/*
 *  linux/kernel/fork.c
 *
 *  Copyright (C) 1991, 1992  Linus Torvalds
 */

/*
 *  'fork.c' contains the help-routines for the 'fork' system call
 * (see also entry.S and others).
 * Fork is rather simple, once you get the hang of it, but the memory
 * management can be a bitch. See 'mm/memory.c': 'copy_page_range()'
 */

#include < linux/config.h>
#include < linux/slab.h>
#include < linux/init.h>
#include < linux/unistd.h>
#include < linux/smp_lock.h>
#include < linux/module.h>
#include < linux/vmalloc.h>
#include < linux/completion.h>
#include < linux/personality.h>

#include < asm/pgtable.h>
#include < asm/pgalloc.h>
#include < asm/uaccess.h>
#include < asm/mmu_context.h>

/* The idle threads do not count.. */
int nr_threads;
int nr_running;	//cxl: 递增、递减

int max_threads;
unsigned long total_forks;	/* Handle normal Linux uptimes. */
int last_pid;

struct task_struct *pidhash[PIDHASH_SZ];

add_wait_queuevoid add_wait_queue(wait_queue_head_t *q, wait_queue_t * wait) //cxl: 插入等待队列
{
	unsigned long flags;

	wait->flags &= ~WQ_FLAG_EXCLUSIVE;
	wq_write_lock_irqsave(&q->lock, flags);
	__add_wait_queue(q, wait);	//cxl: 插入等待队列
	wq_write_unlock_irqrestore(&q->lock, flags);
}

void add_wait_queue_exclusive(wait_queue_head_t *q, wait_queue_t * wait)
{
	unsigned long flags;

	wait->flags |= WQ_FLAG_EXCLUSIVE;
	wq_write_lock_irqsave(&q->lock, flags);
	__add_wait_queue_tail(q, wait);
	wq_write_unlock_irqrestore(&q->lock, flags);
}

void remove_wait_queue(wait_queue_head_t *q, wait_queue_t * wait)	//cxl: 从等待队列删除
{
	unsigned long flags;

	wq_write_lock_irqsave(&q->lock, flags);
	__remove_wait_queue(q, wait);		//cxl: 从等待队列删除
	wq_write_unlock_irqrestore(&q->lock, flags);
}

void __init fork_init(unsigned long mempages)
{
	/*
	 * The default maximum number of threads is set to a safe
	 * value: the thread structures can take up at most half
	 * of memory.
	 */
	max_threads = mempages / (THREAD_SIZE/PAGE_SIZE) / 8;

	init_task.rlim[RLIMIT_NPROC].rlim_cur = max_threads/2;
	init_task.rlim[RLIMIT_NPROC].rlim_max = max_threads/2;
}

/* Protects next_safe and last_pid. */
spinlock_t lastpid_lock = SPIN_LOCK_UNLOCKED;

static int get_pid(unsigned long flags)		//cxl: 分配一个新的、当前未用的pid
{
	static int next_safe = PID_MAX;	//cxl: 使用next_safe来划定可以直接++的范围
	struct task_struct *p;
	int pid;

	if (flags & CLONE_PID)
		return current->pid;

	spin_lock(&lastpid_lock);
	if((++last_pid) & 0xffff8000) {		//cxl: 一般情况下，++就足够了
		last_pid = 300;		/* Skip daemons etc. */		//cxl: 超过最大范围时，从300以后开始找空闲的pid，此时需要慢慢地、小心地搜索
		goto inside;
	}
	if(last_pid >= next_safe) {	//cxl: 超出next_safe范围时，也要小心
inside:		//cxl: 下面的过程用来找到第一个可用的pid并找到next_safe的范围
		next_safe = PID_MAX;
		read_lock(&tasklist_lock);
	repeat:
		for_each_task(p) {
			if(p->pid == last_pid	||
			   p->pgrp == last_pid	||
			   p->tgid == last_pid	||
			   p->session == last_pid) {
				if(++last_pid >= next_safe) {
					if(last_pid & 0xffff8000)
						last_pid = 300;
					next_safe = PID_MAX;
				}
				goto repeat;
			}
			if(p->pid > last_pid && next_safe > p->pid)
				next_safe = p->pid;
			if(p->pgrp > last_pid && next_safe > p->pgrp)
				next_safe = p->pgrp;
			if(p->session > last_pid && next_safe > p->session)
				next_safe = p->session;
		}
		read_unlock(&tasklist_lock);
	}
	pid = last_pid;	//准备返回
	spin_unlock(&lastpid_lock);

