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#include <stdlib.h>
void exit(int status);
void _Exit(int status);

#include <unistd.h>
void _exit(int status);

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struct task_struct {
    volatile long state;	/* -1 unrunnable, 0 runnable, >0 stopped */
	void *stack;
	atomic_t usage;
	unsigned int flags;	/* per process flags, defined below */
	
	int prio, static_prio, normal_prio;
	unsigned int rt_priority;
	const struct sched_class *sched_class;
	struct sched_entity se;
	struct sched_rt_entity rt;

	unsigned int policy;
	int nr_cpus_allowed;
	cpumask_t cpus_allowed;

	struct list_head tasks;  // 进程链表
	struct mm_struct *mm, *active_mm;
	
    /* task state */
	int exit_state;
	int exit_code, exit_signal;
	int pdeath_signal;  /*  The signal sent when the parent dies  */
	
	pid_t pid;
	pid_t tgid;

    /* 
	 * pointers to (original) parent process, youngest child, younger sibling,
	 * older sibling, respectively.  (p->father can be replaced with 
	 * p->real_parent->pid)
	 */
	struct task_struct *real_parent; /* real parent process */
	struct task_struct *parent; /* recipient of SIGCHLD, wait4() reports */
	/*
	 * children/sibling forms the list of my natural children
	 */
	struct list_head children;	/* list of my children */
	struct list_head sibling;	/* linkage in my parent's children list */
	struct task_struct *group_leader;	/* threadgroup leader */

    /* PID/PID hash table linkage. */
	struct pid_link pids[PIDTYPE_MAX];

    /* file system info */
	struct nameidata *nameidata;
    /* filesystem information */
	struct fs_struct *fs;
    /* open file information */
	struct files_struct *files;
 
    /* signal handlers */
	struct signal_struct *signal;
	struct sighand_struct *sighand;

	sigset_t saved_sigmask;	/* restored if set_restore_sigmask() was used */
	struct sigpending pending;
};

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struct thread_info {
	struct task_struct	    *task;		    /* main task structure */
	struct exec_domain	    *exec_domain;	/* execution domain */
	__u32			        flags;		    /* low level flags */
	__u32			        status;		    /* thread synchronous flags */
	__u32			        cpu;		    /* current CPU */
	int			            preempt_count;	/* 0 => preemptable,  <0 => BUG */
	mm_segment_t		    addr_limit;
	struct restart_block    restart_block;
	void __user		        *sysenter_return;
#ifdef CONFIG_X86_32
	unsigned long           previous_esp;   /* ESP of the previous stack in case of nested (IRQ) stacks */
	__u8			        supervisor_stack[0];
#endif
	int			            uaccess_err;
};

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static inline struct thread_info *current_thread_info(void) __attribute_const__;

static inline struct thread_info *current_thread_info(void)
{
	return (struct thread_info *)
		(current_stack_pointer & ~(THREAD_SIZE - 1));   // 按照 THREAD\_SIZE 对齐
}

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struct task_struct *my_parent = current->parent;

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struct task_struct *task; 
struct list_head *list;

list_for_each(list, &current->children) {
    task = list_entry(list, struct task_struct, sibling); 
    /* task now points to one of current’s children */
}

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list_entry(task->tasks.next, struct task_struct, tasks)

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list_entry(task->tasks.prev, struct task_struct, tasks)

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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)
{
	struct task_struct *p;

    p = copy_process(clone_flags, stack_start, regs, stack_size,
			 child_tidptr, NULL, trace);

    /*
	 * Do this prior waking up the new thread - the thread pointer
	 * might get invalid after that point, if the thread exits quickly.
	 */
	if (!IS_ERR(p)) {
		struct completion vfork;

		trace_sched_process_fork(current, p);

		if (clone_flags & CLONE_VFORK) {
			p->vfork_done = &vfork;
			init_completion(&vfork);
		}

