On Thu, 11 Jun 2009, Oliver Neukum wrote:
> Am Donnerstag, 11. Juni 2009 00:01:20 schrieb Rafael J. Wysocki:
> > We have queued up resume requests for the device's parent, its parent etc.,
> > the topmost one goes first. The workqueue is singlethread, so
> > pm_autoresume() is going to be run for all parents before the device
> > itself, so if that were the only resume mechanism, it would be enough to
> > check if the parent is RPM_ACTIVE.
>
> A (IDLE)
> / \
> B (SUSPENDED) C (SUSPENDED)
>
> Suppose C is to be resumed. This means first in case of A the request
> to suspend would be cancelled. Here you drop the locks:
>
> + && (dev->parent->power.runtime_status == RPM_IDLE
> + || dev->parent->power.runtime_status == RPM_SUSPENDING
> + || dev->parent->power.runtime_status == RPM_SUSPENDED)) {
> + spin_unlock_irqrestore(&dev->power.lock, flags);
> + spin_unlock_irqrestore(&dev->parent->power.lock, parent_flags);
> +
> + /* We have to resume the parent first. */
> + pm_request_resume(dev->parent);
>
> But after pm_request_resume() returns there's no means to make sure
> nothing alters it back to RPM_SUSPENDED. The workqueue doesn't help
> you because you've scheduled nothing by that time. The suspension will
> work because C is still in RPM_SUSPENDED.
This is an example where usage counters come in handy.
Alan Stern
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