Re: [RFC][PATCH 0/3] PM / sleep: Avoid resuming runtime-suspended devices during system suspend

Rafael J. Wysocki
2014-05-13T15:46:25Z

On Tuesday, May 13, 2014 11:25:07 AM Alan Stern wrote:
> Crossing emails again...
>
> On Tue, 13 May 2014, Rafael J. Wysocki wrote:
>
> > > There's nothing to prevent a runtime-suspended device from being
> > > resumed in between the ->prepare() and ->suspend() callbacks.
> >
> > I'm moving the barrier from __device_suspend() to device_prepare(), so there
> > shouldn't be surprise resumes in that time frame.
>
> A wakeup request from the hardware can cause a runtime resume, even
> if most threads are in the freezer:
>
> Not all kernel threads get frozen. One of the unfrozen threads
> could respond to the wakeup request by calling
> pm_runtime_resume().
>
> Some runtime PM callbacks are marked as IRQ-safe and can run
> directly within an interrupt handler.
>
> > > Therefore it makes little sense to check the device's runtime status in
> > > device_prepare(). The check should be done in __device_suspend().
> >
> > If we do the barrier in device_prepare(), then I'm not sure what mechanism
> > would cause the device to resume.
>
> See above. A wakeup request can arrive after the barrier has finished.
>
> > If there is one, the whole approach is in danger, because ->prepare() has to
> > check if devices are runtime-suspended and has to be sure that their status
> > won't change after it has returned 1.
>
> ->prepare() cannot guarantee in all cases that a device will remain in
> runtime suspend. Fortunately, it doesn't need to. In fact (as I
> mentioned sometime before), it doesn't even need to check whether the
> device currently is runtime suspended -- it suffices to know that _if_
> the device is runtime suspended _then_ it has the proper settings for
> system suspend.
>
> Regardless, status changes cannot cause a problem. If the device does
> get runtime-resumed after ->prepare(), it will remain that way when
> __device_suspend() runs. The device can't be runtime-suspended again,
> because device_prepare() does pm_get_noresume().
>
> Therefore, if the device is still runtime-suspended when
> __device_suspend() runs, we can be sure that its status and state are
> still the same as when ->prepare() ran.

But if the device is runtime-suspended, we cannot know if it's going to
resume a while later. That's the problem.

Rafael