wait_chldexit,&wait);
   if (infop) {
     if (retval > 0)
     retval = 0;
     else {
       /*
       * For a WNOHANG return, clear out all the fields
       * we would set so the user can easily tell the
       * difference.
       */
       if (!retval)
          retval = put_user(0, &infop->si_signo);
       if (!retval)
          retval = put_user(0, &infop->si_errno);
       if (!retval)
          retval = put_user(0, &infop->si_code);
       if (!retval)
          retval = put_user(0, &infop->si_pid);
       if (!retval)
          retval = put_user(0, &infop->si_uid);
       if (!retval)
          retval = put_user(0, &infop->si_status);
     }
   }
   return retval;
}

這代段碼還是比較簡單。先遍歷進程的子進程,再遍歷被跟蹤出去的進程,再遍歷線程中的線程。我 們分析一下裡面用到的幾個重要的子函數。

eligible_child()用來判斷子進程是否是我們想要wait的子進程.代碼如下:

static int eligible_child(pid_t pid, int options, struct task_struct *p)
{
   int err;
   //根據PID判斷是不是我們要wait的子進程
   //pid >0:等待的子程程的進程號等於pid
   //pid = 0:等待進程組號等於當前進程組號的所有子進程
   //pid < -1 :等待任何進程組號等於pid絕對值的子進程
   //pid == -1 :等待任何子進程
   if (pid > 0) {
if (p->pid != pid)
       return 0;
   } else if (!pid) {
     if (process_group(p) != process_group(current))
       return 0;
   } else if (pid != -1) {
     if (process_group(p) != -pid)
       return 0;
   }
   //如果子進程exit_signal ==-1且沒有被跟蹤.那不會對子進程進行回收
   if (p->exit_signal == -1 && !p->ptrace)
     return 0;
   if (((p->exit_signal != SIGCHLD) ^ ((options & __WCLONE) != 0))
     && !(options & __WALL))
     return 0;
   /*
   * Do not consider thread group leaders that are
   * in a non-empty thread group:
   */
   //如果子進程是進程組leader,且進程組不為空
   if (delay_group_leader(p))
     return 2;
   err = security_task_wait(p);
   if (err)
     return err;
   return 1;
}

對TASK_TRACED和TASK_STOPPED狀態的子進程操作是在wait_task_stopped()中完成的。它的代碼如下:

static int wait_task_stopped(struct task_struct *p, int delayed_group_leader,
          int noreap, struct siginfo __user *infop,
          int __user *stat_addr, struct rusage __user *ru)
{
   int retval, exit_code;
   //進程退出狀態碼為零.沒有相關退出信息
   if (!p->exit_code)
     return 0;
   //
   if (delayed_group_leader && !(p->ptrace & PT_PTRACED) &&
     p->signal && p->signal->group_stop_count > 0)
     return 0;
   //正在取task裡面的信息,為了防止意外釋放,先增加它的引用計數
   get_task_struct(p);
   read_unlock(&tasklist_lock);
   //如果WNOWAIT 被定義
   if (unlikely(noreap)) {
     pid_t pid = p->pid;
     uid_t uid = p->uid;
     int why = (p->ptrace & PT_PTRACED) ? CLD_TRAPPED : CLD_STOPPED;
     exit_code = p->exit_code;
     //退出狀態碼為零,但是過程已經處於退出狀態中(僵屍或者是死進程)
     if (unlikely(!exit_code) || unlikely(p->exit_state))
       goto bail_ref;
     //把子進程的各項信息保存起來
     //返回值是退出子進程的PID
     return wait_noreap_copyout(p, pid, uid,
              why, exit_code,
              infop, ru);
   }
   write_lock_irq(&tasklist_lock);
   //如果子進程沒有退出.只要取子進程的退出信息,再清除子進程的退出信息
   //即可
   exit_code = xchg(&p->exit_code, 0);
   if (unlikely(p->exit_state)) {
     p->exit_code = exit_code;
     exit_code = 0;
   }
   if (unlikely(exit_code == 0)) {
      write_unlock_irq(&tasklist_lock);
bail_ref:
     put_task_struct(p);
     return -EAGAIN;
   }
   //將子進程加到父進程子鏈表的末尾
   remove_parent(p);
   add_parent(p);
   write_unlock_irq(&tasklist_lock);
   //收集相關的信息
   retval = ru ? getrusage(p, RUSAGE_BOTH, ru) : 0;
   if (!retval && stat_addr)
     retval = put_user((exit_code << 8) | 0x7f, stat_addr);
   if (!retval && infop)
     retval = put_user(SIGCHLD, &infop->si_signo);
   if (!retval && infop)
     retval = put_user(0, &infop->si_errno);
   if (!retval && infop)
     retval = put_user((short)((p->ptrace & PT_PTRACED)
             ? CLD_TRAPPED : CLD_STOPPED),
           &infop->si_code);
   if (!retval && infop)
     retval = put_user(exit_code, &infop->si_status);
   if (!retval && infop)
     retval = put_user(p->pid, &infop->si_pid);
   if (!retval && infop)
     retval = put_user(p->uid, &infop->si_uid);
   if (!retval)
     retval = p->pid;
   //減少task的引用計數
   put_task_struct(p);
   BUG_ON(!retval);
   return retval;
}

