fs/file_table.c - kernel/msm-4.9 - Gitiles

 /*
  *  linux/fs/file_table.c
  *
  *  Copyright (C) 1991, 1992  Linus Torvalds
  *  Copyright (C) 1997 David S. Miller (davem@caip.rutgers.edu)
  */

 #include <linux/string.h>
 #include <linux/slab.h>
 #include <linux/file.h>
 #include <linux/fdtable.h>
 #include <linux/init.h>
 #include <linux/module.h>
 #include <linux/fs.h>
 #include <linux/security.h>
 #include <linux/eventpoll.h>
 #include <linux/rcupdate.h>
 #include <linux/mount.h>
 #include <linux/capability.h>
 #include <linux/cdev.h>
 #include <linux/fsnotify.h>
 #include <linux/sysctl.h>
 #include <linux/lglock.h>
 #include <linux/percpu_counter.h>
 #include <linux/percpu.h>
 #include <linux/ima.h>

 #include <linux/atomic.h>

 #include "internal.h"

 /* sysctl tunables... */
 struct files_stat_struct files_stat = {
 	.max_files = NR_FILE
 };

 DECLARE_LGLOCK(files_lglock);
 DEFINE_LGLOCK(files_lglock);

 /* SLAB cache for file structures */
 static struct kmem_cache *filp_cachep __read_mostly;

 static struct percpu_counter nr_files __cacheline_aligned_in_smp;

 static inline void file_free_rcu(struct rcu_head *head)
 {
 	struct file *f = container_of(head, struct file, f_u.fu_rcuhead);

 	put_cred(f->f_cred);
 	kmem_cache_free(filp_cachep, f);
 }

 static inline void file_free(struct file *f)
 {
 	percpu_counter_dec(&nr_files);
 	file_check_state(f);
 	call_rcu(&f->f_u.fu_rcuhead, file_free_rcu);
 }

 /*
  * Return the total number of open files in the system
  */
 static long get_nr_files(void)
 {
 	return percpu_counter_read_positive(&nr_files);
 }

 /*
  * Return the maximum number of open files in the system
  */
 unsigned long get_max_files(void)
 {
 	return files_stat.max_files;
 }
 EXPORT_SYMBOL_GPL(get_max_files);

 /*
  * Handle nr_files sysctl
  */
 #if defined(CONFIG_SYSCTL) && defined(CONFIG_PROC_FS)
 int proc_nr_files(ctl_table *table, int write,
                      void __user *buffer, size_t *lenp, loff_t *ppos)
 {
 	files_stat.nr_files = get_nr_files();
 	return proc_doulongvec_minmax(table, write, buffer, lenp, ppos);
 }
 #else
 int proc_nr_files(ctl_table *table, int write,
                      void __user *buffer, size_t *lenp, loff_t *ppos)
 {
 	return -ENOSYS;
 }
 #endif

 /* Find an unused file structure and return a pointer to it.
  * Returns NULL, if there are no more free file structures or
  * we run out of memory.
  *
  * Be very careful using this.  You are responsible for
  * getting write access to any mount that you might assign
  * to this filp, if it is opened for write.  If this is not
  * done, you will imbalance int the mount's writer count
  * and a warning at __fput() time.
  */
 struct file *get_empty_filp(void)
 {
 	const struct cred *cred = current_cred();
 	static long old_max;
 	struct file * f;

 	/*
 	 * Privileged users can go above max_files
 	 */
 	if (get_nr_files() >= files_stat.max_files && !capable(CAP_SYS_ADMIN)) {
 		/*
 		 * percpu_counters are inaccurate.  Do an expensive check before
 		 * we go and fail.
 		 */
 		if (percpu_counter_sum_positive(&nr_files) >= files_stat.max_files)
 			goto over;
 	}

 	f = kmem_cache_zalloc(filp_cachep, GFP_KERNEL);
 	if (f == NULL)
 		goto fail;

 	percpu_counter_inc(&nr_files);
 	f->f_cred = get_cred(cred);
 	if (security_file_alloc(f))
 		goto fail_sec;