	return pid;
}

static inline int dup_mmap(struct mm_struct * mm)
{
	struct vm_area_struct * mpnt, *tmp, **pprev;
	int retval;

	flush_cache_mm(current->mm);
	mm->locked_vm = 0;
	mm->mmap = NULL;
	mm->mmap_cache = NULL;
	mm->map_count = 0;
	mm->rss = 0;
	mm->cpu_vm_mask = 0;
	mm->swap_address = 0;
	pprev = &mm->mmap;

	/*
	 * Add it to the mmlist after the parent.
	 * Doing it this way means that we can order the list,
	 * and fork() won't mess up the ordering significantly.
	 * Add it first so that swapoff can see any swap entries.
	 */
	spin_lock(&mmlist_lock);
	list_add(&mm->mmlist, ¤t->mm->mmlist);	//cxl: 加入到mmlist中
	mmlist_nr++;
	spin_unlock(&mmlist_lock);

	for (mpnt = current->mm->mmap ; mpnt ; mpnt = mpnt->vm_next) {	//cxl: 复制整个线性区链表
		struct file *file;

		retval = -ENOMEM;
		if(mpnt->vm_flags & VM_DONTCOPY)
			continue;
		tmp = kmem_cache_alloc(vm_area_cachep, SLAB_KERNEL);
		if (!tmp)
			goto fail_nomem;
		*tmp = *mpnt;
		tmp->vm_flags &= ~VM_LOCKED;
		tmp->vm_mm = mm;
		tmp->vm_next = NULL;
		file = tmp->vm_file;
		if (file) {
			struct inode *inode = file->f_dentry->d_inode;
			get_file(file);
			if (tmp->vm_flags & VM_DENYWRITE)
				atomic_dec(&inode->i_writecount);
      
			/* insert tmp into the share list, just after mpnt */
			spin_lock(&inode->i_mapping->i_shared_lock);
			if((tmp->vm_next_share = mpnt->vm_next_share) != NULL)
				mpnt->vm_next_share->vm_pprev_share =
					&tmp->vm_next_share;
			mpnt->vm_next_share = tmp;
			tmp->vm_pprev_share = &mpnt->vm_next_share;
			spin_unlock(&inode->i_mapping->i_shared_lock);
		}

		/*
		 * Link in the new vma and copy the page table entries:
		 * link in first so that swapoff can see swap entries.
		 */
		spin_lock(&mm->page_table_lock);
		*pprev = tmp;
		pprev = &tmp->vm_next;
		mm->map_count++;
		retval = copy_page_range(mm, current->mm, tmp);
		spin_unlock(&mm->page_table_lock);

		if (tmp->vm_ops && tmp->vm_ops->open)
			tmp->vm_ops->open(tmp);

		if (retval)
			goto fail_nomem;
	}
	retval = 0;
	build_mmap_rb(mm);	//cxl: 建立红黑树

fail_nomem:
	flush_tlb_mm(current->mm);
	return retval;
}

spinlock_t mmlist_lock __cacheline_aligned = SPIN_LOCK_UNLOCKED;
int mmlist_nr;

#define allocate_mm()	(kmem_cache_alloc(mm_cachep, SLAB_KERNEL))
#define free_mm(mm)	(kmem_cache_free(mm_cachep, (mm)))

static struct mm_struct * mm_init(struct mm_struct * mm)
{
	atomic_set(&mm->mm_users, 1);
	atomic_set(&mm->mm_count, 1);
	init_rwsem(&mm->mmap_sem);
	mm->page_table_lock = SPIN_LOCK_UNLOCKED;
	mm->pgd = pgd_alloc(mm);	//cxl: 分配页表
	mm->def_flags = 0;
	if (mm->pgd)
		return mm;
	free_mm(mm);
	return NULL;
}
	