對僵屍進程的操作是由wait_task_zombie()完成的。代如如下:

static int wait_task_zombie(struct task_struct *p, int noreap,
         struct siginfo __user *infop,
         int __user *stat_addr, struct rusage __user *ru)
{
   unsigned long state;
   int retval;
   int status;
   //WNOWAIT被設置.不需要釋放子進程的資源,只要取相關信息即可
   if (unlikely(noreap)) {
     pid_t pid = p->pid;
     uid_t uid = p->uid;
     int exit_code = p->exit_code;
     int why, status;
     //子進程不為EXIT_ZOMBIE .異常退出
     if (unlikely(p->exit_state != EXIT_ZOMBIE))
       return 0;
     //沒有退出信號具沒有被跟蹤.退出
     if (unlikely(p->exit_signal == -1 && p->ptrace == 0))
       return 0;
     //增加引用計數
     get_task_struct(p);
     read_unlock(&tasklist_lock);
     if ((exit_code & 0x7f) == 0) {
       why = CLD_EXITED;
       status = exit_code >> 8;
     } else {
       why = (exit_code & 0x80) ? CLD_DUMPED : CLD_KILLED;
       status = exit_code & 0x7f;
     }
//取相關信息
     return wait_noreap_copyout(p, pid, uid, why,
              status, infop, ru);
   }
   /*
   * Try to move the task's state to DEAD
   * only one thread is allowed to do this:
   */
   //將子進程狀態設為EXIT_DEAD狀態
   state = xchg(&p->exit_state, EXIT_DEAD);
   //如果子進程不為EXIT_ZOMBIE狀態,異常退出
   if (state != EXIT_ZOMBIE) {
     BUG_ON(state != EXIT_DEAD);
     return 0;
   }
   //沒有退出信號,且沒有被跟蹤
   if (unlikely(p->exit_signal == -1 && p->ptrace == 0)) {
      return 0;
   }
   //子進程的real_parent等於當前父進程.說明子進程並沒有被跟蹤出去
   if (likely(p->real_parent == p->parent) && likely(p->signal)) {
     struct signal_struct *psig;
     struct signal_struct *sig;
     //更新父進程的一些統計信息
     spin_lock_irq(&p->parent->sighand->siglock);
     psig = p->parent->signal;
     sig = p->signal;
     psig->cutime =
       cputime_add(psig->cutime,
       cputime_add(p->utime,
       cputime_add(sig->utime,
            sig->cutime)));
     psig->cstime =
       cputime_add(psig->cstime,
       cputime_add(p->stime,
       cputime_add(sig->stime,
            sig->cstime)));
     psig->cmin_flt +=
       p->min_flt + sig->min_flt + sig->cmin_flt;
     psig->cmaj_flt +=
       p->maj_flt + sig->maj_flt + sig->cmaj_flt;
     psig->cnvcsw +=
       p->nvcsw + sig->nvcsw + sig->cnvcsw;
     psig->cnivcsw +=
       p->nivcsw + sig->nivcsw + sig->cnivcsw;
     psig->cinblock +=
       task_io_get_inblock(p) +
       sig->inblock + sig->cinblock;
     psig->coublock +=
       task_io_get_oublock(p) +
       sig->oublock + sig->coublock;
     spin_unlock_irq(&p->parent->sighand->siglock);
   }
   /*
   * Now we are sure this task is interesting, and no other
   * thread can reap it because we set its state to EXIT_DEAD.
   */
   //取得相關的退出信息
   read_unlock(&tasklist_lock);
   retval = ru ? getrusage(p, RUSAGE_BOTH, ru) : 0;
   status = (p->signal->flags & SIGNAL_GROUP_EXIT)
     ? p->signal->group_exit_code : p->exit_code;
   if (!retval && stat_addr)
     retval = put_user(status, stat_addr);
   if (!retval && infop)
     retval = put_user(SIGCHLD, &infop->si_signo);
   if (!retval && infop)
     retval = put_user(0, &infop->si_errno);
   if (!retval && infop) {
     int why;
     if ((status & 0x7f) == 0) {
       why = CLD_EXITED;
       status >>= 8;
     } else {
       why = (status & 0x80) ? CLD_DUMPED : CLD_KILLED;
       status &= 0x7f;
     }
     retval = put_user((short)why, &infop->si_code);
     if (!retval)
       retval = put_user(status, &infop->si_status);
   }
   if (!retval && infop)
     retval = put_user(p->pid, &infop->si_pid);
   if (!retval && infop)
     retval = put_user(p->uid, &infop->si_uid);
   if (retval) {
     // TODO: is this safe?
     p->exit_state = EXIT_ZOMBIE;
     return retval;
   }
   retval = p->pid;
   //當前進程不是生父進程.則說明進程是被跟蹤出去了
   // TODO:子進程exit退出的時候,只會向其當前父進程發送信號的哦^_^
   if (p->real_parent != p->parent) {
      write_lock_irq(&tasklist_lock);
     /* Double-check with lock held. */
     if (p->real_parent != p->parent) {
       //將進程從跟蹤鏈表中脫落,並設置父進程為生父進程
       __ptrace_unlink(p);
       // TODO: is this safe?
       //重新設置為EXIT_ZOMBI狀態
       p->exit_state = EXIT_ZOMBIE;
       /*
       * If this is not a detached task, notify the parent.
       * If it's still not detached after that, don't release
       * it now.
       */
       //如果允許發送信息,則給生父進程發送相關信號
       if (p->exit_signal != -1) {
          do_notify_parent(p, p->exit_signal);
          if (p->exit_signal != -1)
            p = NULL;
}
     }
     write_unlock_irq(&tasklist_lock);
   }
   //釋放子進程的剩余資源
   if (p != NULL)
     release_task(p);
   BUG_ON(!retval);
   return retval;
}

