Re: [PATCH 1/4] module: implement module_inhibit_unload()

From: Tejun Heo
Date: Mon Sep 24 2007 - 19:19:59 EST

Jonathan Corbet wrote:
> Hi, Tejun,
> I was just looking over these changes...
>> + /* Don't proceed till inhibition is lifted. */
>> + add_wait_queue(&module_unload_wait, &wait);
>> + set_current_state(TASK_UNINTERRUPTIBLE);
>> + if (atomic_read(&module_unload_inhibit_cnt))
>> + schedule();
>> + __set_current_state(TASK_RUNNING);
>> + remove_wait_queue(&module_unload_wait, &wait);
>> +
>> + mutex_lock(&module_mutex);
> Maybe I'm missing something, but this looks racy to me. There's no
> check after schedule() to see if module_unload_inhibit_cnt is really
> zero, and nothing to keep somebody else from slipping in and raising it
> again afterward.

The unloading can proceed once module_unload_inhibit_cnt reaches zero.
An unloading thread only has to care about inhibition put in effect
before unloading has started, so there's no need to check again.

> Given your description of this tool as a "sledgehammer," might it not be
> easier to just take and hold module_mutex for the duration of the unload
> block?

That would be easier but...

* It would serialize users of the sledgehammer.
* It would block loading modules (which is often more important than
unloading them) when things go south.

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