/*
 * Allocate and initialize an mm_struct.
 */
struct mm_struct * mm_alloc(void)
{
	struct mm_struct * mm;

	mm = allocate_mm();
	if (mm) {
		memset(mm, 0, sizeof(*mm));
		return mm_init(mm);
	}
	return NULL;
}

/*
 * Called when the last reference to the mm
 * is dropped: either by a lazy thread or by
 * mmput. Free the page directory and the mm.
 */
inline void __mmdrop(struct mm_struct *mm)
{
	if (mm == &init_mm) BUG();
	pgd_free(mm->pgd);
	destroy_context(mm);
	free_mm(mm);
}

/*
 * Decrement the use count and release all resources for an mm.
 */
void mmput(struct mm_struct *mm)
{
	if (atomic_dec_and_lock(&mm->mm_users, &mmlist_lock)) {
		extern struct mm_struct *swap_mm;
		if (swap_mm == mm)
			swap_mm = list_entry(mm->mmlist.next, struct mm_struct, mmlist);
		list_del(&mm->mmlist);
		mmlist_nr--;
		spin_unlock(&mmlist_lock);
		exit_mmap(mm);
		mmdrop(mm);
	}
}

/* Please note the differences between mmput and mm_release.
 * mmput is called whenever we stop holding onto a mm_struct,
 * error success whatever.
 *
 * mm_release is called after a mm_struct has been removed
 * from the current process.
 *
 * This difference is important for error handling, when we
 * only half set up a mm_struct for a new process and need to restore
 * the old one.  Because we mmput the new mm_struct before
 * restoring the old one. . .
 * Eric Biederman 10 January 1998
 */
void mm_release(void)
{
	struct task_struct *tsk = current;
	struct completion *vfork_done = tsk->vfork_done;

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

///////////////////////////////////////////////////////////
//cxl: 参照父进程，为新进程分配一个新的内存描述符
//cxl:在do_fork中被调用
///////////////////////////////////////////////////////////
static int copy_mm(unsigned long clone_flags, struct task_struct * tsk)	
{
	struct mm_struct * mm, *oldmm;
	int retval;

	tsk->min_flt = tsk->maj_flt = 0;
	tsk->cmin_flt = tsk->cmaj_flt = 0;
	tsk->nswap = tsk->cnswap = 0;

	tsk->mm = NULL;		//cxl:新进程的内存描述符一开始为NULL
	tsk->active_mm = NULL;

	/*
	 * Are we cloning a kernel thread?
	 *
	 * We need to steal a active VM for that..
	 */
	oldmm = current->mm;	//cxl:找到父进程的内存描述符
	if (!oldmm)
		return 0;

	if (clone_flags & CLONE_VM) {	//cxl:复用？？
		atomic_inc(&oldmm->mm_users);
		mm = oldmm;
		goto good_mm;
	}

	retval = -ENOMEM;
	mm = allocate_mm();	//cxl:分配一个新的内存描述符结构
	if (!mm)
		goto fail_nomem;

	/* Copy the current MM stuff.. */
	memcpy(mm, oldmm, sizeof(*mm));		//cxl:从父进程中复制各个数据项
	if (!mm_init(mm))	//cxl:有些数据项需要重新初始化
		goto fail_nomem;

	down_write(&oldmm->mmap_sem);
	retval = dup_mmap(mm);		//cxl:有些数据项需要进一步复制
	up_write(&oldmm->mmap_sem);

	if (retval)
		goto free_pt;

	/*
	 * child gets a private LDT (if there was an LDT in the parent)
	 */
	copy_segments(tsk, mm);

	if (init_new_context(tsk,mm))
		goto free_pt;

good_mm:
	tsk->mm = mm;
	tsk->active_mm = mm;
	return 0;

