linux-kernel - Re: [linux-pm] Re: [PATCH] Remove process freezer from suspend to RAM pathway

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Date:	Fri, 6 Jul 2007 11:53:03 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Oliver Neukum <oliver@...kum.org>
Cc:	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Alan Stern <stern@...land.harvard.edu>,
	Miklos Szeredi <miklos@...redi.hu>, pavel@....cz,
	paulus@...ba.org, johannes@...solutions.net,
	linux-pm@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
	mjg59@...f.ucam.org
Subject: Re: [linux-pm] Re: [PATCH] Remove process freezer from suspend to RAM pathway

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
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