Re: Re: [PATCH] Remove process freezer from suspend to RAM pathway

Rafael J. Wysocki
2007-07-06T09:53:03Z

On Friday, 6 July 2007 11:31, Oliver Neukum wrote:
> Am Freitag, 6. Juli 2007 schrieb Benjamin Herrenschmidt:
> > On Fri, 2007-07-06 at 09:13 +0200, Rafael J. Wysocki wrote:
> > >
> > > The only reason (I know of) why we don't handle uninterruptible tasks in the
> > > freezer is that we're afraid of the suspend process deadlocking with an
> > > uninterruptible task holding a lock, but AFAICS the probability of such an
> > > event is extremely small.
> >
> > What would deadlock specifically ? One of the drivers trying to acquire
> > that lock ? It would be a driver bug then.
>
> Your driver's write method looks like:
>
> mutex_lock();
> poke_some_hardware();
> wait_event_uninterruptible(); //for result
> res = evaluate_result();
> mutex_unlock();
> return res;
>
> If you put a task into the refrigerator at wait_event_interruptible()
> you will deadlock if you need this lock for the driver to go to suspend.
> The suspend method then must not take the lock _and_ it must be
> aware that there may be an ongoing operation.

s/interruptible/uninterruptible/

> you will deadlock if you need this lock for the driver to go to suspend.
> The suspend method then must not take the lock _and_ it must be
> aware that there may be an ongoing operation.

Well, is there any driver in the tree that works like that _and_ has a
.suspend() method requiring the same lock?

Besides, I'm not going to put the task into the refrigerator at that point.

Please read http://lkml.org/lkml/2007/7/6/71

Moreover, I claim that, in the context of your example, _if_ the task is stuck
at the wait_event_uninterruptible(), _then_ the freezerless suspend will
deadlock with the task.

Greetings,
Rafael

--
"Premature optimization is the root of all evil." - Donald Knuth