free_pt:
	mmput(mm);
fail_nomem:
	return retval;
}

static inline struct fs_struct *__copy_fs_struct(struct fs_struct *old)	//cxl: 创建并复制一个fs结构
{
	struct fs_struct *fs = kmem_cache_alloc(fs_cachep, GFP_KERNEL);	//cxl: 创建一个fs结构
	/* We don't need to lock fs - think why ;-) */
	if (fs) {
		atomic_set(&fs->count, 1);
		fs->lock = RW_LOCK_UNLOCKED;
		fs->umask = old->umask;
		read_lock(&old->lock);
		fs->rootmnt = mntget(old->rootmnt);
		fs->root = dget(old->root);	//cxl: 根目录
		fs->pwdmnt = mntget(old->pwdmnt);
		fs->pwd = dget(old->pwd);	//cxl: 当前目录
		if (old->altroot) {
			fs->altrootmnt = mntget(old->altrootmnt);
			fs->altroot = dget(old->altroot);
		} else {
			fs->altrootmnt = NULL;
			fs->altroot = NULL;
		}	
		read_unlock(&old->lock);
	}
	return fs;
}

struct fs_struct *copy_fs_struct(struct fs_struct *old)
{
	return __copy_fs_struct(old);
}

static inline int copy_fs(unsigned long clone_flags, struct task_struct * tsk)	//cxl: 根据clone_flags复制进程的文件系统相关信息
{
	if (clone_flags & CLONE_FS) {	//cxl: 若CLONE_FS，则共享父进程的fs数据结构
		atomic_inc(& current->fs->count);
		return 0;
	}
	tsk->fs = __copy_fs_struct(current->fs);	//cxl: 否则为新进程创建并复制fs结构
	if (!tsk->fs)
		return -1;
	return 0;
}

static int count_open_files(struct files_struct *files, int size)
{
	int i;
	
	/* Find the last open fd */
	for (i = size/(8*sizeof(long)); i > 0; ) {
		if (files->open_fds->fds_bits[--i])
			break;
	}
	i = (i+1) * 8 * sizeof(long);
	return i;
}

static int copy_files(unsigned long clone_flags, struct task_struct * tsk)	//cxl: 根据clone_flags从父进程复制进程的打开文件相关信息到子进程
{
	struct files_struct *oldf, *newf;
	struct file **old_fds, **new_fds;
	int open_files, nfds, size, i, error = 0;

	/*
	 * A background process may not have any files ...
	 */
	oldf = current->files;
	if (!oldf)
		goto out;

	if (clone_flags & CLONE_FILES) {	//cxl: 若CLONE_FILES，则共享父进程的打开文件数据结构
		atomic_inc(&oldf->count);
		goto out;
	}

	tsk->files = NULL;
	error = -ENOMEM;
	newf = kmem_cache_alloc(files_cachep, SLAB_KERNEL);	//cxl: 分配一个新的数据结构
	if (!newf) 
		goto out;

	atomic_set(&newf->count, 1);

	newf->file_lock	    = RW_LOCK_UNLOCKED;
	newf->next_fd	    = 0;
	newf->max_fds	    = NR_OPEN_DEFAULT;
	newf->max_fdset	    = __FD_SETSIZE;
	newf->close_on_exec = &newf->close_on_exec_init;
	newf->open_fds	    = &newf->open_fds_init;
	newf->fd	    = &newf->fd_array[0];

	/* We don't yet have the oldf readlock, but even if the old
           fdset gets grown now, we'll only copy up to "size" fds */
	size = oldf->max_fdset;
	if (size > __FD_SETSIZE) {
		newf->max_fdset = 0;
		write_lock(&newf->file_lock);
		error = expand_fdset(newf, size-1);
		write_unlock(&newf->file_lock);
		if (error)
			goto out_release;
	}
	read_lock(&oldf->file_lock);

	open_files = count_open_files(oldf, size);

	/*
	 * Check whether we need to allocate a larger fd array.
	 * Note: we're not a clone task, so the open count won't
	 * change.
	 */
	nfds = NR_OPEN_DEFAULT;
	if (open_files > nfds) {
		read_unlock(&oldf->file_lock);
		newf->max_fds = 0;
		write_lock(&newf->file_lock);
		error = expand_fd_array(newf, open_files-1);
		write_unlock(&newf->file_lock);
		if (error) 
			goto out_release;
		nfds = newf->max_fds;
		read_lock(&oldf->file_lock);
	}

	old_fds = oldf->fd;
	new_fds = newf->fd;

	memcpy(newf->open_fds->fds_bits, oldf->open_fds->fds_bits, open_files/8);
	memcpy(newf->close_on_exec->fds_bits, oldf->close_on_exec->fds_bits, open_files/8);

	for (i = open_files; i != 0; i--) {
		struct file *f = *old_fds++;
		if (f)
			get_file(f);
		*new_fds++ = f;
	}
	read_unlock(&oldf->file_lock);

