Linux ARM系统调用过程分析(三)——Linux中open系统调用实现原理

Linux ARM系统调用过程分析(三)——Linux中open系统调用实现原理

备注：
  1. Kernel版本：5.4
  2. 使用工具：Source Insight 4.0
  3. 参考博客：
  （1）arm-linux 系统调用流程
  （2）ARM Linux上的系统调用代码分析

sys_open的定义

sys_open是经过宏替换定义的，源码在fs/open.c中。

SYSCALL_DEFINE3(open, const char __user *, filename, int, flags, umode_t, mode)
{
	if (force_o_largefile())
		flags |= O_LARGEFILE;

	return do_sys_open(AT_FDCWD, filename, flags, mode);
}

SYSCALL_DEFINEx宏定义

//include/linux/syscalls.h
#define SYSCALL_DEFINE1(name, ...) SYSCALL_DEFINEx(1, _##name, __VA_ARGS__)
#define SYSCALL_DEFINE2(name, ...) SYSCALL_DEFINEx(2, _##name, __VA_ARGS__)
#define SYSCALL_DEFINE3(name, ...) SYSCALL_DEFINEx(3, _##name, __VA_ARGS__)
#define SYSCALL_DEFINE4(name, ...) SYSCALL_DEFINEx(4, _##name, __VA_ARGS__)
#define SYSCALL_DEFINE5(name, ...) SYSCALL_DEFINEx(5, _##name, __VA_ARGS__)
#define SYSCALL_DEFINE6(name, ...) SYSCALL_DEFINEx(6, _##name, __VA_ARGS__)

#define SYSCALL_DEFINE_MAXARGS	6

#define SYSCALL_DEFINEx(x, sname, ...)				\
	SYSCALL_METADATA(sname, x, __VA_ARGS__)			\
	__SYSCALL_DEFINEx(x, sname, __VA_ARGS__)

注意：这里name已经变成了”_name”,加上了下划线了，所以”open”变成了“_open”了。

SYSCALL_METADATA宏定义

如果未定义了CONFIG_FTRACE_SYSCALLS，则SYSCALL_METADATA为空，否则如下：

#ifdef CONFIG_FTRACE_SYSCALLS
#define __SC_STR_ADECL(t, a)	#a
#define __SC_STR_TDECL(t, a)	#t

extern struct trace_event_class event_class_syscall_enter;
extern struct trace_event_class event_class_syscall_exit;
extern struct trace_event_functions enter_syscall_print_funcs;
extern struct trace_event_functions exit_syscall_print_funcs;

#define SYSCALL_TRACE_ENTER_EVENT(sname)				\
	static struct syscall_metadata __syscall_meta_##sname;		\
	static struct trace_event_call __used				\
	  event_enter_##sname = {					\
		.class			= &event_class_syscall_enter,	\
		{							\
			.name                   = "sys_enter"#sname,	\
		},							\
		.event.funcs            = &enter_syscall_print_funcs,	\
		.data			= (void *)&__syscall_meta_##sname,\
		.flags                  = TRACE_EVENT_FL_CAP_ANY,	\
	};								\
	static struct trace_event_call __used				\
	  __attribute__((section("_ftrace_events")))			\
	 *__event_enter_##sname = &event_enter_##sname;

#define SYSCALL_TRACE_EXIT_EVENT(sname)					\
	static struct syscall_metadata __syscall_meta_##sname;		\
	static struct trace_event_call __used				\
	  event_exit_##sname = {					\
		.class			= &event_class_syscall_exit,	\
		{							\
			.name                   = "sys_exit"#sname,	\
		},							\
		.event.funcs		= &exit_syscall_print_funcs,	\
		.data			= (void *)&__syscall_meta_##sname,\
		.flags                  = TRACE_EVENT_FL_CAP_ANY,	\
	};								\
	static struct trace_event_call __used				\
	  __attribute__((section("_ftrace_events")))			\
	*__event_exit_##sname = &event_exit_##sname;

