如何在ngx_lua中调用lfs模块？

我想在我的 ngx_lua 程序中使用 lfs 模块，我从 Github 上下载了源码包，使用 luajit2.0.3 编译，并将静态文件放在 /usr/local/lib 中。我的 nginx.conf 内容如下：

http {
    lua_shared_dict  my_cache 10m;
    lua_package_cpath '/usr/local/lib/lfs.so';

    init_worker_by_lua_file  'conf/cop/check1.lua';
    ...
}

check1.lua 的内容如下：

local time_at = ngx.timer.at
local shared = ngx.shared
local log = ngx.log
local delay = 5
local ngx_err = ngx.ERR
local handler, updateip, checkfile
local my_cache = shared["my_cache"]
local filename = '/home/nginx/conf/cop/ip.txt'

local lfs = require "lfs"
local attribute = lfs.attribute

if not my_cache then
    log(ngx_err, "共享字典消失")
    return
end

checkfile = function (filename)
    local attr = attributes(filename)
    if attr then
        return attr.modification
    else
        return nil
    end
end

updateip = function (filename)
    my_cache:flush_all()
    local fp = io.open(filename, "r")
    if not fp then
        log(ngx_err, "打开文件失败")
        return
    end
    for line in fp:lines() do
        my_cache:set(line, line)
    end
end

handler = function (premature, filename)
    -- 像 cron 一样在 Lua 中执行一些例行工作
    if premature then
        return
    end
    local mod
    local md5result, flags = my_cache:get("md5")
    if not md5result then
        mod = checkfile(filename)
        updateip(filename)
        my_cache:set("md5", mod)
    else
        mod = checkfile(filename)
        if mod ~= md5result then
            updateip(filename)
            my_cache:set("md5", mod)
        end
    end
    local ok, err = time_at(delay, handler, filename)
    if not ok then
        log(ngx_err, "创建定时器失败：", err)
        return
    end
end

local ok, err = time_at(delay, handler, filename)
if not ok then
    log(ngx_err, "创建定时器失败：", err)
    return
end

但当我启动 nginx 时，出现错误，内容如下：

2014/04/28 16:36:54 [error] 9437#0: lua entry thread aborted: runtime error:

/home/nginx/conf/cop/check1.lua:22: attempt to call upvalue    'attributes' (a nil value) stack traceback: coroutine 0:
/home/nginx/conf/cop/check1.lua: in function 'checkfile'
/home/nginx/conf/cop/check1.lua:50: in function </home/nginx/conf/cop/check1.lua:42>, context: ngx.timer

这是怎么回事呢？但如果我使用 luajit 命令行，就没有问题：

LuaJIT 2.0.3 -- Copyright (C) 2005-2014 Mike Pall. http://luajit.org/
JIT: ON CMOV SSE2 SSE3 SSE4.1 fold cse dce fwd dse narrow loop abc sink fuse
> lfs = require "lfs"
> attr = lfs.attributes('check1.lua')
> print(attr.modification)
1398678137
>