 	INIT_LIST_HEAD(&f->f_u.fu_list);
 	atomic_long_set(&f->f_count, 1);
 	rwlock_init(&f->f_owner.lock);
 	spin_lock_init(&f->f_lock);
 	eventpoll_init_file(f);
 	/* f->f_version: 0 */
 	return f;

 over:
 	/* Ran out of filps - report that */
 	if (get_nr_files() > old_max) {
 		pr_info("VFS: file-max limit %lu reached\n", get_max_files());
 		old_max = get_nr_files();
 	}
 	goto fail;

 fail_sec:
 	file_free(f);
 fail:
 	return NULL;
 }

 /**
  * alloc_file - allocate and initialize a 'struct file'
  * @mnt: the vfsmount on which the file will reside
  * @dentry: the dentry representing the new file
  * @mode: the mode with which the new file will be opened
  * @fop: the 'struct file_operations' for the new file
  *
  * Use this instead of get_empty_filp() to get a new
  * 'struct file'.  Do so because of the same initialization
  * pitfalls reasons listed for init_file().  This is a
  * preferred interface to using init_file().
  *
  * If all the callers of init_file() are eliminated, its
  * code should be moved into this function.
  */
 struct file *alloc_file(struct path *path, fmode_t mode,
 		const struct file_operations *fop)
 {
 	struct file *file;

 	file = get_empty_filp();
 	if (!file)
 		return NULL;

 	file->f_path = *path;
 	file->f_mapping = path->dentry->d_inode->i_mapping;
 	file->f_mode = mode;
 	file->f_op = fop;

 	/*
 	 * These mounts don't really matter in practice
 	 * for r/o bind mounts.  They aren't userspace-
 	 * visible.  We do this for consistency, and so
 	 * that we can do debugging checks at __fput()
 	 */
 	if ((mode & FMODE_WRITE) && !special_file(path->dentry->d_inode->i_mode)) {
 		file_take_write(file);
 		WARN_ON(mnt_clone_write(path->mnt));
 	}
 	if ((mode & (FMODE_READ | FMODE_WRITE)) == FMODE_READ)
 		i_readcount_inc(path->dentry->d_inode);
 	return file;
 }
 EXPORT_SYMBOL(alloc_file);

 /**
  * drop_file_write_access - give up ability to write to a file
  * @file: the file to which we will stop writing
  *
  * This is a central place which will give up the ability
  * to write to @file, along with access to write through
  * its vfsmount.
  */
 void drop_file_write_access(struct file *file)
 {
 	struct vfsmount *mnt = file->f_path.mnt;
 	struct dentry *dentry = file->f_path.dentry;
 	struct inode *inode = dentry->d_inode;

 	put_write_access(inode);

 	if (special_file(inode->i_mode))
 		return;
 	if (file_check_writeable(file) != 0)
 		return;
 	mnt_drop_write(mnt);
 	file_release_write(file);
 }
 EXPORT_SYMBOL_GPL(drop_file_write_access);

 /* the real guts of fput() - releasing the last reference to file
  */
 static void __fput(struct file *file)
 {
 	struct dentry *dentry = file->f_path.dentry;
 	struct vfsmount *mnt = file->f_path.mnt;
 	struct inode *inode = dentry->d_inode;

 	might_sleep();

 	fsnotify_close(file);
 	/*
 	 * The function eventpoll_release() should be the first called
 	 * in the file cleanup chain.
 	 */
 	eventpoll_release(file);
 	locks_remove_flock(file);

