The upload path does not appear to be valid ???????

[eluser]Unknown[/eluser]
Hi every one,
I am having this problem in uploading image and seriously making my hairs out. I have googled and gone thorough many forums but could not solved this out.

Here is the code for uploading image

Code:

function do_upload()
{
  
if(is_dir('./uploads/'))
  {
  // echo "there is directory <br/>";
   chmod('./uploads/',0777);
  }
  else echo "<br>there is no directory</br>";
  
  $config['upload_path'] ='./uploads/';
  $config['allowed_types'] = 'gif|jpg|png';
  
  $this->load->library('upload', $config);
        
  
  if (!$this->upload->do_upload())
  {
  // echo "<br/>noupload<br/>";
   $error = array('error' => $this->upload->display_errors());
   print_r($error);
  }
  else
  {
   //echo "<br/>yesupload";
   $userfile=$this->$_FILES['userfile']['name'];;
   $data = array('upload_data' => $this->upload->data($userfile));
   $image_path = $data['upload_data']['file_name'];
  
   return $image_path;
  
  }
}

But it shows the error The upload path does not appear to be valid. I have gone through the forums and it said to add

$this->upload->initialize($config);

but whenever i add that code nothing happens even doesn't display any errors.

Please help me out of this. Thanks in advance