I can loop dev_base only once. Why?

I can loop dev_base only once. Why?

[Kirill Ratkin]

Hi. I have a problem.

I write module which scans network device list. 
I made:

int init(void)
{
  struct net_device *dev, **dp;

  printk("<1>Ok\n");
  while ((dev = *dp) != NULL) {
    printk("<1>Searching ...\n"); 
    write_lock_bh(&dev_base_lock);

    /* ... here I change one fileld ... */

    *dp = dev->next;
    write_unlock_bh(&dev_base_lock);
  }
  return 0;
}

void cleanup(void)
{
  while ((dev = *dp) != NULL) {
    write_lock_bh(&dev_base_lock);

    /* ... here turn change back  ... */

   *dp = dev->next;
   write_unlock_bh(&dev_base_lock);
 }
}

The problem:
This code works only once. Then I remove module and
start it again I see only 'Ok' string. Could you
explain me where is problem?

Regards,
Kirill.

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