On Sat, 31 Oct 2015, Rafael J. Wysocki wrote:

Runtime PM uses a freezable workqueue, allocated in pm_start_workqueue().

That's because we don't want async runtime PM to happen during system
suspend/resume and for good reasons, so if you want to remove the freezing
mechanism, you need to stop that workqueue at the beginning of dpm_prepare
and start it again at the end of dpm_complete().

The same sort of thing is true for the USB hub driver's workqueue.  
Since it registers new devices (as a result of hotplugs), it must stop
running at the beginning of the dpm_prepare stage.  Of course, that's
also a workqueue and not a kthread.

Alan Stern