linux-kernel - Re: [PATCH 1/4] module: implement module_inhibit

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Date:	Tue, 25 Sep 2007 10:24:03 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Tejun Heo <htejun@...il.com>
cc:	Jonathan Corbet <corbet@....net>, <ebiederm@...ssion.com>,
	<cornelia.huck@...ibm.com>, <greg@...ah.com>,
	<kay.sievers@...y.org>, <linux-kernel@...r.kernel.org>,
	<rusty@...tcorp.com.au>
Subject: Re: [PATCH 1/4] module: implement module_inhibit_unload()

On Tue, 25 Sep 2007, Tejun Heo wrote:

> 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.

You haven't fully answered Jon's question.  Suppose
module_unload_inhibit_cnt is nonzero, so the task adds itself to the
module_unload_wait queue, changes to TASK_UNINTERRUPTIBLE, and calls
schedule.  There's nothing to prevent somebody else from waking the
task back up before the original inhibition has been lifted.

Alan Stern

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