 	if (unlikely(file->f_flags & FASYNC)) {
 		if (file->f_op && file->f_op->fasync)
 			file->f_op->fasync(-1, file, 0);
 	}
 	if (file->f_op && file->f_op->release)
 		file->f_op->release(inode, file);
 	security_file_free(file);
 	ima_file_free(file);
 	if (unlikely(S_ISCHR(inode->i_mode) && inode->i_cdev != NULL &&
 		     !(file->f_mode & FMODE_PATH))) {
 		cdev_put(inode->i_cdev);
 	}
 	fops_put(file->f_op);
 	put_pid(file->f_owner.pid);
 	file_sb_list_del(file);
 	if ((file->f_mode & (FMODE_READ | FMODE_WRITE)) == FMODE_READ)
 		i_readcount_dec(inode);
 	if (file->f_mode & FMODE_WRITE)
 		drop_file_write_access(file);
 	file->f_path.dentry = NULL;
 	file->f_path.mnt = NULL;
 	file_free(file);
 	dput(dentry);
 	mntput(mnt);
 }

 void fput(struct file *file)
 {
 	if (atomic_long_dec_and_test(&file->f_count))
 		__fput(file);
 }

 EXPORT_SYMBOL(fput);

 struct file *fget(unsigned int fd)
 {
 	struct file *file;
 	struct files_struct *files = current->files;

 	rcu_read_lock();
 	file = fcheck_files(files, fd);
 	if (file) {
 		/* File object ref couldn't be taken */
 		if (file->f_mode & FMODE_PATH ||
 		    !atomic_long_inc_not_zero(&file->f_count))
 			file = NULL;
 	}
 	rcu_read_unlock();

 	return file;
 }

 EXPORT_SYMBOL(fget);

 struct file *fget_raw(unsigned int fd)
 {
 	struct file *file;
 	struct files_struct *files = current->files;

 	rcu_read_lock();
 	file = fcheck_files(files, fd);
 	if (file) {
 		/* File object ref couldn't be taken */
 		if (!atomic_long_inc_not_zero(&file->f_count))
 			file = NULL;
 	}
 	rcu_read_unlock();

 	return file;
 }

 EXPORT_SYMBOL(fget_raw);

 /*
  * Lightweight file lookup - no refcnt increment if fd table isn't shared.
  *
  * You can use this instead of fget if you satisfy all of the following
  * conditions:
  * 1) You must call fput_light before exiting the syscall and returning control
  *    to userspace (i.e. you cannot remember the returned struct file * after
  *    returning to userspace).
  * 2) You must not call filp_close on the returned struct file * in between
  *    calls to fget_light and fput_light.
  * 3) You must not clone the current task in between the calls to fget_light
  *    and fput_light.
  *
  * The fput_needed flag returned by fget_light should be passed to the
  * corresponding fput_light.
  */
 struct file *fget_light(unsigned int fd, int *fput_needed)
 {
 	struct file *file;
 	struct files_struct *files = current->files;

 	*fput_needed = 0;
 	if (atomic_read(&files->count) == 1) {
 		file = fcheck_files(files, fd);
 		if (file && (file->f_mode & FMODE_PATH))
 			file = NULL;
 	} else {
 		rcu_read_lock();
 		file = fcheck_files(files, fd);
 		if (file) {
 			if (!(file->f_mode & FMODE_PATH) &&
 			    atomic_long_inc_not_zero(&file->f_count))
 				*fput_needed = 1;
 			else
 				/* Didn't get the reference, someone's freed */
 				file = NULL;
 		}
 		rcu_read_unlock();
 	}

 	return file;
 }

 struct file *fget_raw_light(unsigned int fd, int *fput_needed)
 {
 	struct file *file;
 	struct files_struct *files = current->files;

 	*fput_needed = 0;
 	if (atomic_read(&files->count) == 1) {
 		file = fcheck_files(files, fd);
 	} else {
 		rcu_read_lock();
 		file = fcheck_files(files, fd);
 		if (file) {
 			if (atomic_long_inc_not_zero(&file->f_count))
 				*fput_needed = 1;
 			else
 				/* Didn't get the reference, someone's freed */
 				file = NULL;
 		}
 		rcu_read_unlock();
 	}

 	return file;
 }

 void put_filp(struct file *file)
 {
 	if (atomic_long_dec_and_test(&file->f_count)) {
 		security_file_free(file);
 		file_sb_list_del(file);
 		file_free(file);
 	}
 }

 static inline int file_list_cpu(struct file *file)
 {
 #ifdef CONFIG_SMP
 	return file->f_sb_list_cpu;
 #else
 	return smp_processor_id();
 #endif
 }

