Re: Async suspend-resume patch w/ rwsems (was: Re: [GIT PULL] PM updates for 2.6.33)

Alan Stern
2009-12-09T22:38:33Z

On Wed, 9 Dec 2009, Rafael J. Wysocki wrote:

> On Wednesday 09 December 2009, Alan Stern wrote:
> > On Tue, 8 Dec 2009, Rafael J. Wysocki wrote:
> >
> > > For completness, below is the full async suspend/resume patch with rwlocks,
> > > that has been (very slightly) tested and doesn't seem to break things.
> > >
> > > [Note to Alan: lockdep doesn't seem to complain about the not annotated nested
> > > locks.]
> >
> > I can't imagine why not. And wouldn't lockdep get confused by the fact
> > that in the async case, the rwsems are released by a different process
> > from the one that acquired them?
>
> /me looks at the .config
>
> I have CONFIG_LOCKDEP_SUPPORT set, is there anything else I need to set
> in .config?

How about CONFIG_PROVE_LOCKING? If lockdep really does start
complaining then switching to completions would be a simple way to
appease it.

> > > @@ -683,10 +835,12 @@ static int dpm_suspend(pm_message_t stat
> > >
> > > INIT_LIST_HEAD(&list);
> > > mutex_lock(&dpm_list_mtx);
> > > + pm_transition = state;
> > > while (!list_empty(&dpm_list)) {
> > > struct device *dev = to_device(dpm_list.prev);
> > >
> > > get_device(dev);
> > > + dev->power.status = DPM_OFF;
> >
> > What's that for? dev->power.status is supposed to be DPM_SUSPENDING
> > until the suspend method is successfully completed.
>
> If the suspend is run asynchronoysly, the main thread will always get a
> "success" from device_suspend(), so it can't change power.status on this
> basis. I thought we could set power.status to DPM_OFF upfront and change
> it back when error is returned.
>
> The alternative would be to move the modification of power.status to
> device_suspend() and async_suspend(). Well, maybe that's better.

Yes, I think so. Or into __device_suspend(). And the same thing in
dpm_suspend_noirq().

> > How about exporting a wait_for_device_to_resume() routine? Drivers
> > could call it for non-tree resume constraints:
> >
> > void wait_for_device_to_resume(struct device *other)
> > {
> > down_read(&other->power.rwsem);
> > up_read(&other->power.rwsem);
> > }
> >
> > Unfortunately there is no equivalent for non-tree suspend constraints.
>
> If we use completions, it will be possible to just export something like
>
> dpm_wait(dev)
> {
> if (dev)
> wait_for_completion(dev->power.completion);
> }
>
> I think. It appears that will also work for suspend, unless I'm missing
> something.

It will.

Alan Stern