file: Rename fcheck lookup_fd_rcu

   be held.

4. To look up the file structure given an fd, a reader

   must use either fcheck() or files_lookup_fd_rcu() APIs. These

   must use either lookup_fd_rcu() or files_lookup_fd_rcu() APIs. These

   take care of barrier requirements due to lock-free lookup.

   An example::

	struct file *file;

	rcu_read_lock();

	file = fcheck(fd);

	file = lookup_fd_rcu(fd);

	if (file) {

		...

	}

   lock-free, they must be installed using rcu_assign_pointer()

   API. If they are looked up lock-free, rcu_dereference()

   must be used. However it is advisable to use files_fdtable()

   and fcheck()/files_lookup_fd_rcu() which take care of these issues.

   and lookup_fd_rcu()/files_lookup_fd_rcu() which take care of these issues.

7. While updating, the fdtable pointer must be looked up while

   holding files->file_lock. If ->file_lock is dropped, then

	*fd = n - 1;

	rcu_read_lock();

	file = fcheck(*fd);

	file = lookup_fd_rcu(*fd);

	ctx = SPUFS_I(file_inode(file))->i_ctx;

	get_spu_context(ctx);

	rcu_read_unlock();

	}

	rcu_read_lock();

	f = fcheck(fd);

	f = lookup_fd_rcu(fd);

	rcu_read_unlock();

	/* if (f != filp) means that we lost a race and another task/thread

	return files_lookup_fd_raw(files, fd);

}

/*

 * Check whether the specified fd has an open file.

 */

#define fcheck(fd)	files_lookup_fd_rcu(current->files, fd)

static inline struct file *lookup_fd_rcu(unsigned int fd)

{

	return files_lookup_fd_rcu(current->files, fd);

}

struct task_struct;