 /* helper for file_sb_list_add to reduce ifdefs */
 static inline void __file_sb_list_add(struct file *file, struct super_block *sb)
 {
 	struct list_head *list;
 #ifdef CONFIG_SMP
 	int cpu;
 	cpu = smp_processor_id();
 	file->f_sb_list_cpu = cpu;
 	list = per_cpu_ptr(sb->s_files, cpu);
 #else
 	list = &sb->s_files;
 #endif
 	list_add(&file->f_u.fu_list, list);
 }

 /**
  * file_sb_list_add - add a file to the sb's file list
  * @file: file to add
  * @sb: sb to add it to
  *
  * Use this function to associate a file with the superblock of the inode it
  * refers to.
  */
 void file_sb_list_add(struct file *file, struct super_block *sb)
 {
 	lg_local_lock(files_lglock);
 	__file_sb_list_add(file, sb);
 	lg_local_unlock(files_lglock);
 }

 /**
  * file_sb_list_del - remove a file from the sb's file list
  * @file: file to remove
  * @sb: sb to remove it from
  *
  * Use this function to remove a file from its superblock.
  */
 void file_sb_list_del(struct file *file)
 {
 	if (!list_empty(&file->f_u.fu_list)) {
 		lg_local_lock_cpu(files_lglock, file_list_cpu(file));
 		list_del_init(&file->f_u.fu_list);
 		lg_local_unlock_cpu(files_lglock, file_list_cpu(file));
 	}
 }

 #ifdef CONFIG_SMP

 /*
  * These macros iterate all files on all CPUs for a given superblock.
  * files_lglock must be held globally.
  */
 #define do_file_list_for_each_entry(__sb, __file)		\
 {								\
 	int i;							\
 	for_each_possible_cpu(i) {				\
 		struct list_head *list;				\
 		list = per_cpu_ptr((__sb)->s_files, i);		\
 		list_for_each_entry((__file), list, f_u.fu_list)

 #define while_file_list_for_each_entry				\
 	}							\
 }

 #else

 #define do_file_list_for_each_entry(__sb, __file)		\
 {								\
 	struct list_head *list;					\
 	list = &(sb)->s_files;					\
 	list_for_each_entry((__file), list, f_u.fu_list)

 #define while_file_list_for_each_entry				\
 }

 #endif

 /**
  *	mark_files_ro - mark all files read-only
  *	@sb: superblock in question
  *
  *	All files are marked read-only.  We don't care about pending
  *	delete files so this should be used in 'force' mode only.
  */
 void mark_files_ro(struct super_block *sb)
 {
 	struct file *f;

 retry:
 	lg_global_lock(files_lglock);
 	do_file_list_for_each_entry(sb, f) {
 		struct vfsmount *mnt;
 		if (!S_ISREG(f->f_path.dentry->d_inode->i_mode))
 		       continue;
 		if (!file_count(f))
 			continue;
 		if (!(f->f_mode & FMODE_WRITE))
 			continue;
 		spin_lock(&f->f_lock);
 		f->f_mode &= ~FMODE_WRITE;
 		spin_unlock(&f->f_lock);
 		if (file_check_writeable(f) != 0)
 			continue;
 		file_release_write(f);
 		mnt = mntget(f->f_path.mnt);
 		/* This can sleep, so we can't hold the spinlock. */
 		lg_global_unlock(files_lglock);
 		mnt_drop_write(mnt);
 		mntput(mnt);
 		goto retry;
 	} while_file_list_for_each_entry;
 	lg_global_unlock(files_lglock);
 }

 void __init files_init(unsigned long mempages)
 {
 	unsigned long n;

 	filp_cachep = kmem_cache_create("filp", sizeof(struct file), 0,
 			SLAB_HWCACHE_ALIGN | SLAB_PANIC, NULL);