#define SYSCALL_METADATA(sname, nb, ...)			\
	static const char *types_##sname[] = {			\
		__MAP(nb,__SC_STR_TDECL,__VA_ARGS__)		\
	};							\
	static const char *args_##sname[] = {			\
		__MAP(nb,__SC_STR_ADECL,__VA_ARGS__)		\
	};							\
	SYSCALL_TRACE_ENTER_EVENT(sname);			\
	SYSCALL_TRACE_EXIT_EVENT(sname);			\
	static struct syscall_metadata __used			\
	  __syscall_meta_##sname = {				\
		.name 		= "sys"#sname,			\
		.syscall_nr	= -1,	/* Filled in at boot */	\
		.nb_args 	= nb,				\
		.types		= nb ? types_##sname : NULL,	\
		.args		= nb ? args_##sname : NULL,	\
		.enter_event	= &event_enter_##sname,		\
		.exit_event	= &event_exit_##sname,		\
		.enter_fields	= LIST_HEAD_INIT(__syscall_meta_##sname.enter_fields), \
	};							\
	static struct syscall_metadata __used			\
	  __attribute__((section("__syscalls_metadata")))	\
	 *__p_syscall_meta_##sname = &__syscall_meta_##sname;

static inline int is_syscall_trace_event(struct trace_event_call *tp_event)
{
	return tp_event->class == &event_class_syscall_enter ||
	       tp_event->class == &event_class_syscall_exit;
}

#else
#define SYSCALL_METADATA(sname, nb, ...)

static inline int is_syscall_trace_event(struct trace_event_call *tp_event)
{
	return 0;
}
#endif

__SYSCALL_DEFINEx宏定义

#define __SYSCALL_DEFINEx(x, name, ...)					\
	__diag_push();							\
	__diag_ignore(GCC, 8, "-Wattribute-alias",			\
		      "Type aliasing is used to sanitize syscall arguments");\
	asmlinkage long sys##name(__MAP(x,__SC_DECL,__VA_ARGS__))	\
		__attribute__((alias(__stringify(__se_sys##name))));	\
	ALLOW_ERROR_INJECTION(sys##name, ERRNO);			\
	static inline long __do_sys##name(__MAP(x,__SC_DECL,__VA_ARGS__));\
	asmlinkage long __se_sys##name(__MAP(x,__SC_LONG,__VA_ARGS__));	\
	asmlinkage long __se_sys##name(__MAP(x,__SC_LONG,__VA_ARGS__))	\
	{								\
		long ret = __do_sys##name(__MAP(x,__SC_CAST,__VA_ARGS__));\
		__MAP(x,__SC_TEST,__VA_ARGS__);				\
		__PROTECT(x, ret,__MAP(x,__SC_ARGS,__VA_ARGS__));	\
		return ret;						\
	}								\
	__diag_pop();							\
	static inline long __do_sys##name(__MAP(x,__SC_DECL,__VA_ARGS__))

asmlinkage long sys##name(__MAP(x,__SC_DECL,VA_ARGS))，此时已变成了“sys_open”。
经过宏最终的展开，最后的形式如下：

asmlinkage long sys_open(const char __user*  filename,int flags,umode_t mode)
{
	if (force_o_largefile())
		flags |= O_LARGEFILE;

	return do_sys_open(AT_FDCWD, filename, flags, mode);
}

sys_open的实现

SYSCALL_DEFINE3(open, const char __user *, filename, int, flags, umode_t, mode)
{
    
        //判断系统是否支持大文件，即判断long的位数，如果64则表示支持大文件；
	if (force_o_largefile())
		flags |= O_LARGEFILE;

    
        //完成主要的open工作，AT_FDCWD表示从当前目录开始查找
 	return do_sys_open(AT_FDCWD, filename, flags, mode);
}

其中先调用force_o_largefile()来判断是否需要设置大文件标识，然后调用do_sys_open来完成具体的工作。其中force_o_largefile()函数如下：

#ifndef force_o_largefile
#define force_o_largefile() (!IS_ENABLED(CONFIG_ARCH_32BIT_OFF_T))
#endif

只有在32位的OS上此处才为false，所以64位的系统上才有可能使能该寄存器。

因此文件最大大小受索引节点中表示文件大小的32位的i_size的影响，只能访问2的32次方字节，即4GB（实际高位一般不用，所以通常只有2G）。加上O_LAGEFILE之后启用索引节点的i_dir_acl字段也可以一起表示文件的大小了，这样位数就变成了64位，2的64位就4GB*4GB，单个文件这么大已经很大了16T了。

do_sys_open函数

该函数主要分为如下几个步骤来完成打开文件的操作：

• 1. 将文件名参数从用户态拷贝至内核，调用函数get_name()；
• 2. 从进程的文件表中找到一个空闲的文件表指针，调用了函数get_unused_fd_flgas()；
• 3. 完成真正的打开操作，调用函数do_filp_open()；
• 4. 将打开的文件添加到进程的文件表数组中，调用函数fd_install()；