		/*
		 * 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);
		}

		if (clone_flags & CLONE_VFORK) {
			wait_for_completion(&vfork);
		}
	} else {
		nr = PTR_ERR(p);
	}
	return nr;
}

static struct task_struct *copy_process(unsigned long clone_flags,
					unsigned long stack_start,
					struct pt_regs *regs,
					unsigned long stack_size,
					int __user *child_tidptr,
					struct pid *pid,
					int trace)
{
	int retval;
	struct task_struct *p;

    p = dup_task_struct(current);

    if (atomic_read(&p->real_cred->user->processes) >=
			task_rlimit(p, RLIMIT_NPROC)) {
		if (!capable(CAP_SYS_ADMIN) && !capable(CAP_SYS_RESOURCE) &&
		    p->real_cred->user != INIT_USER)
			goto bad_fork_free;
	}

    p->did_exec = 0;
	delayacct_tsk_init(p);	/* Must remain after dup_task_struct() */
	copy_flags(clone_flags, p);
	INIT_LIST_HEAD(&p->children);
	INIT_LIST_HEAD(&p->sibling);
	rcu_copy_process(p);
	p->vfork_done = NULL;
	spin_lock_init(&p->alloc_lock);

	init_sigpending(&p->pending);

	p->utime = cputime_zero;
	p->stime = cputime_zero;
	p->gtime = cputime_zero;
	p->utimescaled = cputime_zero;
	p->stimescaled = cputime_zero;
#ifndef CONFIG_VIRT_CPU_ACCOUNTING
	p->prev_utime = cputime_zero;
	p->prev_stime = cputime_zero;
#endif
    
    /* 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_namespaces(clone_flags, p)))
		goto bad_fork_cleanup_mm;
	if ((retval = copy_io(clone_flags, p)))
		goto bad_fork_cleanup_namespaces;
	retval = copy_thread(clone_flags, stack_start, stack_size, p, regs);
	if (retval)
		goto bad_fork_cleanup_io;

    if (pid != &init_struct_pid) {
		retval = -ENOMEM;
		pid = alloc_pid(p->nsproxy->pid_ns);
		if (!pid)
			goto bad_fork_cleanup_io;

		if (clone_flags & CLONE_NEWPID) {
			retval = pid_ns_prepare_proc(p->nsproxy->pid_ns);
			if (retval < 0)
				goto bad_fork_free_pid;
		}
	}

	p->pid = pid_nr(pid);
	p->tgid = p->pid;
	if (clone_flags & CLONE_THREAD)
		p->tgid = current->tgid;  // 同一进程内的所有线程的 tgid 相同


    /* 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);

    /* CLONE_PARENT re-uses the old parent */
	if (clone_flags & (CLONE_PARENT|CLONE_THREAD)) {
		p->real_parent = current->real_parent;
		p->parent_exec_id = current->parent_exec_id;
	} else {
		p->real_parent = current;
		p->parent_exec_id = current->self_exec_id;
	}

    return p;
}

static struct task_struct *dup_task_struct(struct task_struct *orig)
{
	struct task_struct *tsk;
	struct thread_info *ti;
	unsigned long *stackend;

	int err;

	prepare_to_copy(orig);

	tsk = alloc_task_struct();
	if (!tsk)
		return NULL;

	ti = alloc_thread_info(tsk);
	if (!ti) {
		free_task_struct(tsk);
		return NULL;
	}

 	err = arch_dup_task_struct(tsk, orig);
	if (err)
		goto out;

	tsk->stack = ti;

	setup_thread_stack(tsk, orig);
	stackend = end_of_stack(tsk);
	*stackend = STACK_END_MAGIC;	/* for overflow detection */

	/* One for us, one for whoever does the "release_task()" (usually parent) */
	atomic_set(&tsk->usage,2);
	atomic_set(&tsk->fs_excl, 0);

	return tsk;
}

// x86 下的实现
int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src)
{
	*dst = *src;
	if (src->thread.xstate) {
		dst->thread.xstate = kmem_cache_alloc(task_xstate_cachep,
						      GFP_KERNEL);
		if (!dst->thread.xstate)
			return -ENOMEM;
		WARN_ON((unsigned long)dst->thread.xstate & 15);
		memcpy(dst->thread.xstate, src->thread.xstate, xstate_size);
	}
	return 0;
}

#define task_thread_info(task)	((struct thread_info *)(task)->stack)

#define setup_thread_stack(p, org) \
	*task_thread_info(p) = *task_thread_info(org); \
	task_thread_info(p)->task = (p);

static void copy_flags(unsigned long clone_flags, struct task_struct *p)
{
	unsigned long new_flags = p->flags;

	new_flags &= ~PF_SUPERPRIV;
	new_flags |= PF_FORKNOEXEC;
	new_flags |= PF_STARTING;
	p->flags = new_flags;
	clear_freeze_flag(p);
}

void wake_up_new_task(struct task_struct *p, unsigned long clone_flags)
{
	unsigned long flags;
	struct rq *rq;
	int cpu __maybe_unused = get_cpu();