 	/*
 	 * One file with associated inode and dcache is very roughly 1K.
 	 * Per default don't use more than 10% of our memory for files.
 	 */

 	n = (mempages * (PAGE_SIZE / 1024)) / 10;
 	files_stat.max_files = max_t(unsigned long, n, NR_FILE);
 	files_defer_init();
 	lg_lock_init(files_lglock);
 	percpu_counter_init(&nr_files, 0);
 }
	/*
	* linux/fs/file_table.c
	*
	* Copyright (C) 1991, 1992 Linus Torvalds
	* Copyright (C) 1997 David S. Miller (davem@caip.rutgers.edu)
	*/

	#include <linux/string.h>
	#include <linux/slab.h>
	#include <linux/file.h>
	#include <linux/fdtable.h>
	#include <linux/init.h>
	#include <linux/module.h>
	#include <linux/fs.h>
	#include <linux/security.h>
	#include <linux/eventpoll.h>
	#include <linux/rcupdate.h>
	#include <linux/mount.h>
	#include <linux/capability.h>
	#include <linux/cdev.h>
	#include <linux/fsnotify.h>
	#include <linux/sysctl.h>
	#include <linux/lglock.h>
	#include <linux/percpu_counter.h>
	#include <linux/percpu.h>
	#include <linux/ima.h>

	#include <linux/atomic.h>

	#include "internal.h"

	/* sysctl tunables... */
	struct files_stat_struct files_stat = {
	.max_files = NR_FILE
	};

	DECLARE_LGLOCK(files_lglock);
	DEFINE_LGLOCK(files_lglock);

	/* SLAB cache for file structures */
	static struct kmem_cache *filp_cachep __read_mostly;

	static struct percpu_counter nr_files __cacheline_aligned_in_smp;

	static inline void file_free_rcu(struct rcu_head *head)
	{
	struct file *f = container_of(head, struct file, f_u.fu_rcuhead);

	put_cred(f->f_cred);
	kmem_cache_free(filp_cachep, f);
	}

	static inline void file_free(struct file *f)
	{
	percpu_counter_dec(&nr_files);
	file_check_state(f);
	call_rcu(&f->f_u.fu_rcuhead, file_free_rcu);
	}

	/*
	* Return the total number of open files in the system
	*/
	static long get_nr_files(void)
	{
	return percpu_counter_read_positive(&nr_files);
	}

	/*
	* Return the maximum number of open files in the system
	*/
	unsigned long get_max_files(void)
	{
	return files_stat.max_files;
	}
	EXPORT_SYMBOL_GPL(get_max_files);

	/*
	* Handle nr_files sysctl
	*/
	#if defined(CONFIG_SYSCTL) && defined(CONFIG_PROC_FS)
	int proc_nr_files(ctl_table *table, int write,
	void __user buffer, size_t lenp, loff_t *ppos)
	{
	files_stat.nr_files = get_nr_files();
	return proc_doulongvec_minmax(table, write, buffer, lenp, ppos);
	}
	#else
	int proc_nr_files(ctl_table *table, int write,
	void __user buffer, size_t lenp, loff_t *ppos)
	{
	return -ENOSYS;
	}
	#endif

	/* Find an unused file structure and return a pointer to it.
	* Returns NULL, if there are no more free file structures or
	* we run out of memory.
	*
	* Be very careful using this. You are responsible for
	* getting write access to any mount that you might assign
	* to this filp, if it is opened for write. If this is not
	* done, you will imbalance int the mount's writer count
	* and a warning at __fput() time.
	*/
	struct file *get_empty_filp(void)
	{
	const struct cred *cred = current_cred();
	static long old_max;
	struct file * f;

	/*
	* Privileged users can go above max_files
	*/
	if (get_nr_files() >= files_stat.max_files && !capable(CAP_SYS_ADMIN)) {
	/*
	* percpu_counters are inaccurate. Do an expensive check before
	* we go and fail.
	*/
	if (percpu_counter_sum_positive(&nr_files) >= files_stat.max_files)
	goto over;
	}

	f = kmem_cache_zalloc(filp_cachep, GFP_KERNEL);
	if (f == NULL)
	goto fail;

	percpu_counter_inc(&nr_files);
	f->f_cred = get_cred(cred);
	if (security_file_alloc(f))
	goto fail_sec;