源码：fs/open.c

long do_sys_open(int dfd, const char __user *filename, int flags, umode_t mode)
{
	struct open_flags op;
	int fd = build_open_flags(flags, mode, &op);
	struct filename *tmp;

	if (fd)
		return fd;

	//将欲打开的文件名拷贝到内核中
	tmp = getname(filename);
	if (IS_ERR(tmp))
		return PTR_ERR(tmp);

	//从进程的文件表中找到一个空闲的文件表指针，如果出错，则返回
	fd = get_unused_fd_flags(flags);
	if (fd >= 0) {
		struct file *f = do_filp_open(dfd, tmp, &op);
		if (IS_ERR(f)) {
			put_unused_fd(fd);
			fd = PTR_ERR(f);
		} else {
			fsnotify_open(f);

			//把file和fd关联起来，添加打开的文件表f到当前进程的文件表数组中
			fd_install(fd, f);
		}
	}
	putname(tmp);
	return fd;
}

getname函数：
源码：fs/namei.c

struct filename *
getname(const char __user * filename)
{
	return getname_flags(filename, 0, NULL);
}

struct filename *
getname_flags(const char __user *filename, int flags, int *empty)
{
	struct filename *result;
	char *kname;
	int len;

	result = audit_reusename(filename);
	if (result)
		return result;

	result = __getname(); //从内核缓存中分配空间；
	if (unlikely(!result))
		return ERR_PTR(-ENOMEM);

	/*
	 * First, try to embed the struct filename inside the names_cache
	 * allocation
	 */
	kname = (char *)result->iname;
	result->name = kname;

	//将文件名从用户态拷贝至内核态；
	len = strncpy_from_user(kname, filename, EMBEDDED_NAME_MAX);
	if (unlikely(len < 0)) {
		__putname(result); //如果拷贝失败，则调用__putname()释放__getname()中申请的空间；
		return ERR_PTR(len);
	}

	/*
	 * Uh-oh. We have a name that's approaching PATH_MAX. Allocate a
	 * separate struct filename so we can dedicate the entire
	 * names_cache allocation for the pathname, and re-do the copy from
	 * userland.
	 */
	if (unlikely(len == EMBEDDED_NAME_MAX)) {
		const size_t size = offsetof(struct filename, iname[1]);
		kname = (char *)result;

		/*
		 * size is chosen that way we to guarantee that
		 * result->iname[0] is within the same object and that
		 * kname can't be equal to result->iname, no matter what.
		 */
		result = kzalloc(size, GFP_KERNEL);
		if (unlikely(!result)) {
			__putname(kname);
			return ERR_PTR(-ENOMEM);
		}
		result->name = kname;
		len = strncpy_from_user(kname, filename, PATH_MAX);
		if (unlikely(len < 0)) {
			__putname(kname);
			kfree(result);
			return ERR_PTR(len);
		}
		if (unlikely(len == PATH_MAX)) {
			__putname(kname);
			kfree(result);
			return ERR_PTR(-ENAMETOOLONG);
		}
	}

	result->refcnt = 1;
	/* The empty path is special. */
	if (unlikely(!len)) {
		if (empty)
			*empty = 1;
		if (!(flags & LOOKUP_EMPTY)) {
			putname(result);
			return ERR_PTR(-ENOENT);
		}
	}

	result->uptr = filename;
	result->aname = NULL;
	audit_getname(result);
	return result;
}

get_unused_fd_flags函数：
源码：fs/file.c

int get_unused_fd_flags(unsigned flags)
{
	//获得当前进程的files_struct 结构——current->files
	return __alloc_fd(current->files, 0, rlimit(RLIMIT_NOFILE), flags);
}
EXPORT_SYMBOL(get_unused_fd_flags);

/*
 * allocate a file descriptor, mark it busy.
 */
int __alloc_fd(struct files_struct *files,
	       unsigned start, unsigned end, unsigned flags)
{
	unsigned int fd;
	int error;
	struct fdtable *fdt;

	spin_lock(&files->file_lock);
repeat:
	fdt = files_fdtable(files);
	fd = start;					// 从上一次打开的fd的下一个fd开始搜索空闲的fd