	/* compute the remainder to be cleared */
	size = (newf->max_fds - open_files) * sizeof(struct file *);

	/* This is long word aligned thus could use a optimized version */ 
	memset(new_fds, 0, size); 

	if (newf->max_fdset > open_files) {
		int left = (newf->max_fdset-open_files)/8;
		int start = open_files / (8 * sizeof(unsigned long));
		
		memset(&newf->open_fds->fds_bits[start], 0, left);
		memset(&newf->close_on_exec->fds_bits[start], 0, left);
	}

	tsk->files = newf;
	error = 0;
out:
	return error;

out_release:
	free_fdset (newf->close_on_exec, newf->max_fdset);
	free_fdset (newf->open_fds, newf->max_fdset);
	kmem_cache_free(files_cachep, newf);
	goto out;
}

static inline int copy_sighand(unsigned long clone_flags, struct task_struct * tsk)
{
	struct signal_struct *sig;

	if (clone_flags & CLONE_SIGHAND) {
		atomic_inc(¤t->sig->count);
		return 0;
	}
	sig = kmem_cache_alloc(sigact_cachep, GFP_KERNEL);
	tsk->sig = sig;
	if (!sig)
		return -1;
	spin_lock_init(&sig->siglock);
	atomic_set(&sig->count, 1);
	memcpy(tsk->sig->action, current->sig->action, sizeof(tsk->sig->action));
	return 0;
}

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

	new_flags &= ~(PF_SUPERPRIV | PF_USEDFPU);
	new_flags |= PF_FORKNOEXEC;
	if (!(clone_flags & CLONE_PTRACE))
		p->ptrace = 0;
	p->flags = new_flags;
}

/*
 *  Ok, this is the main fork-routine. It copies the system process
 * information (task[nr]) and sets up the necessary registers. It also
 * copies the data segment in its entirety.  The "stack_start" and
 * "stack_top" arguments are simply passed along to the platform
 * specific copy_thread() routine.  Most platforms ignore stack_top.
 * For an example that's using stack_top, see
 * arch/ia64/kernel/process.c.
 */
int do_fork(unsigned long clone_flags, unsigned long stack_start,
	    struct pt_regs *regs, unsigned long stack_size)	//cxl: sys_fork, sys_vfork, sys_clone
{
	int retval;
	struct task_struct *p;
	struct completion vfork;

	retval = -EPERM;

	/* 
	 * CLONE_PID is only allowed for the initial SMP swapper
	 * calls
	 */
	if (clone_flags & CLONE_PID) {
		if (current->pid)
			goto fork_out;
	}

	retval = -ENOMEM;
	p = alloc_task_struct();  //cxl:以获得8KB的union task_union内存区
						//cxl:来存放进程描述符和新进程的内核态堆栈
	if (!p)
		goto fork_out;

	*p = *current;		//cxl:让当前指针指向父进程描述符，并把父进程
					//cxl:描述符的内容拷贝到刚刚分配的内存区的新
					//cxl:进程描述符中

	retval = -EAGAIN;
	
	//cxl:检查几个值，确认用户可以创建一个新进程
	/*
	 * Check if we are over our maximum process limit, but be sure to
	 * exclude root. This is needed to make it possible for login and
	 * friends to set the per-user process limit to something lower
	 * than the amount of processes root is running. -- Rik
	 */
	if (atomic_read(&p->user->processes) >= p->rlim[RLIMIT_NPROC].rlim_cur
	              && !capable(CAP_SYS_ADMIN) && !capable(CAP_SYS_RESOURCE))
		goto bad_fork_free;

	atomic_inc(&p->user->__count);
	atomic_inc(&p->user->processes);