	INIT_LIST_HEAD(&f->f_u.fu_list);
	atomic_long_set(&f->f_count, 1);
	rwlock_init(&f->f_owner.lock);
	spin_lock_init(&f->f_lock);
	eventpoll_init_file(f);
	/* f->f_version: 0 */
	return f;

	over:
	/* Ran out of filps - report that */
	if (get_nr_files() > old_max) {
	pr_info("VFS: file-max limit %lu reached\n", get_max_files());
	old_max = get_nr_files();
	}
	goto fail;

	fail_sec:
	file_free(f);
	fail:
	return NULL;
	}

	/**
	* alloc_file - allocate and initialize a 'struct file'
	* @mnt: the vfsmount on which the file will reside
	* @dentry: the dentry representing the new file
	* @mode: the mode with which the new file will be opened
	* @fop: the 'struct file_operations' for the new file
	*
	* Use this instead of get_empty_filp() to get a new
	* 'struct file'. Do so because of the same initialization
	* pitfalls reasons listed for init_file(). This is a
	* preferred interface to using init_file().
	*
	* If all the callers of init_file() are eliminated, its
	* code should be moved into this function.
	*/
	struct file alloc_file(struct path path, fmode_t mode,
	const struct file_operations *fop)
	{
	struct file *file;

	file = get_empty_filp();
	if (!file)
	return NULL;

	file->f_path = *path;
	file->f_mapping = path->dentry->d_inode->i_mapping;
	file->f_mode = mode;
	file->f_op = fop;

	/*
	* These mounts don't really matter in practice
	* for r/o bind mounts. They aren't userspace-
	* visible. We do this for consistency, and so
	* that we can do debugging checks at __fput()
	*/
	if ((mode & FMODE_WRITE) && !special_file(path->dentry->d_inode->i_mode)) {
	file_take_write(file);
	WARN_ON(mnt_clone_write(path->mnt));
	}
	if ((mode & (FMODE_READ \| FMODE_WRITE)) == FMODE_READ)
	i_readcount_inc(path->dentry->d_inode);
	return file;
	}
	EXPORT_SYMBOL(alloc_file);

	/**
	* drop_file_write_access - give up ability to write to a file
	* @file: the file to which we will stop writing
	*
	* This is a central place which will give up the ability
	* to write to @file, along with access to write through
	* its vfsmount.
	*/
	void drop_file_write_access(struct file *file)
	{
	struct vfsmount *mnt = file->f_path.mnt;
	struct dentry *dentry = file->f_path.dentry;
	struct inode *inode = dentry->d_inode;

	put_write_access(inode);

	if (special_file(inode->i_mode))
	return;
	if (file_check_writeable(file) != 0)
	return;
	mnt_drop_write(mnt);
	file_release_write(file);
	}
	EXPORT_SYMBOL_GPL(drop_file_write_access);

	/* the real guts of fput() - releasing the last reference to file
	*/
	static void __fput(struct file *file)
	{
	struct dentry *dentry = file->f_path.dentry;
	struct vfsmount *mnt = file->f_path.mnt;
	struct inode *inode = dentry->d_inode;

	might_sleep();

	fsnotify_close(file);
	/*
	* The function eventpoll_release() should be the first called
	* in the file cleanup chain.
	*/
	eventpoll_release(file);
	locks_remove_flock(file);