#ifdef CONFIG_SMP
	/*
	 * Fork balancing, do it here and not earlier because:
	 *  - cpus_allowed can change in the fork path
	 *  - any previously selected cpu might disappear through hotplug
	 *
	 * We still have TASK_WAKING but PF_STARTING is gone now, meaning
	 * ->cpus_allowed is stable, we have preemption disabled, meaning
	 * cpu_online_mask is stable.
	 */
	cpu = select_task_rq(p, SD_BALANCE_FORK, 0);
	set_task_cpu(p, cpu);
#endif

	/*
	 * Since the task is not on the rq and we still have TASK_WAKING set
	 * nobody else will migrate this task.
	 */
	rq = cpu_rq(cpu);
	raw_spin_lock_irqsave(&rq->lock, flags);

	BUG_ON(p->state != TASK_WAKING);
	p->state = TASK_RUNNING;  // 设置成TASK_RUNNING状态  
	update_rq_clock(rq);
	activate_task(rq, p, 0);  // activate_task 会把该 task 放入 cpu 的 runqueue
	trace_sched_wakeup_new(rq, p, 1);
	check_preempt_curr(rq, p, WF_FORK);
#ifdef CONFIG_SMP
	if (p->sched_class->task_woken)
		p->sched_class->task_woken(rq, p);
#endif
	task_rq_unlock(rq, &flags);
	put_cpu();
}


/*
 * activate_task - move a task to the runqueue.
 */
static void activate_task(struct rq *rq, struct task_struct *p, int wakeup)
{
	if (task_contributes_to_load(p))
		rq->nr_uninterruptible--;

	enqueue_task(rq, p, wakeup, false);  // 放入 runqueue
	inc_nr_running(rq);
}

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void mm_release(struct task_struct *tsk, struct mm_struct *mm)
{
	struct completion *vfork_done = tsk->vfork_done;

	/* notify parent sleeping on vfork() */
	if (vfork_done) {
		tsk->vfork_done = NULL;
		complete(vfork_done);
	}
}

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clone(CLONE_VM | CLONE_FS | CLONE_FILES | CLONE_SIGHAND, 0);

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NORET_TYPE void do_exit(long code)
{
	struct task_struct *tsk = current;
    int group_dead;

    /*
	 * We're taking recursive faults here in do_exit. Safest is to just
	 * leave this task alone and wait for reboot.
	 */
	if (unlikely(tsk->flags & PF_EXITING)) {
		printk(KERN_ALERT
			"Fixing recursive fault but reboot is needed!\n");
		/*
		 * We can do this unlocked here. The futex code uses
		 * this flag just to verify whether the pi state
		 * cleanup has been done or not. In the worst case it
		 * loops once more. We pretend that the cleanup was
		 * done as there is no way to return. Either the
		 * OWNER_DIED bit is set by now or we push the blocked
		 * task into the wait for ever nirwana as well.
		 */
		tsk->flags |= PF_EXITPIDONE;
		set_current_state(TASK_UNINTERRUPTIBLE);
		schedule();
	}

    exit_signals(tsk);  /* sets PF_EXITING */

    acct_update_integrals(tsk);

    group_dead = atomic_dec_and_test(&tsk->signal->live);

    tsk->exit_code = code;
	
    exit_mm(tsk);

    exit_sem(tsk);
	exit_files(tsk);
	exit_fs(tsk);
	check_stack_usage();
	exit_thread();
	cgroup_exit(tsk, 1);

    exit_notify(tsk, group_dead);

    /*
	 * We can do this unlocked here. The futex code uses this flag
	 * just to verify whether the pi state cleanup has been done
	 * or not. In the worst case it loops once more.
	 */
	tsk->flags |= PF_EXITPIDONE;
    preempt_disable();
	exit_rcu();
	/* causes final put_task_struct in finish_task_switch(). */
	tsk->state = TASK_DEAD;
	schedule();
}

static void exit_mm(struct task_struct * tsk)
{
	struct mm_struct *mm = tsk->mm;
	struct core_state *core_state;

	mm_release(tsk, mm);
	if (!mm)
		return;
	
	down_read(&mm->mmap_sem);
	atomic_inc(&mm->mm_count);
	BUG_ON(mm != tsk->active_mm);
	/* more a memory barrier than a real lock */
	task_lock(tsk);
	tsk->mm = NULL;
	up_read(&mm->mmap_sem);
	enter_lazy_tlb(mm, current);
	/* We don't want this task to be frozen prematurely */
	clear_freeze_flag(tsk);
	task_unlock(tsk);
	mm_update_next_owner(mm);
	mmput(mm);
}

