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SubjectRe: [RFC][PATCH] PM: Introduce new top level suspend and hibernation callbacks (rev. 2)
On Sat, 22 Mar 2008, Rafael J. Wysocki wrote:

> > Yep. The only thing to watch out for is in device_pm_remove(); it
> > would be a disaster if somehow a device was removed while it was being
> > prepared/suspended/resumed/completed/whatever. I know that's not
> > supposed to happen but there's nothing to prevent it, especially if
> > the device in question doesn't have a driver. No doubt you can invent
> > a way to allow this to happen safely.
> Well, that's a separate issue that IMO should be addressed in a separate patch.
> Something like the one below comes to mind.
> The comment removed by the patch is wrong IMO, because it implies that
> device_add() may be called with the device semaphore held and that might
> deadlock in bus_attach_device().

Are you talking about this comment?

> - * Since device_pm_add() may be called with a device semaphore held,
> - * we must never try to acquire a device semaphore while holding
> - * dpm_list_mutex.

It isn't wrong. device_add() may indeed be called with a device
semaphore held -- just not the semaphore for the device being added.
Quite often it is called with device's parent's semaphore held. The
implication is not that we may deadlock in bus_attach_device(); rather
it is that the order of acquisition must always be device semaphore
first, dev_list_mutex second.

> Thus, I think we can acquire dev->sem
> in device_pm_add() and in device_pm_remove().

No, you have missed the entire point. The problem doesn't exist in the
current code; it exists only if we switch over to using a single list.
Routines like dpm_suspend() won't be able to use list_for_each_entry()
to traverse the list because entries may be removed by other threads
during the traversal. Even list_for_each_entry_safe() won't work
correctly without careful attention to details.

Alan Stern

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