	if (unlikely(file->f_flags & FASYNC)) {
	if (file->f_op && file->f_op->fasync)
	file->f_op->fasync(-1, file, 0);
	}
	if (file->f_op && file->f_op->release)
	file->f_op->release(inode, file);
	security_file_free(file);
	ima_file_free(file);
	if (unlikely(S_ISCHR(inode->i_mode) && inode->i_cdev != NULL &&
	!(file->f_mode & FMODE_PATH))) {
	cdev_put(inode->i_cdev);
	}
	fops_put(file->f_op);
	put_pid(file->f_owner.pid);
	file_sb_list_del(file);
	if ((file->f_mode & (FMODE_READ \| FMODE_WRITE)) == FMODE_READ)
	i_readcount_dec(inode);
	if (file->f_mode & FMODE_WRITE)
	drop_file_write_access(file);
	file->f_path.dentry = NULL;
	file->f_path.mnt = NULL;
	file_free(file);
	dput(dentry);
	mntput(mnt);
	}

	void fput(struct file *file)
	{
	if (atomic_long_dec_and_test(&file->f_count))
	__fput(file);
	}

	EXPORT_SYMBOL(fput);

	struct file *fget(unsigned int fd)
	{
	struct file *file;
	struct files_struct *files = current->files;

	rcu_read_lock();
	file = fcheck_files(files, fd);
	if (file) {
	/* File object ref couldn't be taken */
	if (file->f_mode & FMODE_PATH \|\|
	!atomic_long_inc_not_zero(&file->f_count))
	file = NULL;
	}
	rcu_read_unlock();

	return file;
	}

	EXPORT_SYMBOL(fget);

	struct file *fget_raw(unsigned int fd)
	{
	struct file *file;
	struct files_struct *files = current->files;

	rcu_read_lock();
	file = fcheck_files(files, fd);
	if (file) {
	/* File object ref couldn't be taken */
	if (!atomic_long_inc_not_zero(&file->f_count))
	file = NULL;
	}
	rcu_read_unlock();

	return file;
	}

	EXPORT_SYMBOL(fget_raw);

	/*
	* Lightweight file lookup - no refcnt increment if fd table isn't shared.
	*
	* You can use this instead of fget if you satisfy all of the following
	* conditions:
	* 1) You must call fput_light before exiting the syscall and returning control
	* to userspace (i.e. you cannot remember the returned struct file * after
	* returning to userspace).
	* 2) You must not call filp_close on the returned struct file * in between
	* calls to fget_light and fput_light.
	* 3) You must not clone the current task in between the calls to fget_light
	* and fput_light.
	*
	* The fput_needed flag returned by fget_light should be passed to the
	* corresponding fput_light.
	*/
	struct file fget_light(unsigned int fd, int fput_needed)
	{
	struct file *file;
	struct files_struct *files = current->files;

	*fput_needed = 0;
	if (atomic_read(&files->count) == 1) {
	file = fcheck_files(files, fd);
	if (file && (file->f_mode & FMODE_PATH))
	file = NULL;
	} else {
	rcu_read_lock();
	file = fcheck_files(files, fd);
	if (file) {
	if (!(file->f_mode & FMODE_PATH) &&
	atomic_long_inc_not_zero(&file->f_count))
	*fput_needed = 1;
	else
	/* Didn't get the reference, someone's freed */
	file = NULL;
	}
	rcu_read_unlock();
	}

	return file;
	}

	struct file fget_raw_light(unsigned int fd, int fput_needed)
	{
	struct file *file;
	struct files_struct *files = current->files;

	*fput_needed = 0;
	if (atomic_read(&files->count) == 1) {
	file = fcheck_files(files, fd);
	} else {
	rcu_read_lock();
	file = fcheck_files(files, fd);
	if (file) {
	if (atomic_long_inc_not_zero(&file->f_count))
	*fput_needed = 1;
	else
	/* Didn't get the reference, someone's freed */
	file = NULL;
	}
	rcu_read_unlock();
	}

	return file;
	}

	void put_filp(struct file *file)
	{
	if (atomic_long_dec_and_test(&file->f_count)) {
	security_file_free(file);
	file_sb_list_del(file);
	file_free(file);
	}
	}

	static inline int file_list_cpu(struct file *file)
	{
	#ifdef CONFIG_SMP
	return file->f_sb_list_cpu;
	#else
	return smp_processor_id();
	#endif
	}