	/*
	 * Counter increases are protected by
	 * the kernel lock so nr_threads can't
	 * increase under us (but it may decrease).
	 */
	if (nr_threads >= max_threads)
		goto bad_fork_cleanup_count;
	
	get_exec_domain(p->exec_domain);

	if (p->binfmt && p->binfmt->module)
		__MOD_INC_USE_COUNT(p->binfmt->module);

	p->did_exec = 0;
	p->swappable = 0;
	p->state = TASK_UNINTERRUPTIBLE;

	copy_flags(clone_flags, p);	//cxl:更新一些从父进程拷贝来的标志域的标志
	
	//cxl:现在子进程已经从父进程那里获得了几乎所有的东西，
	//cxl:剩下的就是建立子进程的新资源，并让内核知道这个
	//cxl:新进程已经诞生。
	p->pid = get_pid(clone_flags);	//cxl:为进程分配一个PID

	p->run_list.next = NULL;	//cxl:子进程还没有插入到运行队列
	p->run_list.prev = NULL;

	p->p_cptr = NULL;
	init_waitqueue_head(&p->wait_chldexit);
	p->vfork_done = NULL;
	if (clone_flags & CLONE_VFORK) {
		p->vfork_done = &vfork;
		init_completion(&vfork);
	}
	spin_lock_init(&p->alloc_lock);

	p->sigpending = 0;
	init_sigpending(&p->pending);

	p->it_real_value = p->it_virt_value = p->it_prof_value = 0;
	p->it_real_incr = p->it_virt_incr = p->it_prof_incr = 0;
	init_timer(&p->real_timer);
	p->real_timer.data = (unsigned long) p;

	p->leader = 0;		/* session leadership doesn't inherit */
	p->tty_old_pgrp = 0;
	p->times.tms_utime = p->times.tms_stime = 0;
	p->times.tms_cutime = p->times.tms_cstime = 0;
#ifdef CONFIG_SMP
	{
		int i;
		p->cpus_runnable = ~0UL;
		p->processor = current->processor;
		/* ?? should we just memset this ?? */
		for(i = 0; i < smp_num_cpus; i++)
			p->per_cpu_utime[i] = p->per_cpu_stime[i] = 0;
		spin_lock_init(&p->sigmask_lock);
	}
#endif
	p->lock_depth = -1;		/* -1 = no lock */
	p->start_time = jiffies;

	INIT_LIST_HEAD(&p->local_pages);

	retval = -ENOMEM;
	//cxl:根据参数flag，调用copy_file(),copy_fs(),copy_sighand()
	//cxl:和copy_mm()来创建新的数据结构，并把父进程相应的数据结构拷贝过来
	/* copy all the process information */
	if (copy_files(clone_flags, p))	拷贝进程打开文件的相关信息
		goto bad_fork_cleanup;
	if (copy_fs(clone_flags, p))		拷贝进程的文件系统相关信息
		goto bad_fork_cleanup_files;
	if (copy_sighand(clone_flags, p))
		goto bad_fork_cleanup_fs;
	if (copy_mm(clone_flags, p))
		goto bad_fork_cleanup_sighand;
		
	//cxl: 用包含在CPU寄存器中的值初始化子进程的内核态堆栈，
	//cxl: 然后手工修改eax寄存器对应的域为0。用内核态堆栈的
	//cxl: 基地址初始化子进程TSS的tss.esp域，并且把汇编语言
	//cxl: 函数[ret_from_fork()]的地址保存在tss.eip域		
	retval = copy_thread(0, clone_flags, stack_start, stack_size, p, regs);
	if (retval)
		goto bad_fork_cleanup_mm;
	p->semundo = NULL;
	
	/* 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->swappable = 1;
	p->exit_signal = clone_flags & CSIGNAL;
	p->pdeath_signal = 0;
	
	/*
	 * "share" dynamic priority between parent and child, thus the
	 * total amount of dynamic priorities in the system doesnt change,
	 * more scheduling fairness. This is only important in the first
	 * timeslice, on the long run the scheduling behaviour is unchanged.
	 */
	p->counter = (current->counter + 1) >> 1;
	current->counter >>= 1;		//cxl: 继承一半时间片
	if (!current->counter)
		current->need_resched = 1;

	/*
	 * Ok, add it to the run-queues and make it
	 * visible to the rest of the system.
	 *
	 * Let it rip!
	 */
	retval = p->pid;
	p->tgid = retval;
	INIT_LIST_HEAD(&p->thread_group);