至此,我們看到了繼子進程退出之後的完整處理。在此,值得注意的是。子進程在退出的時候會給父 進程發送相應的信號(例如SIG_CHILD),默認的信號處理函數也會進行相應的處理。

七:等待隊列的操作

在這裡,我們第一次接觸到了等待隊列,我們就以上面的代碼做為例子來分析一下。

1:申請一個等待隊列:

DECLARE_WAITQUEUE():
//name:等待隊列的名字。Tsk:所要操作的task
#define DECLARE_WAITQUEUE(name, tsk)                    \
     wait_queue_t name = __WAITQUEUE_INITIALIZER(name, tsk)
#define __WAITQUEUE_INITIALIZER(name, tsk) {                 \
     .private = tsk,                           \
     .func         = default_wake_function,             \
     .task_list    = { NULL, NULL } }

default_wake_function()為默認的喚醒處理函數。

2:添加等待隊列。

在上面的代碼中,有:

add_wait_queue(&current->signal->wait_chldexit,&wait);

它的意思是將wait添加至&current->signal->wait_chldexit中。代碼如下:

void fastcall add_wait_queue(wait_queue_head_t *q, wait_queue_t *wait)
{
   unsigned long flags;
   wait->flags &= ~WQ_FLAG_EXCLUSIVE;
   //為了防止競爭。加鎖
   spin_lock_irqsave(&q->lock, flags);
   //添加至隊列
   __add_wait_queue(q, wait);
   //解鎖
   spin_unlock_irqrestore(&q->lock, flags);
}

3:喚醒操作:

在do_notify_parent()中有這樣的代碼片段:

……
__wake_up_parent(tsk, tsk->parent);
……
__wake_up_parent()的代碼如下:
static inline void __wake_up_parent(struct task_struct *p,
            struct task_struct *parent)
{
   wake_up_interruptible_sync(&parent->signal->wait_chldexit);
}

parent->signal->wait_chldexit這個隊列很熟吧?我們在父進程中添加的等待隊列就是添加在 這裡哦。^_^

喚醒隊列的操作是由wake_up_interruptible_sync()完成的,代碼如下:

wake_up_interruptible_sync() à __wake_up_sync()à__wake_up_common():
static void __wake_up_common(wait_queue_head_t *q, unsigned int mode,
          int nr_exclusive, int sync, void *key)
{
   struct list_head *tmp, *next;
   list_for_each_safe(tmp, next, &q->task_list) {
     wait_queue_t *curr = list_entry(tmp, wait_queue_t, task_list);
     unsigned flags = curr->flags;
     if (curr->func(curr, mode, sync, key) &&
          (flags & WQ_FLAG_EXCLUSIVE) && !--nr_exclusive)
       break;
   }
}

上述操作會遍歷整個等待隊列,然後運行對應的函數。我們在前面申請等待隊列的時候,默認的函數 為:default_wake_function()。它會將操作的task放入運行隊列,並將狀態設為RUNING這個函數等之 後我們分析進程切換與調度的時候再來分析。

八:小結

通過分析進程的創建,執行與消息等過程,可以對子程管理子系統有一個大概的了解。該子系統與其 它子系統關系十分密切。對進程資源的管理和釋放是理解這個子系統的難點。在下一個小節點,我們接著 分析進程的切換與調度。

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