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SubjectRe: [RFC][PATCH] PM: Introduce new top level suspend and hibernation callbacks (rev. 3)
On Mon, 24 Mar 2008, Rafael J. Wysocki wrote:

> > 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?

I don't think so. What I have in mind is situations where there
accessed has already been synchronized by other means, while the
prepare() method is running. For example:

Task 0 Task 1
------ ------
->prepare() is called
Waits for currently-running
registration in task 1
to finish
Does other stuff
Receives a request to register
a new child under dev
Sees that dev->power.state is
still DPM_ON, so goes ahead
with the child's registration
->prepare() returns
dev->power.state is set to
DPM_SUSPENDING
device_pm_add() checks
dev->power.state and fails
the registration

If dev->power.state had been set to DPM_PREPARING before ->prepare()
was called, then task 1 would have avoided trying to register the
child.

> > > + 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().

True, it can't be removed at this point. But it _can_ be removed
between the calls to resume_device() and mutex_lock().

> > > }
> > > - 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?

It will still work correctly. If dpm_list is empty then dpm_list->prev
is equal to &dpm_list, so it will do the same thing as your current
code does.


I just thought of another problem. At the point where
local_irq_disable() is called, in between device_suspend() and
device_power_down(), it is possible in a preemptible kernel that
another task is holding dpm_list_mtx and is in the middle of updating
the list pointers. This would mess up the traversal in
device_power_down().

I'm not sure about the best way to prevent this. Is it legal to call
unlock_mutex() while interrupts or preemption are disabled?

Alan Stern



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