	// 寻找空闲的fd，返回值为空闲的fd
	if (fd < files->next_fd)
		fd = files->next_fd;

	if (fd < fdt->max_fds)
		fd = find_next_fd(fdt, fd);

	/*
	 * N.B. For clone tasks sharing a files structure, this test
	 * will limit the total number of files that can be opened.
	 */
	error = -EMFILE;
	if (fd >= end)
		goto out;

	//如果有必要，即打开的fd超过max_fds,则需要expand当前进程的fd表；
	//返回值error<0表示出错，error=0表示无需expand，error=1表示进行了expand；
	error = expand_files(files, fd);
	if (error < 0)
		goto out;

	/*
	 * If we needed to expand the fs array we
	 * might have blocked - try again.
	 */
	//error=1表示进行了expand，那么此时需要重新去查找空闲的fd；
	if (error)
		goto repeat;

	//设置下一次查找的起始fd，即本次找到的空闲的fd的下一个fd，记录在files->next_fd中
	if (start <= files->next_fd)
		files->next_fd = fd + 1;

	__set_open_fd(fd, fdt);
	if (flags & O_CLOEXEC)
		__set_close_on_exec(fd, fdt);
	else
		__clear_close_on_exec(fd, fdt);
	error = fd;
#if 1
	/* Sanity check */
	if (rcu_access_pointer(fdt->fd[fd]) != NULL) {
		printk(KERN_WARNING "alloc_fd: slot %d not NULL!\n", fd);
		rcu_assign_pointer(fdt->fd[fd], NULL);
	}
#endif

out:
	spin_unlock(&files->file_lock);
	return error;
}

do_filp_open函数：

do_filep_open函数的主要功能，就是通过路径名来分配并填充这个文件对应的文件对象。do_filep_open(dfd, pathname, flags, mode, acc_mode)函数主要执行如下操作：
1，设置一堆访问模式标志
2，调用get_empty_filep()函数从名为filp_cachep的slab缓存中分配一个struct file*的文件对象。
3，如果flags中有O_CREATE标志，跳到，否则到4
4，调用do_path_lookup(dfd, pathname, flags, &nd)做目录查找，将查找结果填充到struct nameidata nd中。还记得目录查找么？见这里
5，调用finish_open(nd, flags, mode)做一些合法性验验证并从nd->intent.open.file中获取到struct file filep
6，调用release_open_intent(nd)做一些清理工作。主要是减少nd->intent.open.file中的一些引用计数。
7，返回filep
8，到这一步说明flags中有O_CREATE标志，需要在目录查找过程中逐级创建对应的目录和文件，这一步依次调用path_init_rcu(), path_walk_rcu()和path_finish_rcu()完成创建文件的目录查找工作，最终依然是将查找结果填充到struct nameidata *nd中。（在标准的目录查找do_path_lookup()的实现中，主干流程也是依次调用着三个函数做查找工作）
9，调用do_last(&nd, &path, flags, acc_mode, mode, pathname)函数获取最终的struct file filep结构。在这个函数中，内核会根据nd->last_type做不同的处理，对于普通文件，会调用finish_open(nd, flags, mode)做一些合法性验验证并从nd->intent.open.file中获取到struct file filep
10，调用release_open_intent(nd)做一些清理工作。主要是减少nd->intent.open.file中的一些引用计数。
11，返回filep
源码：fs/namei.c

struct file *do_filp_open(int dfd, struct filename *pathname,
		const struct open_flags *op)
{
	struct nameidata nd;
	int flags = op->lookup_flags;
	struct file *filp;

	set_nameidata(&nd, dfd, pathname);
	filp = path_openat(&nd, op, flags | LOOKUP_RCU);
	if (unlikely(filp == ERR_PTR(-ECHILD)))
		filp = path_openat(&nd, op, flags);
	if (unlikely(filp == ERR_PTR(-ESTALE)))
		filp = path_openat(&nd, op, flags | LOOKUP_REVAL);
	restore_nameidata();
	return filp;
}

//源码：fs/namei.c

static struct file *path_openat(struct nameidata *nd,
			const struct open_flags *op, unsigned flags)
{
	struct file *file;
	int error;

