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

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Message-ID: <25380.1190671205@lwn.net>
Date:	Mon, 24 Sep 2007 16:00:05 -0600
From:	corbet@....net (Jonathan Corbet)
To:	ebiederm@...ssion.com, cornelia.huck@...ibm.com, greg@...ah.com,
	stern@...land.harvard.edu, kay.sievers@...y.org,
	linux-kernel@...r.kernel.org, rusty@...tcorp.com.au
Subject: Re: [PATCH 1/4] module: implement module_inhibit_unload() 

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.

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?

jon

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