Re: Async suspend-resume patch w/ completions (was: Re: Async suspend-resume patch w/ rwsems)

Alan Stern
2009-12-10T15:31:05Z

On Thu, 10 Dec 2009, Rafael J. Wysocki wrote:

> > How about CONFIG_PROVE_LOCKING? If lockdep really does start
> > complaining then switching to completions would be a simple way to
> > appease it.
>
> Ah, that one is not set. I guess I'll try it later, although I've already
> decided to use completions anyway.

You should see how badly lockdep complains about the rwsems. If it
really doesn't like them then using completions makes sense.

> Index: linux-2.6/drivers/base/power/main.c
> ===================================================================
> --- linux-2.6.orig/drivers/base/power/main.c
> +++ linux-2.6/drivers/base/power/main.c
> @@ -56,6 +58,7 @@ static bool transition_started;
> void device_pm_init(struct device *dev)
> {
> dev->power.status = DPM_ON;
> + init_completion(&dev->power.completion);
> pm_runtime_init(dev);
> }

You need a matching complete_all() in device_pm_remove(), in case
someone else is waiting for the device when it gets unregistered.

> +/**
> + * dpm_synchronize - Wait for PM callbacks of all devices to complete.
> + */
> +static void dpm_synchronize(void)
> +{
> + struct device *dev;
> +
> + async_synchronize_full();
> +
> + mutex_lock(&dpm_list_mtx);
> + list_for_each_entry(dev, &dpm_list, power.entry)
> + INIT_COMPLETION(dev->power.completion);
> + mutex_unlock(&dpm_list_mtx);
> +}

I agree with Linus, initializing the completions here is weird. You
should initialize them just before using them.

> @@ -683,6 +786,7 @@ 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);
>
> @@ -697,13 +801,18 @@ static int dpm_suspend(pm_message_t stat
> put_device(dev);
> break;
> }
> - dev->power.status = DPM_OFF;
> if (!list_empty(&dev->power.entry))
> list_move(&dev->power.entry, &list);
> put_device(dev);
> + error = atomic_read(&async_error);
> + if (error)
> + break;
> }
> list_splice(&list, dpm_list.prev);

Here's something you might want to do in a later patch. These awkward
list-pointer manipulations can be simplified as follows:

static bool dpm_iterate_forward;
static struct device *dpm_next;

In device_pm_remove():

mutex_lock(&dpm_list_mtx);
if (dev == dpm_next)
dpm_next = to_device(dpm_iterate_forward ?
dev->power.entry.next : dev->power.entry.prev);
list_del_init(&dev->power.entry);
mutex_unlock(&dpm_list_mtx);

In dpm_resume():

dpm_iterate_forward = true;
list_for_each_entry_safe(dev, dpm_next, dpm_list, power.entry) {
...

In dpm_suspend():

dpm_iterate_forward = false;
list_for_each_entry_safe_reverse(dev, dpm_next, dpm_list,
power.entry) {
...

Whether this really is better is a matter of opinion; I like it.

Alan Stern