	// 拿到一个初始化好的file结构体
	file = alloc_empty_file(op->open_flag, current_cred());
	if (IS_ERR(file))
		return file;

	if (unlikely(file->f_flags & __O_TMPFILE)) {
		error = do_tmpfile(nd, flags, op, file);
	} else if (unlikely(file->f_flags & O_PATH)) {
		error = do_o_path(nd, flags, file);
	} else {
		//找到需要打开文件的父目录的dentry结构，这个过程我们已经在sys_mount过程中分析；
		//为什么要查找父目录的struct dentry，我想是因为open的文件不一定存在，所以先找到父目录的dentry；
		const char *s = path_init(nd, flags);
		while (!(error = link_path_walk(s, nd)) &&
			(error = do_last(nd, file, op)) > 0) {
			nd->flags &= ~(LOOKUP_OPEN|LOOKUP_CREATE|LOOKUP_EXCL);
			s = trailing_symlink(nd);
		}
		terminate_walk(nd);
	}
	if (likely(!error)) {
		if (likely(file->f_mode & FMODE_OPENED))
			return file;
		WARN_ON(1);
		error = -EINVAL;
	}
	fput(file);
	if (error == -EOPENSTALE) {
		if (flags & LOOKUP_RCU)
			error = -ECHILD;
		else
			error = -ESTALE;
	}
	return ERR_PTR(error);
}

//源码：fs/namei.c

/*
 * Handle the last step of open()
 */
static int do_last(struct nameidata *nd,
		   struct file *file, const struct open_flags *op)
{
	struct dentry *dir = nd->path.dentry;
	kuid_t dir_uid = nd->inode->i_uid;
	umode_t dir_mode = nd->inode->i_mode;
	int open_flag = op->open_flag;
	bool will_truncate = (open_flag & O_TRUNC) != 0;
	bool got_write = false;
	int acc_mode = op->acc_mode;
	unsigned seq;
	struct inode *inode;
	struct path path;
	int error;

	nd->flags &= ~LOOKUP_PARENT;
	nd->flags |= op->intent;

	//nd->last_type:记录路径名的最后一个分量的类型；
	if (nd->last_type != LAST_NORM) {
		error = handle_dots(nd, nd->last_type);
		if (unlikely(error))
			return error;
		goto finish_open;
	}

	if (!(open_flag & O_CREAT)) {
		if (nd->last.name[nd->last.len])
			nd->flags |= LOOKUP_FOLLOW | LOOKUP_DIRECTORY;
		/* we _can_ be in RCU mode here */
		error = lookup_fast(nd, &path, &inode, &seq);
		if (likely(error > 0))
			goto finish_lookup;

		if (error < 0)
			return error;

		BUG_ON(nd->inode != dir->d_inode);
		BUG_ON(nd->flags & LOOKUP_RCU);
	} else {
		/* create side of things */
		/*
		 * This will *only* deal with leaving RCU mode - LOOKUP_JUMPED
		 * has been cleared when we got to the last component we are
		 * about to look up
		 */
		error = complete_walk(nd);
		if (error)
			return error;

		audit_inode(nd->name, dir, AUDIT_INODE_PARENT);
		/* trailing slashes? */
		if (unlikely(nd->last.name[nd->last.len]))
			return -EISDIR;
	}

	// 以下处理的是没有设置O_CREAT标志位的情况，即简单的查找，调用函数do_lookup()；
	// 如果没有找到struct dentry，那么返回说目录项不存在；
	// 如果找到nd->last对应的dentry，那么跳转到ok；
	if (open_flag & (O_CREAT | O_TRUNC | O_WRONLY | O_RDWR)) {
		error = mnt_want_write(nd->path.mnt);
		if (!error)
			got_write = true;
		/*
		 * do _not_ fail yet - we might not need that or fail with
		 * a different error; let lookup_open() decide; we'll be
		 * dropping this one anyway.
		 */
	}

