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

[Tejun Heo <htejun@gmail.com>]

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.

-- 
tejun