	/* Need tasklist lock for parent etc handling! */
	write_lock_irq(&tasklist_lock);

	/* CLONE_PARENT and CLONE_THREAD re-use the old parent */
	p->p_opptr = current->p_opptr;
	p->p_pptr = current->p_pptr;
	if (!(clone_flags & (CLONE_PARENT | CLONE_THREAD))) {
		p->p_opptr = current;
		if (!(p->ptrace & PT_PTRACED))
			p->p_pptr = current;
	}

	if (clone_flags & CLONE_THREAD) {
		p->tgid = current->tgid;
		list_add(&p->thread_group, ¤t->thread_group);
	}

	SET_LINKS(p);		//cxl: 把新的进程描述符插入进程链表
	hash_pid(p);		//cxl: 把新的进程描述符插入pidhash散列表
	nr_threads++;
	write_unlock_irq(&tasklist_lock);

	if (p->ptrace & PT_PTRACED)
		send_sig(SIGSTOP, p, 1);

	//cxl: 子进程状态设置成TASK_RUNNING，并调用
	//cxl: wake_up_process()把子进程插入到运行队列中
	wake_up_process(p);		/* do this last */
	++total_forks;
	if (clone_flags & CLONE_VFORK)
		wait_for_completion(&vfork);	//cxl: 如果指定了CLONE_VFORK标志，就要将父
						//cxl：进程挂起，直到子进程释放了它的内存
						//cxl：地址空间，即要么子进程结束了，要么
						//cxl: 子进程执行了一个新的程序

fork_out:
	return retval;		//cxl:返回子进程的PID，这个PID最终由用户态下的父进程读取

bad_fork_cleanup_mm:
	exit_mm(p);
bad_fork_cleanup_sighand:
	exit_sighand(p);
bad_fork_cleanup_fs:
	exit_fs(p); /* blocking */
bad_fork_cleanup_files:
	exit_files(p); /* blocking */
bad_fork_cleanup:
	put_exec_domain(p->exec_domain);
	if (p->binfmt && p->binfmt->module)
		__MOD_DEC_USE_COUNT(p->binfmt->module);
bad_fork_cleanup_count:
	atomic_dec(&p->user->processes);
	free_uid(p->user);
bad_fork_free:
	free_task_struct(p);
	goto fork_out;
}

/* SLAB cache for signal_struct structures (tsk->sig) */
kmem_cache_t *sigact_cachep;

/* SLAB cache for files_struct structures (tsk->files) */
kmem_cache_t *files_cachep;

/* SLAB cache for fs_struct structures (tsk->fs) */
kmem_cache_t *fs_cachep;

/* SLAB cache for vm_area_struct structures */
kmem_cache_t *vm_area_cachep;

/* SLAB cache for mm_struct structures (tsk->mm) */
kmem_cache_t *mm_cachep;

void __init proc_caches_init(void)
{
	sigact_cachep = kmem_cache_create("signal_act",
			sizeof(struct signal_struct), 0,
			SLAB_HWCACHE_ALIGN, NULL, NULL);
	if (!sigact_cachep)
		panic("Cannot create signal action SLAB cache");

	files_cachep = kmem_cache_create("files_cache", 
			 sizeof(struct files_struct), 0, 
			 SLAB_HWCACHE_ALIGN, NULL, NULL);
	if (!files_cachep) 
		panic("Cannot create files SLAB cache");

	fs_cachep = kmem_cache_create("fs_cache", 
			 sizeof(struct fs_struct), 0, 
			 SLAB_HWCACHE_ALIGN, NULL, NULL);
	if (!fs_cachep) 
		panic("Cannot create fs_struct SLAB cache");
 
	vm_area_cachep = kmem_cache_create("vm_area_struct",
			sizeof(struct vm_area_struct), 0,
			SLAB_HWCACHE_ALIGN, NULL, NULL);
	if(!vm_area_cachep)
		panic("vma_init: Cannot alloc vm_area_struct SLAB cache");

	mm_cachep = kmem_cache_create("mm_struct",
			sizeof(struct mm_struct), 0,
			SLAB_HWCACHE_ALIGN, NULL, NULL);
	if(!mm_cachep)
		panic("vma_init: Cannot alloc mm_struct SLAB cache");
}