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Date: Mon, 24 Mar 2008 22:18:46 +0100
From: "Rafael J. Wysocki" <rjw@...k.pl>
To: Alan Stern <stern@...land.harvard.edu>
Cc: pm list <linux-pm@...ts.linux-foundation.org>,
ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
Greg KH <greg@...ah.com>, Len Brown <lenb@...nel.org>,
LKML <linux-kernel@...r.kernel.org>,
Alexey Starikovskiy <astarikovskiy@...e.de>,
David Brownell <david-b@...bell.net>,
Pavel Machek <pavel@....cz>,
Benjamin Herrenschmidt <benh@...nel.crashing.org>
Subject: Re: [RFC][PATCH] PM: Introduce new top level suspend and hibernation callbacks (rev. 3)
On Monday, 24 of March 2008, Alan Stern wrote:
> On Mon, 24 Mar 2008, Rafael J. Wysocki wrote:
>
> > Hi,
> >
> > This is the 3rd revision of the patch introducing new callbacks for suspend
> > and hibernation. It has been tested on x86-64.
> ...
> > * The registrations of parentless devices are disabled before the first
> > ->prepare() method is called and enabled before the first ->resume() method
> > is called
>
> It would be okay to wait until after the last prepare() method is
> called. I don't know if it makes any difference in the end, however.
>
> > +enum dpm_state {
> > + DPM_ON,
> > + DPM_RESUMING,
> > + DPM_SUSPENDING,
> > + DPM_OFF,
> > + DPM_OFF_IRQ,
> > +};
>
> Can we also have a DPM_PREPARING state, set when ->prepare() is about
> to be called? The PM core wouldn't make use of it but some drivers
> would. (I can't think of any use at all for the analogous
> DPM_COMPLETING state, however.)
Hmm. dev->power.status is protected by dpm_list_mtx. Do you think it would be
useful to have an accessor function for reading it under the lock?
> > @@ -68,22 +59,30 @@ int device_pm_add(struct device *dev)
> ...
> > + if (dev->parent) {
> > + if (dev->parent->power.status > DPM_RESUMING) {
>
> Clearer to say: if (dev->parent->power.status >= DPM_SUSPENDING) {
OK
> ...
> > + } else if (transition_started) {
> > + /*
> > + * We refuse to register parentless devices while a PM
> > + * transition is in progress in order to avoid leaving them
> > + * unhandled down the road
> > + */
>
> Log a warning here? If this ever happened, it would be the sort of
> unexpected regression that people get all excited about.
The WARN_ON() below 'Refuse' will trigger. I think that's sufficient.
> > + goto Refuse;
> > }
> ...
>
> > +static void dpm_resume(pm_message_t state)
> > +{
> > + struct list_head list;
> > +
> > + INIT_LIST_HEAD(&list);
> > + mutex_lock(&dpm_list_mtx);
> > + transition_started = false;
> > + while (!list_empty(&dpm_list)) {
> > + struct device *dev = to_device(dpm_list.next);
> > +
> > + if (dev->power.status > DPM_SUSPENDING) {
>
> Clearer to say: if (dev->power.status >= DPM_OFF) {
OK
> Note that if dev->power.status is equal to DPM_SUSPENDING then you
> don't want to call resume_device(), but you still do want to change
> dev->power.status to DPM_RESUMING so that new children can be
> registered.
Ah, I overlooked that. Will fix.
> > + dev->power.status = DPM_RESUMING;
> > + get_device(dev);
> > + mutex_unlock(&dpm_list_mtx);
> > +
> > + resume_device(dev, state);
> > +
> > + mutex_lock(&dpm_list_mtx);
> > + put_device(dev);
> > + }
> > + if (!list_empty(&dev->power.entry))
> > + list_move_tail(&dev->power.entry, &list);
>
> A little problem here: You refer to dev after calling put_device().
The device can't be removed at this point, because we hold dpm_list_mtx, which
is needed by device_del(). Still, I'll move the put_device() to avoid
confusion (as well as in all of the other places).
> > + }
> > + list_splice(&list, &dpm_list);
>
> This isn't the way I imagined doing it (your extra "list"), but it's
> fine.
>
> ...
> > +static void dpm_complete(pm_message_t state)
> > {
> ...
> > + complete_device(dev, state);
> > +
> > + mutex_lock(&dpm_list_mtx);
> > + put_device(dev);
> > + }
> > + if (!list_empty(&dev->power.entry))
> > + list_move(&dev->power.entry, &list);
>
> Same problem with use-after-put. Also present in dpm_prepare().
>
> > }
> > + list_splice(&list, &dpm_list);
> > mutex_unlock(&dpm_list_mtx);
> > }
>
> ...
> > static int dpm_suspend(pm_message_t state)
> > {
> ...
> > error = suspend_device(dev, state);
> > +
> > mutex_lock(&dpm_list_mtx);
> > + put_device(dev);
> > if (error) {
> > printk(KERN_ERR "Could not suspend device %s: "
> > - "error %d%s\n",
> > - kobject_name(&dev->kobj),
> > - error,
> > - (error == -EAGAIN ?
> > - " (please convert to suspend_late)" :
> > - ""));
> > - dev->power.sleeping = false;
> > + "error %d\n", kobject_name(&dev->kobj), error);
> > + list_splice_init(&dpm_list, &list);
> > break;
> > }
> > - if (!list_empty(&dev->power.entry))
> > - list_move(&dev->power.entry, &dpm_off);
> > + if (!list_empty(&dev->power.entry)) {
> > + dev->power.status = DPM_OFF;
> > + list_move(&dev->power.entry, &list);
> > + }
>
> Use-after-put again.
>
> > }
> > - if (!error)
> > - all_sleeping = true;
> > + list_splice(&list, &dpm_list);
>
> Instead you could eliminate the list_splice_init() above and put here:
>
> list_splice(&list, dpm_list->prev);
>
> This will move the entries from list to the end of dpm_list.
dpm_list may be empty at this point. Wouldn't that cause any trouble?
> > mutex_unlock(&dpm_list_mtx);
> > + return error;
> > +}
>
> On the whole it looks quite good.
Okay, thanks for the comments.
Rafael
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