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SubjectRe: [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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