/*
 * Send signals to all our closest relatives so that they know
 * to properly mourn us..
 */
static void exit_notify(struct task_struct *tsk, int group_dead)
{
	int signal;
	void *cookie;

	/*
	 * 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)
	 */
	forget_original_parent(tsk);
	exit_task_namespaces(tsk);

	write_lock_irq(&tasklist_lock);
	if (group_dead)
		kill_orphaned_pgrp(tsk->group_leader, NULL);

	/* 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 && !task_detached(tsk) &&
	    (tsk->parent_exec_id != tsk->real_parent->self_exec_id ||
	     tsk->self_exec_id != tsk->parent_exec_id))
		tsk->exit_signal = SIGCHLD;

	signal = tracehook_notify_death(tsk, &cookie, group_dead);
	if (signal >= 0)
		signal = do_notify_parent(tsk, signal);  // 给父进程发信号。如果父进程处理 SIGCHLD 信号，返回 DEATH_REAP

	tsk->exit_state = signal == DEATH_REAP ? EXIT_DEAD : EXIT_ZOMBIE;  

	/* mt-exec, de_thread() is waiting for us */
	if (thread_group_leader(tsk) &&
	    tsk->signal->group_exit_task &&
	    tsk->signal->notify_count < 0)
		wake_up_process(tsk->signal->group_exit_task);

	write_unlock_irq(&tasklist_lock);

	tracehook_report_death(tsk, signal, cookie, group_dead);

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

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void release_task(struct task_struct * p)
{
	struct task_struct *leader;
	int zap_leader;

repeat:
    __exit_signal(p);
    zap_leader = 0;
	leader = p->group_leader;
	if (leader != p && thread_group_empty(leader) && leader->exit_state == EXIT_ZOMBIE) {
		BUG_ON(task_detached(leader));
		do_notify_parent(leader, leader->exit_signal);
		/*
		 * If we were the last child thread and the leader has
		 * exited already, and the leader's parent ignores SIGCHLD,
		 * then we are the one who should release the leader.
		 *
		 * do_notify_parent() will have marked it self-reaping in
		 * that case.
		 */
		zap_leader = task_detached(leader);

		/*
		 * This maintains the invariant that release_task()
		 * only runs on a task in EXIT_DEAD, just for sanity.
		 */
		if (zap_leader)
			leader->exit_state = EXIT_DEAD;
	}

	write_unlock_irq(&tasklist_lock);
	release_thread(p);
	call_rcu(&p->rcu, delayed_put_task_struct);

	p = leader;
	if (unlikely(zap_leader))
		goto repeat;
}


/*
 * This function expects the tasklist_lock write-locked.
 */
static void __exit_signal(struct task_struct *tsk)
{
	struct signal_struct *sig = tsk->signal;
	struct sighand_struct *sighand;

	BUG_ON(!sig);
	BUG_ON(!atomic_read(&sig->count));

	
	posix_cpu_timers_exit(tsk);
	if (atomic_dec_and_test(&sig->count))
		posix_cpu_timers_exit_group(tsk);
	else {
		/*
		 * If there is any task waiting for the group exit
		 * then notify it:
		 */
		if (sig->group_exit_task && atomic_read(&sig->count) == sig->notify_count)
			wake_up_process(sig->group_exit_task);

		if (tsk == sig->curr_target)
			sig->curr_target = next_thread(tsk);
		/*
		 * Accumulate here the counters for all threads but the
		 * group leader as they die, so they can be added into
		 * the process-wide totals when those are taken.
		 * The group leader stays around as a zombie as long
		 * as there are other threads.  When it gets reaped,
		 * the exit.c code will add its counts into these totals.
		 * We won't ever get here for the group leader, since it
		 * will have been the last reference on the signal_struct.
		 */
		sig->utime = cputime_add(sig->utime, tsk->utime);
		sig->stime = cputime_add(sig->stime, tsk->stime);
		sig->gtime = cputime_add(sig->gtime, tsk->gtime);
		sig->min_flt += tsk->min_flt;
		sig->maj_flt += tsk->maj_flt;
		sig->nvcsw += tsk->nvcsw;
		sig->nivcsw += tsk->nivcsw;
		sig->inblock += task_io_get_inblock(tsk);
		sig->oublock += task_io_get_oublock(tsk);
		task_io_accounting_add(&sig->ioac, &tsk->ioac);
		sig->sum_sched_runtime += tsk->se.sum_exec_runtime;
		sig = NULL; /* Marker for below. */
	}

