Re: Re: Suspend without the freezer

Rafael J. Wysocki
2007-07-31T19:08:11Z

On Tuesday, 31 July 2007 17:24, Alan Stern wrote:
> On Tue, 31 Jul 2007, Rafael J. Wysocki wrote:
>
> > > > One thought was to have the PM core acquire and hold the dpm_list_mutex
> > > > throughout the suspend. This would block registration attempts at the
> > > > point where the new device is added to the PM core's device-list.
> > > >
> > >
> > > I think blocking at this point is too late - many drivers muck with
> > > the device in different ways before registering the "result" with
> > > driver core. The device may be half-awaken by then.
>
> > An alternative could be to have a rwsem taken for writing by the PM core and
> > for reading by registration/binding/unbinding (and other suspend-sensitive code
> > paths).
>
> I think this is subject to the same weakness Dmitry mentions: By the
> time the driver would block on the new rwsem, it has already started
> mucking with the device. Worse yet, it may hold a mutex that the
> suspend method needs, thereby deadlocking the suspend. (That's what
> would happen with serio->drv_mutex in the input layer.)

Not if the rule is that the "suspend" rwsem should be acquired before any
other locks and before the device is actually accessed.

> Maybe the best answer is simply to fail all attempts at device
> registration while a suspend is underway. At least that is a known
> error path which drivers are prepared (in theory) to deal with. It
> could be implemented quite easily with an rwsem, by making the
> registration code use down_read_trylock.

Yes.

> Binding and unbinding aren't an issue once the PM core owns all the
> device semaphores.

Yes, but currently they aren't held accross the entire suspend.

Greetings,
Rafael

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