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

From: Alan Stern
Date: Tue Sep 25 2007 - 11:09:20 EST

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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