	__unhash_process(tsk);

	/*
	 * Do this under ->siglock, we can race with another thread
	 * doing sigqueue_free() if we have SIGQUEUE_PREALLOC signals.
	 */
	flush_sigqueue(&tsk->pending);

	tsk->signal = NULL;
	tsk->sighand = NULL;
	spin_unlock(&sighand->siglock);

	__cleanup_sighand(sighand);
	clear_tsk_thread_flag(tsk,TIF_SIGPENDING);
	if (sig) {
		flush_sigqueue(&sig->shared_pending);
		taskstats_tgid_free(sig);
		/*
		 * Make sure ->signal can't go away under rq->lock,
		 * see account_group_exec_runtime().
		 */
		task_rq_unlock_wait(tsk);
		__cleanup_signal(sig);
	}
}

static void __unhash_process(struct task_struct *p)
{
	nr_threads--;
	detach_pid(p, PIDTYPE_PID);
	if (thread_group_leader(p)) {
		detach_pid(p, PIDTYPE_PGID);
		detach_pid(p, PIDTYPE_SID);

		list_del_rcu(&p->tasks);  // 没理解 为什么需要是 group_header 才从 tasks_list 上删掉？
		list_del_init(&p->sibling);
		__get_cpu_var(process_counts)--;
	}
	list_del_rcu(&p->thread_group);
}


void release_task(struct task_struct *p)
{
	struct task_struct *leader;

    __exit_signal(p);

    zap_leader = 0;
	leader = p->group_leader;
	if (leader != p && thread_group_empty(leader)
			&& leader->exit_state == EXIT_ZOMBIE) {
		/*
		 * If we were the last child thread and the leader has
		 * exited already, and the leader's parent ignores SIGCHLD,
		 * then we are the one who should release the leader.
		 */
		zap_leader = do_notify_parent(leader, leader->exit_signal);
		if (zap_leader)
			leader->exit_state = EXIT_DEAD;
	}

    release_thread(p);
}

void detach_pid(struct task_struct *task, enum pid_type type)
{
	__change_pid(task, type, NULL);
}

static void __change_pid(struct task_struct *task, enum pid_type type,
			struct pid *new)
{
	struct pid_link *link;
	struct pid *pid;
	int tmp;

	link = &task->pids[type];
	pid = link->pid;

	hlist_del_rcu(&link->node);
	link->pid = new;

	for (tmp = PIDTYPE_MAX; --tmp >= 0; )
		if (!hlist_empty(&pid->tasks[tmp]))
			return;

	free_pid(pid);
}

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static void forget_original_parent(struct task_struct *father)
{
	struct task_struct *p, *n, *reaper;
	LIST_HEAD(dead_children);

	exit_ptrace(father);

	write_lock_irq(&tasklist_lock);
	reaper = find_new_reaper(father);

	list_for_each_entry_safe(p, n, &father->children, sibling) {
		struct task_struct *t = p;
		do {
			t->real_parent = reaper;
			if (t->parent == father) {
				BUG_ON(task_ptrace(t));
				t->parent = t->real_parent;
			}
		} while_each_thread(p, t);
		reparent_leader(father, p, &dead_children);
	}
	write_unlock_irq(&tasklist_lock);

	BUG_ON(!list_empty(&father->children));

	list_for_each_entry_safe(p, n, &dead_children, sibling) {
		list_del_init(&p->sibling);
		release_task(p);
	}
}

static struct task_struct *find_new_reaper(struct task_struct *father)
{
	struct pid_namespace *pid_ns = task_active_pid_ns(father);
	struct task_struct *thread;

	thread = father;
	while_each_thread(father, thread) {
		if (thread->flags & PF_EXITING)
			continue;
		if (unlikely(pid_ns->child_reaper == father))
			pid_ns->child_reaper = thread;
		return thread;
	}

	if (unlikely(pid_ns->child_reaper == father)) {
		write_unlock_irq(&tasklist_lock);
		if (unlikely(pid_ns == &init_pid_ns))
			panic("Attempted to kill init!");

		zap_pid_ns_processes(pid_ns);
		write_lock_irq(&tasklist_lock);
		/*
		 * We can not clear ->child_reaper or leave it alone.
		 * There may by stealth EXIT_DEAD tasks on ->children,
		 * forget_original_parent() must move them somewhere.
		 */
		pid_ns->child_reaper = init_pid_ns.child_reaper;
	}

	return pid_ns->child_reaper;
}