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

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Date:	Tue, 25 Sep 2007 11:09:07 -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:

> Alan Stern wrote:
> >> 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.
> 
> Hmmm... I might be missing something here.  Who else can wake up a
> thread in uninterruptible sleep?

In principle, anything can.  There has never been any guarantee in the 
kernel that a task sleeping on a waitqueue will remain asleep until 
the waitqueue is signalled.  That's part of the reason why things like 
__wait_event() are coded as loops.

Alan Stern

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