	// 以下进入O_CREAT标志位被置位的处理情况
	if (open_flag & O_CREAT)
		inode_lock(dir->d_inode);
	else
		inode_lock_shared(dir->d_inode);
		
	error = lookup_open(nd, &path, file, op, got_write);
	if (open_flag & O_CREAT)
		inode_unlock(dir->d_inode);
	else
		inode_unlock_shared(dir->d_inode);

	if (error)
		goto out;

	if (file->f_mode & FMODE_OPENED) {
		if ((file->f_mode & FMODE_CREATED) ||
		    !S_ISREG(file_inode(file)->i_mode))
			will_truncate = false;

		audit_inode(nd->name, file->f_path.dentry, 0);
		goto opened;
	}

	if (file->f_mode & FMODE_CREATED) {
		/* Don't check for write permission, don't truncate */
		open_flag &= ~O_TRUNC;
		will_truncate = false;
		acc_mode = 0;
		path_to_nameidata(&path, nd);
		goto finish_open_created;
	}

	/*
	 * If atomic_open() acquired write access it is dropped now due to
	 * possible mount and symlink following (this might be optimized away if
	 * necessary...)
	 */
	if (got_write) {
		mnt_drop_write(nd->path.mnt);
		got_write = false;
	}

	error = follow_managed(&path, nd);
	if (unlikely(error < 0))
		return error;

	if (unlikely(d_is_negative(path.dentry))) {
		path_to_nameidata(&path, nd);
		return -ENOENT;
	}

	/*
	 * create/update audit record if it already exists.
	 */
	audit_inode(nd->name, path.dentry, 0);

	if (unlikely((open_flag & (O_EXCL | O_CREAT)) == (O_EXCL | O_CREAT))) {
		path_to_nameidata(&path, nd);
		return -EEXIST;
	}

	seq = 0;	/* out of RCU mode, so the value doesn't matter */
	inode = d_backing_inode(path.dentry);
finish_lookup:
	error = step_into(nd, &path, 0, inode, seq);
	if (unlikely(error))
		return error;
finish_open:
	/* Why this, you ask?  _Now_ we might have grown LOOKUP_JUMPED... */
	error = complete_walk(nd);
	if (error)
		return error;
	audit_inode(nd->name, nd->path.dentry, 0);
	if (open_flag & O_CREAT) {
		error = -EISDIR;
		if (d_is_dir(nd->path.dentry))
			goto out;
		error = may_create_in_sticky(dir_mode, dir_uid,
					     d_backing_inode(nd->path.dentry));
		if (unlikely(error))
			goto out;
	}
	error = -ENOTDIR;
	if ((nd->flags & LOOKUP_DIRECTORY) && !d_can_lookup(nd->path.dentry))
		goto out;
	if (!d_is_reg(nd->path.dentry))
		will_truncate = false;

	if (will_truncate) {
		error = mnt_want_write(nd->path.mnt);
		if (error)
			goto out;
		got_write = true;
	}
finish_open_created:
	error = may_open(&nd->path, acc_mode, open_flag);
	if (error)
		goto out;
	BUG_ON(file->f_mode & FMODE_OPENED); /* once it's opened, it's opened */
	error = vfs_open(&nd->path, file);
	if (error)
		goto out;
opened:
	error = ima_file_check(file, op->acc_mode);
	if (!error && will_truncate)
		error = handle_truncate(file);
out:
	if (unlikely(error > 0)) {
		WARN_ON(1);
		error = -EINVAL;
	}
	if (got_write)
		mnt_drop_write(nd->path.mnt);
	return error;
}

fd_install函数：
fd_install()将分配的fd和代表该文件的struct file结构建立关联。
源码：fs/file.c

void fd_install(unsigned int fd, struct file *file)
{
	__fd_install(current->files, fd, file);
}

EXPORT_SYMBOL(fd_install);

void __fd_install(struct files_struct *files, unsigned int fd,
		struct file *file)
{
	struct fdtable *fdt;

	rcu_read_lock_sched();

	if (unlikely(files->resize_in_progress)) {
		rcu_read_unlock_sched();
		spin_lock(&files->file_lock);
		fdt = files_fdtable(files);
		BUG_ON(fdt->fd[fd] != NULL);
		rcu_assign_pointer(fdt->fd[fd], file);
		spin_unlock(&files->file_lock);
		return;
	}
	/* coupled with smp_wmb() in expand_fdtable() */
	smp_rmb();
	fdt = rcu_dereference_sched(files->fdt);
	BUG_ON(fdt->fd[fd] != NULL);
	rcu_assign_pointer(fdt->fd[fd], file);
	rcu_read_unlock_sched();
}