	/* helper for file_sb_list_add to reduce ifdefs */
	static inline void __file_sb_list_add(struct file file, struct super_block sb)
	{
	struct list_head *list;
	#ifdef CONFIG_SMP
	int cpu;
	cpu = smp_processor_id();
	file->f_sb_list_cpu = cpu;
	list = per_cpu_ptr(sb->s_files, cpu);
	#else
	list = &sb->s_files;
	#endif
	list_add(&file->f_u.fu_list, list);
	}

	/**
	* file_sb_list_add - add a file to the sb's file list
	* @file: file to add
	* @sb: sb to add it to
	*
	* Use this function to associate a file with the superblock of the inode it
	* refers to.
	*/
	void file_sb_list_add(struct file file, struct super_block sb)
	{
	lg_local_lock(files_lglock);
	__file_sb_list_add(file, sb);
	lg_local_unlock(files_lglock);
	}

	/**
	* file_sb_list_del - remove a file from the sb's file list
	* @file: file to remove
	* @sb: sb to remove it from
	*
	* Use this function to remove a file from its superblock.
	*/
	void file_sb_list_del(struct file *file)
	{
	if (!list_empty(&file->f_u.fu_list)) {
	lg_local_lock_cpu(files_lglock, file_list_cpu(file));
	list_del_init(&file->f_u.fu_list);
	lg_local_unlock_cpu(files_lglock, file_list_cpu(file));
	}
	}

	#ifdef CONFIG_SMP

	/*
	* These macros iterate all files on all CPUs for a given superblock.
	* files_lglock must be held globally.
	*/
	#define do_file_list_for_each_entry(__sb, __file) \
	{ \
	int i; \
	for_each_possible_cpu(i) { \
	struct list_head *list; \
	list = per_cpu_ptr((__sb)->s_files, i); \
	list_for_each_entry((__file), list, f_u.fu_list)

	#define while_file_list_for_each_entry \
	} \
	}

	#else

	#define do_file_list_for_each_entry(__sb, __file) \
	{ \
	struct list_head *list; \
	list = &(sb)->s_files; \
	list_for_each_entry((__file), list, f_u.fu_list)

	#define while_file_list_for_each_entry \
	}

	#endif

	/**
	* mark_files_ro - mark all files read-only
	* @sb: superblock in question
	*
	* All files are marked read-only. We don't care about pending
	* delete files so this should be used in 'force' mode only.
	*/
	void mark_files_ro(struct super_block *sb)
	{
	struct file *f;

	retry:
	lg_global_lock(files_lglock);
	do_file_list_for_each_entry(sb, f) {
	struct vfsmount *mnt;
	if (!S_ISREG(f->f_path.dentry->d_inode->i_mode))
	continue;
	if (!file_count(f))
	continue;
	if (!(f->f_mode & FMODE_WRITE))
	continue;
	spin_lock(&f->f_lock);
	f->f_mode &= ~FMODE_WRITE;
	spin_unlock(&f->f_lock);
	if (file_check_writeable(f) != 0)
	continue;
	file_release_write(f);
	mnt = mntget(f->f_path.mnt);
	/* This can sleep, so we can't hold the spinlock. */
	lg_global_unlock(files_lglock);
	mnt_drop_write(mnt);
	mntput(mnt);
	goto retry;
	} while_file_list_for_each_entry;
	lg_global_unlock(files_lglock);
	}

	void __init files_init(unsigned long mempages)
	{
	unsigned long n;

	filp_cachep = kmem_cache_create("filp", sizeof(struct file), 0,
	SLAB_HWCACHE_ALIGN \| SLAB_PANIC, NULL);

	/*
	* One file with associated inode and dcache is very roughly 1K.
	* Per default don't use more than 10% of our memory for files.
	*/

	n = (mempages * (PAGE_SIZE / 1024)) / 10;
	files_stat.max_files = max_t(unsigned long, n, NR_FILE);
	files_defer_init();
	lg_lock_init(files_lglock);
	percpu_counter_init(&nr_files, 0);
	}