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

["Rafael J. Wysocki" <rjw@rjwysocki.net>]

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